| WEATHER GLOSSARY |
Advection
The horizontal transfer of any property in the atmosphere by the
movement of air. Examples include heat and moisture advection.
Air
This is considered the misture of gases that make up the earth's
atmosphere. The principle gases that compose dry air are Nitrogen at
78.084%, Oxygen at 20.946%, Argon at .93%, and Carbon Dioxide .03%
Airmass
An extensive body of air throughout which the horizontal temperature and
moisture characteristics are similar.
ASOS
Acronym for Automated Surace Observing Systen. This system is a
collection of automated surface weather instruments that collect data.
It performs surface observations in places that either do not have a
human observer, or an observer 24-hours a day.
Barometer
An instrument used to measure atmospheric pressure. Examples include the
aneroid barometer and the mercurial barometer.
Barometric Pressure
The pressure exerted by the atmophere at a given point. The measurement
can be expressed in millibars(mb) or in inches of mercury(Hg).
Blizzard
A severe weather condition characterized by low tempratures, winds 35mph
of greater, blowing snow that can reduce visibilities to 1/4 mile or
less for more than 3 hours. A severe blizzard is characterized by
temperatures at or below 10 degrees fahrenheit, winds exceeding 45mph,
and visibility reduced by snow to near zero.
Blue Norther
Refers to a fast-moving cold front in the southern Great Plains , marked
by a dark, blue-black sky, strong north winds and temperatures that may
drop 20-30 degrees fahrenheit in a matter of minutes
Calm
Atmospheric conditions devoid of wind or any other air motion.
CAPE
Acronym for Convective Available Potential Energy. The amount of energy
available to create convection. Higher values indicate an increasing
possibility of severe weather.
Ceiling
The lowest cloud layer that is reported as broken or overcast. If the
sky is totally obscured for example by fog, then the ceiling is defined
by vertical visibility.
Chinook
A type of foehn wind. Refers to the warm downslope wind in the Rocky
Mountains that may occur after an intense cold spell when the
temperatures may rise 20-40 degrees in a matter of minutes.
Cirrus
High clouds usally above 18,000 feet, composed of ice crystals
Climate
The historical record of average daily and seasonal weather events.
Statistics are generally drawn over several decades. The word is derived
from the Greek "klima" meaning inclination, and reflects the importance
early scholars attributed to the sun's influence.
Climate Prediction Center
A branch of the National Centers for Environmental Prediction. The
center maintains a continuous watch on short-term climate fluctuations
and diagnoses and predicts them.
Climatology
The study of climate. Includes climatic data, the analysis of the causes
of the differences in climate, and the application of climatic data to
the solution of specific design or operational problems.
Cloudburst
A sudden, heavy rainfall of a showery nature. Related
Coalescence
The merging of two water drops into a single larger drop.
Cold Air Funnel
Funnel clouds, usually short-lived, that develop from relatively small
showers or thunderstorms when the air aloft is very cold. Cold air
funnels tend to touch down briefly, but in general are less violent than
most other types of tornadoes.
Cold Front
The leading edge of an advancing cold air mass that is under running and
displacing the warmer air in its path. Generally, when a cold front
passes the temperature and humidity decrease, the pressure rises, and
the wind shifts from southwest to northwest. Precipitation is usually
along or ahead of the front in the form of thunderstorms.
Condensation
The process by which water vapor undergoes a change in state from a gas
to a liquid. It's opposite is evaporation.
Convection
Motions in a fluid that transport and mix the properties of the fluid.
These properties could be heat and/or moisture. Often the term
convection is used to describe upward motion of water vapor(moisture)
forced to rise by surface heating in turn creating rain or thunderstorms
Convergence
Wind movement that results in a horizontal net inflow of air into a
particular region. Convergent winds at lower levels are associated with
upward motion.
Cooling Degree Days
A cooling degree day is a unit used to relate the day's temperature to
the energy demands of air conditioning. Cooling degree days are
calculated by subtracting 65 from a day's average temperature. For
example, if the day's high is 90°F and the day's low is 70°F, the day's
average is 80°F. Eighty minus 65 is 15 cooling degree days. Cooling
degree days can be used to compare the current summer to past summers.
It is a good way to generally keep track of how much demand there has
been for energy needed for either heating or cooling buildings. The
cooler (warmer) the weather, the larger the number of "heating (cooling)
degree days"... and the larger the number of heating (cooling) degree
days, the heavier the demand for energy needed to heat (cool) buildings.
It can also be used to compare the heat in one part of the country with
another. See also Degree Days and Heating Degree Days.
Coriolis Force
A force per unit mass that arises solely from the earth's rotation,
acting as a deflecting force. It is dependent on the latitude and speed
of the moving object. In the Northern Hemisphere the air is deflected to
the right, and in the Southern Hemisphere to the left. The coriolis
effect is almost non-existent at the equator.
Cumulonimbus Cloud
A vertically developed cloud, often capped by an anvil shaped cloud.
This cloud is otherwise known as a thundercloud. A cumulonimbus cloud
can produce tornadoes, hail, lightning, strong winds and heavy rain.
Cyclone
An area of closed pressure circulation with rotating and converging
winds. The circulation is counterclockwise in the Northern Hemisphere
and clockwise in the Southern Hemisphere. Also called a low pressure
system and the term used for tropical cyclone in the Indian Ocean .
Other phenomena with cyclonic flow may be referred to as dust devils,
tornadoes, tropical and extratropical systems.
Data Buoys
Buoys placed throughout the Gulf of Mexice and along the Atlantic and
Pacific coasts of the U.S. that relay information on air and water
temperature, air pressure, wind speed and wave conditions via radio
signals.
Degree Days
Degree Days are a practical method for determining cumulative
temperatures over the course of a season. Originally designed to
evaluate energy demand and consumption, degree days are based on how far
the average temperature departs from a human comfort level of 65 °F *.
Simply put, each degree of temperature above 65 °F is counted as one
cooling degree day, and each degree of temperature below 65°F is counted
as one heating degree day. For example, a day with an average
temperature of 80 °F will have 15 cooling degree days. The number of
degree days accumulated in a day are proportional to the amount of
heating/cooling you would have to do to a building to reach the human
comfort level of 65 °F. The degree days are accumulated each day over
the course of a heating/cooling season, and can be compared to a long
term (multi-year) average, or normal, to see if that season was warmer
or cooler than usual. See also Cooling Degree Days and Heating Degree
Days.
Depression
In meteorology it is another name for an area of low pressure, a low or
trough. It also applies to the initial stage of a developing tropical
cyclone.
Dew
Condensation in the form of small water drops that form on grass and
other obect near the ground when the temperature has fallen to the
dewpoint. Dew generally forms during the nighttime hours and evaporates
by mid to late morning.
Dewpoint
The temperature to which air must be cooled at a constant pressure to
become saturated. Example: If the air temperature is 70 degrees and the
dewpoint temperature is 70 degrees the air is saturated and dew will
form i.e, the relative humidity is 100%
Difluence
A rate at which wind flow spreads apart along an axis oriented normal to
the flow in question.
Divergence
Wind movement that results in a horizontal net outlow of air from a
particular region. Divergence at lower levels is assiciated with a
downward movement of air from aloft.
Doppler Radar
Weather radar that measures the direction and speed of a moving object,
such as drops of precipitation, by determining whether atmospheric
motion is horizontally toward or away from the radar.
Downburst
A severe localized downdraft from a thunderstorm or shower. This outward
burst of cool air creates damaging winds at or near the surface.
Drought
Abnormal, dry weather for a specific area that is prolonged and causes
serious hydrological imbalance.
Dry Line
The boundary between the dry desert air mass of the southwestern U.S.
and moist air mass from the Gulf of Mexico . It usually lies north-south
across the central and southern High Plains states druing the spring and
summer months. When a dry line passes it results in a decrease in
humidity, clearing skies, and wind shift from east/southeasterly to
west/southwesterly. Its presence influences severe weather development
in the Great Plains .
Easterlies
Usually applied to the broad patterns of persistent winds with an
easterly component, such as the easterly trade winds.
Easterly Wave
An inverted, migratory wave-like disturbance or trough in the tropical
region that moves from east to west, generally creating only a shift in
winds and rain. The low level convergence and associated convective
weather occur on the eastern side of the wave axis. It is often
associated with possible tropical cyclone development and is also known
as a tropical wave.
Echo
The energy return of a radar signal after it has hit the target. Related
radar echo.
El Nino
The cyclical warming of East Pacific Ocean sea water temperatures off
the western coast of South America that can result in significant
changes in weather patterns in the United States and elsewhere. This
occurs when warm equatorial Pacific waters move in and displace the
colder waters, cutting off the upwelling process.
Environment
The sum total of all the external conditions that effect an organism,
community, materail, or energy.
Equator
The geographic circle at 0 degrees latitude on the earth's surface. It
is equal distance from the North and South Poles and divides the
Northern Hemisphere from the Southern.
Equinox
The point at which the ecliptic intersects the celestial equator. Days
and nights are most nearly equal in duration. In the Northern
Hemisphere, the vernal equinox falls on or about March 20 and the
autumnal equinox on or about September 22.
Evaporation
The physical process by which a liquid, such as water is transformed
into a gaseous state, such as water vapor. It is the opposite physical
process of condensation.
Evapotranspiration
The total amount of water that is transferred from the earth's surface
to the atmosphere. It is made up of the evaporation of liquid or solid
water plus the transpiration from plants.
Extratropical Cyclone
Any cyclone not of tropical origin. Generally considered to be a
migratory frontal cyclone found in the middle and high latitudes.
Eye
The center of a tropical storm or hurricane, characterized by a roughly
circular area of light winds and rain-free skies. An eye will usually
develop when the maximum sustained wind speeds exceed 78 mph. It can
range in size from as small as 5 miles up to 60 miles, but the average
size is 20 miles. In general, when the eye begins to shrink in size, the
storm is intensifying.
Eyewall
An organized band of convection surrounding the eye, or center, of a
tropical cyclone. It contains cumulonimbus clouds, intense rainfall and
very strong winds.
Fahrenheit Temperature Scale
A temperature scale where water at sea level has a freezing point of +32
degrees F and a boiling point of +212 degrees F. More commonly used in
areas that observe the English system of measurement. Created in 1714 by
Gabriel Daniel Fahrenheit (1696-1736), a German physicist, who also
invented the alcohol and mercury thermometers.
Fair
This is a subjective description. Considered as pleasant weather
conditions with regard to the time of year and the physical conditions.
Feeder Bands
In tropical parlance, the lines or bands of thunderstorms that spiral
into and around the center of a tropical system. Also known as outer
convective bands, a typical hurricane may have three or more of these
bands. They occur in advance of the main rain shield and are usually 40
to 80 miles apart. In thunderstorm development, they are the lines or
bands of low level clouds that move or feed into the updraft region of a
thunderstorm.
Flanking Line
A line of attached cumulus or towering cumulus clouds of descending
height, appearing as stair steps (usually on the southwest side) of the
most active part of a supercell.
Flash Flood
A flood that rises and falls quite rapidly with little or no advance
warning, usually as the result of intense rainfall over a relatively
small area. Flash floods can be caused by situations such as a sudden
excessive rainfall, the failure of a dam, or the thaw of an ice jam.
Flood
High water flow or an overflow of rivers or streams from their natural
or artificial banks, inundating adjacent low lying areas.
Foehn
A warm dry wind on the lee side of a mountain range, whose temperature
is increased as the wind descends down the slope. It is created when air
flows downhill from a high elevation, raising the temperature by
adiabatic compression. Classified as a katabatic wind.
Fog
A visible aggregate of minute water droplets suspended in the atmosphere
at or near the surface of the earth, reducing horizontal visibility to
less than 5/8 statute miles. It is created when the temperature and the
dew point of the air have become the same, or nearly the same, and
sufficient condensation nuclei are present. It is reported as "FG" in an
observation and on the METAR.
Forecast
A statement of expected future occurrences. Weather forecasting includes
the use of objective models based on certain atmospheric parameters,
along with the skill and experience of a meteorologist.
Freezing Drizzle
Drizzle, falling as a liquid, but freezing on impact with the colder
ground or other exposed surfaces. It is reported as "FZDZ" in an
observation and on the METAR.
Freezing Rain
Rain that falls as liquid and freezes upon impact to form a coating of
glaze on the colder ground or other exposed surfaces. It is reported as
"FZRA" in an observation and on the METAR.
Front
The transition zone or interface between two air masses of different
densities, which usually means different temperatures. For example, the
area of convergence between warm, moist air and cool, dry air.
Frost
The covering of ice crystals that forms by direct sublimation on exposed
surfaces whose temperature is below freezing.
Fujita-Pearson Scale
A scale that classifies the severity of wind damage intensity based on
the degree of destruction as it relates to the wind speed as well as
path length and path width of the event. It is normally used to identify
the most intense damage exhibited by a tornado. Developed by T. Theodore
Fujita and Allen Pearson.
Funnel Cloud
A violent, rotating column of air visibly extending from the base of a
towering cumulus or cumulonimbus toward the ground, but not in contact
with it.
Geostationary Satellite
An orbiting weather satellite that maintains the same position over the
equator during the earth's rotation. Also known as GOES, an acronym for
Geostationary Operational Evnvironmental Satellite.
Geostrophic Wind
A steady horizontal motion of air along straight, parallel isobars or
contours in an unchanging pressure or contour field. It is assumed that
there is no friction, that the flow is straight with no curvature and
there is no divergence or convergence with no vertical acceleration.
Graupel
A form of frozen precipitation consisting of snowflakes or ice crystals
and supercooled water droplets frozen together.
Gravity
The force of attraction of the earth on an object. The direction is
downward relative to the earth, and it decreases with elevation or
altitude away from the earth's surface.
Green Flash
A brilliant green coloration of the upper edge of the sun, occasionally
seen as the sun's apparent disk is about to set below a clear horizon.
Greenhouse Effect
The overall warming of the earth's lower atmosphere primarily due to
carbon dioxide and water vapor which permit the sun's rays to heat the
earth, but then restrict some heat-energy from escaping back into space.
Greenwich Mean Time (GMT)
The name of the twenty-four hour time scale which is used throughout the
scientific and military communities. This is the Prime Meridian of
Longitude. The globe is divided into twenty-four time zones of 15
degrees of arc, or one hour in time apart. To the east of this meridian,
time zones are numbered from 1 to 12 and prefixed with a minus (-),
indicting the number of hours to be subtracted to obtain Greenwich Time
(GMT). To the west, the time zones are also numbered 1 through 12, but
are prefixed with a plus (+), indicating the number of hours to be added
to obtain GMT.
Ground Clutter
A pattern of radar echoes reflecting off fixed ground targets such as
buildings or hills near the radar. This may hide or confuse the proper
return echo signifying actual precipitation.
Gulf Stream
The warm, well-defined, swift, relatively narrow ocean current which
exists off the east coast of the United States , beginning near Cape
Hatteras . The term also applies to the oceanic system of currents that
dominate the western and northern Atlantic Ocean: the Florida current,
which flows through the Florida Straits between the Florida Keys and
Cuba and northwards; the Gulf Stream, which begins around Cape Hatteras
and flows northeasterly off the continental slope into the North
Atlantic; and the North Atlantic current, which begins around the Grand
Banks off Newfoundland and continues east-northeastwards towards the
British Isles.
Gust
A sudden significant increase in or rapid fluctuations of wind speed.
Peak wind must reach at least 16 knots (18 miles per hour) and the
variation between peaks and lulls is at least 10 knots (11.5 miles per
hour). The duration is usually less than twenty seconds.
Gust Front
The leading edge of the cool, gusty surface winds produced by
thunderstorm downdrafts. Sometimes confused with an outflow boundary.
Gustnado
A weak, and usually short-lived, tornado that forms along the gust front
of a thunderstorm, appearing as a temporary dust whirl or debris cloud.
Hail
Precipitation that originates in convective clouds, such as
cumulonimbus, in the form of balls or irregular pieces of ice, which
comes in different shapes and sizes. Hail is considered to have a
diameter of 5 millimeter or more; smaller bits of ice are classified as
ice pellets, snow pellets, or graupel. Individual lumps are caled
hailstones.
Haze
A suspension of fine dust and/or smoke particles in the air. Invisible
to the naked eye, the particles reduce visibility by being sufficiently
numerous to give the air an opalescent appearance.
Heat
A form of energy transferred between two systems by virtue of a
difference in temperature. The first law of thermodynamics demonstrated
that the heat absorbed by a system may be used by the system to do work
or to raise its internal energy.
Heat Exhaustion
The effect of excessive heat, particularly when combined with high
humidity on a human being. Signs of heat exhaustion include a general
weakness, heavy sweating and clammy skin, dizziness and/or fainting, and
muscle cramps.
Heat Index
T he perceived temperature to the human body based on both air
temperature and the amount of moisture (humidity) present in the air.
The body dissipates heat by varying the rate and depth of blood
circulation, by losing water through the skin and sweat glands, and as a
last resort, by panting, when blood is heated above 98.6°F (37° C).
Sweating cools the body through evaporation. However, high relative
humidity retards evaporation, robbing the body of its ability to cool
itself. When heat gain exceeds the level the body can remove, body
temperature begins to rise, and heat related illnesses and disorders may
develop. This is not the actual air temperature. As relative humidity
increases, the air seems warmer than it actually is because the body is
less able to cool itself via evaporation of perspiration. As the heat
index rises, so do health risks. When the heat index is 90°-105°F, heat
exhaustion is possible. When it is above 105°F, it is probable.
Heatstroke is possible when the heat index is above 105°F, and very
likely when it is 130°F and above. Physical activity and prolonged
exposure to the heat increase the risks. Sometimes called "apparent
temperature."
Heat Lightning
Lightning that appears as a glowing flash on the horizon. It is actually
lightning occurring in distant thunderstorms, just over the horizon and
too far away for thunder to be heard.
Heat Stroke
Introduced to the body by overexposure to high temperatures,
particularly when accompanied by high humidity. The signs of heat stroke
include when an individual's body temperature is greater than 105
degrees Fahrenheit, the skin is hot and dry, there is a rapid and
irregular pulse, perspiration has stopped, and one has lost
consciousness. Seek immediate medical aid. May be called a sun-stroke
when caused by direct exposure to the sun.
Heat Wave
A period of abnormally and uncomfortably hot weather. It could last from
several days to several weeks.
Heating Degree Day
Indicator of household energy consumption for space heating. It was
found that for an average outdoor temperature of 65 degrees Fahrenheit,
most buildings require heat to maintain a 70 degree temperature inside.
Similarly, for an average outdoor temperature of 65 degrees or more,
most buildings require air-conditioning to maintain a 70 degree
temperature inside. Example: If the weather has been cool, and the mean
temperature is, say, 55 degrees, then there have 10 heating degree days
(65 minus 55 equals 10). It is a good way to generally keep track of how
much demand there has been for energy needed for either heating or
cooling buildings. The cooler (warmer) the weather, the larger the
number of "heating (cooling) degree days"... and the larger the number
of heating (cooling) degree days, the heavier the demand for energy
needed to heat (cool) buildings. Also see Degree Days and Cooling Degree
Days.
High Clouds
A term used to signify cirriform clouds that are composed of ice
crystals and generally have bases above 20,000 feet. The main types of
high clouds are cirrus, cirrocumulus, and cirrostratus. This altitude
applies to the temperate zone. In the polar regions, these clouds may be
found at lower altitudes. In the tropics, the defining altitudes for
cloud types are generally higher.
High Pressure System
An area of relative pressure maximum that has diverging winds and a
rotation opposite to the earth's rotation. This is clockwise in the
Northern Hemisphere and counterclockwise in the Southern Hemisphere. It
is the opposite of an area of low pressure or a cyclone.
Hook Echo
A radar reflectivity pattern observed in a thunderstorm, appearing like
a fish hook and indicating favorable conditions for tornadic
development. However, hook echoes and tornadoes do not always accompany
each other.
Humidex
The humidex is a measurement used by Canadian meteorologists to reflect
the combined effect of heat and humidity. It differs from the heat index
used in the United States in using dew point rather than relative
humidity. Humidex values are generally higher than the heat index.
Humidity
The amount of water vapor in the air. It is often confused with relative
humidity or dew point.
Hurricane
The name for a tropical cyclone with sustained winds of 74 miles per
hour (65 knots) or greater in the North Atlantic Ocean, Caribbean Sea,
Gulf of Mexico, and in the eastern North Pacific Ocean. This same
tropical cyclone is known as a typhoon in the western Pacific and a
cyclone in the Indian Ocean .
Hurricane Warning
A formal advisory issued by forecasters at the National Hurricane Center
when they have determined that hurricane conditions are expected in a
coastal area or group of islands within a 24 hour period. A warning is
used to inform the public and marine interests of the storm's location,
intensity, and movement.
Hurricane Watch
A formal advisory issued by forecasters at the National Hurricane Center
when they have determined that hurricane conditions are a potential
threat to a coastal area or group of islands within a 24 to 36 hour
period. A watch is used to inform the public and marine interests of the
storm's location, intensity, and movement.
Hydrologic Cycle
Often called the water cycle, it is the vertical and horizontal
transport of water in all its states between the earth, the atmosphere,
and the seas.
Hydrometeor
Any form of atmospheric water vapor, including those blown by the wind
off the earth's surface. Liquid or solid water formation that is
suspended in the air includes clouds, fog, ice fog, and mist. Drizzle
and rain are examples of liquid precipitation, while freezing drizzle
and freezing rain are examples of freezing precipitation. Solid or
frozen precipitation includes ice pellets, hail, snow, snow pellets,
snow grains, and ice crystals. Water vapor that evaporates before
reaching the ground is virga. Examples of liquid or solid water
particles that are lifted off the earth's surface by the wind includes
drifting and blowing snow and blowing spary. Dew, frost, rime, and glaze
are examples of liquid or solid water deposits on exposed objects.
Hygrometer
An instrument that measures the water vapor content of the atmosphere.
Hypothermia
This situation occurs when the core temperature of one's body falls
below normal. It is the failure of the body to maintain adequate
production of heat under conditions of extreme cold.
Ice
Water in a solid state. It can be found in the atmosphere in the form of
ice crystals, snow, ice pellets, and hail.
Inches of Mercury
The name comes from the use of mercurial barometers which equate the
height of a column of mercury with air pressure. One inch of mercury is
equivalent to 33.86 millibars. First devised in 1644 by Evangelista
Torricelli, an Italian physicist and mathematician, to explain the
fundamental principles of hydromechanics.
Infrared
The long wave, electromagnetic radiation of radiant heat emitted by all
hot objects. On the electromagnetic spectrum, it can be found between
microwave radiation and visible light. Water vapor, ozone, and carbon
dioxide are efficient at absorbing or transmitting infrared radiation.
Insolation
Solar radiation or heating received at the earth's surface. The name is
derived from INcoming SOLar radiATION.
Instability
It is the condition of the atmosphere when spontaneous convection and
severe weather can occur. Air parcels, when displaced vertically, will
accelerate upward.
Instrument Flight Rules (IFR)
Refers to the general weather conditions a pilot can expect at the
surface and applies to the weather situations at an airport during which
a pilot must use instruments to assist in take off and landing.
Instrument Shelter
A boxlike structure designed to protect temperature measuring
instruments from exposure to direct sunshine, precipitaion, and
condensation, while at the same time time providing adequate
ventilation.
Intertropical Convergence Zone (ITCZ)
An area where the Northern and Southern Hemispheric trade winds
converge. It is a broad area of low pressure where both the coriolis
force and the low-level pressure gradient are weak, occasionally
allowing tropical disturbances to form. The ITCZ fluctuates, moving
northward over the south Atlantic during the Northern Hemipshere summer.
Inversion
It refers to an increase in an atmospheric property with height. For
example...A temperature inversion is when the temperature increases with
altitude, which is a departure from the usual decrease of temperature
with height.
Ionisphere
An atmospheric zone of ionized gases that extends between 50 and 400
miles above the surface of the Earth. It is located between the
mesosphere and the exosphere.
Isallobar
The line of equal change in atmospheric pressure during a certain time
period. It marks the change in pressure tendedncy.
Isobar
The line drawn on a weather map connecting points of equal barometric
pressure.
Isodrosotherm
The line drawn on a weather map connecting points of equal dew point.
Isohyet
THe line drawn through geographic points recording equl amounts of
rainfall during a given time or for a give storm.
Isopleth
A line connecting equal points of value.
Isotach
A line connecting equal wind speeds.
Isotherm
The line of equal or constant temperature.
Jetstreak
A region of accelerated wind speed along the axis of a jet stream.
Jetstream
A area of strong winds that are concentrated in a realatively narrow
band in the upper troposphere of the Northern and Southern Hemispheres.
Flowing in a semi-continuous band around the globe from west to east, it
is caused by the changes in air temperature where the cold polar air
moving towards the equator meets the warmer equatorial air moving
northward toward the poles.
K Index
The measure of thunderstorm potential based on the vertical temperature
lapse rate, the mosture content of the lower atmosphere and the vertical
extent of the moist layer.
Katabatic Wind
A wind that is created by air flowing downhill. When the air is warm, it
may be called a foehn wind, and regionally may be known as a Chinook, or
Santa Ana . When this air is cool, it is called a drainage wind,
mountain breeze or glacier wind.
Knot
A nautical unit of wind speed equal to the velocity at which one
nautical mile is traveled in one hour. Used primarily by marine
interests and in weather observations. 1 knot = 1.151 statute miles per
hour.
Land Breeze
A diurnal coastal breeze that blows offshore, from the land to the sea.
It is caused by the termpeature difference when the sea sruface is
warmer than the adjacent land. Predominate during the night, it reaches
its maximum around dawn.
Lapse Rate
The change of an atmospheric variable, usually temperature, with height.
A steep lapse rate implies a rapid decrease in temperature with height.
This is a sign of instability.
Latitude
The location north or south in reference to the equator, which is
designated at zero degrees. Parallel lines that circle the globe both
north and south of the equator. The poles are at 90 degrees North and
South latitude.
Lee/Leeside
The side of an object, such as a ship's sail, a mountain, or a hill,
furthest away from the wind, and therefore protected from the direct
force of the wind.
Lifted Index
A measure of atmospheric instability that is obtained by computing the
temperature that the air near the ground would have if it were lifted to
a higher level and compared to the actual temperature at that altitude.
Positive values indicate more stable air and negative values indicate
instability.
Lightning
A rapid, visible discharge of electricity hotter than the surface of the
sun. Lightning is caused by the build up of electrical potential between
cloud and ground, between clouds, or between clouds and the surrounding
air.
Longitude
The location east or west in reference to the Prime Meridian, which is
designated at 0 degrees longitude. The distance between lines of
longitude are greater at the equator and smaller at the higher
latitudes. Time zones are correlated to longitude.
Low Clouds
A term used to describe clouds with bases below 6,000 feet. Types of low
clouds include stratus, stratocumulus, cumulus and cumulonimbus.
Low Level Jet
Strong winds that are concertrated in relatively narrow bands in the
lower part of the atmosphere. It is often amplified at night.
Low Pressure System
An area of a relative pressure minimum that has converging winds and
rotates in the same direction as the Earth...counterclockwise in the
Northern Hemisphere and clockwise in the Southern Hemisphere. Also known
as a cyclone.
Lunar Eclipse
A eclipse of the moon occurs when the Earth is in a direct line between
the sun and the moon. The moon does not have any light of its own,
instead it reflects the sun's light. During a lunar eclipse the moon is
in the earth's shadow.
Macroburst
A large downburst iwht an outflow diameter of 2.5miles or larger and
damaging winds.
Macroscale
The meteorological scale covering an area ranging from the size of a
continent to the entire globe.
Mare's Tail
The name given to thin, wispy cirrus clouds composed of ice crystals
that appear as veil patches of strands, often resembling a horse's tail.
Mean Sea Level
The average height of the sea surface water level. For the United States
, it is computed by averaging the levels of all tide stages over a
ninteen year period, determined from hourly height readings measured
from a fixed predetermined reference level. It is used as a basis for
determining elevations, as the referece for all altitudes in upper air
measurements, and as the level above which altitude is measure by a
pressure altimeter for aviation. Often referred to as MSL
Mean Sea Level
The average height of the sea surface water level. For the United States
, it is computed by averaging the levels of all tide stages over a
nineteen year period, determined from hourly height readings measured
from a fix, predetermined reference level. It is used as a basis for
determining elevations, as the reference for all altitudes in upper air
measurements, and as the level above which altitude is measured by a
pressure altimeter for aviation. Often referred to as MSL.
Mercurial Barometer
An instrument used for measuring the change in atmospheric pressure. It
uses a long glass tube, open at one end and closed at the other. After
first filling the open end with mercury, it is then temporarily sealed
and placed into a cistern of mercury. A nearly perfect vacuum is
established at the closed end after the mercury descends. The height of
the column of mercury in the tube is a measurement of air pressure. As
atmospheric pressure increases, the mercury is forced from the cistern
up the tube; when the atmospheric pressure decreases, the mercury flows
back into the cistern. Measurement is taken in inches of mercury. First
used by Evangelista Torricelli (1608-1647), an Italian physicist and
mathematician, to explain the fundamental principles of hydromechanics.
Mesocyclone
An area of rotation of storm size that may often be found on the
southwest part of a supercell. Its circulation can be larger than the
tornado that may develop within it, but not necessarily. Originally a
radar term for a rotation signature that met certain criteria, it is
best seen on Doppler radar.
Mesoscale
The scale of meteorological phenomena that range in size from several
kilometers to around 100 kilometers. This includes MCCs, MCSs, and
squall lines. Smaller phenomena are classified as microscale while
larger are classified as synoptic-scale.
Mesoscale Convective Complex (MCC)
A large mesoscale convective system (MCS) which is about the size of the
state of Ohio or Iowa and lasts at least 6 hours. Generally forming
during the afternoon and evening, the complex normally reaches its peak
intensity at night when heavy rainfall and flooding become the primary
threat. Severe weather may occur at anytime.
Mesoscale Convective System (MCS)
A large organized convective weather system comprised of a number of
individual thunderstorms. It normally persists of several hours and may
be rounded or linear in shape. This term is often used to describe a
cluster of thunderstorms that does not meet the criteria of a mesoscale
convective complex (MCC).
Metar
Acronym for METeorological Aerodrome Report. It is the primary
observation code used in the United States to satisfy requirements for
reporting surface meteorological data. Minimum reporting requirements
includes wind, visibility, runway visual range, present weather, sky
condition, temperature, dew point, and altimeter setting.
Meteorology/Meteorologist
The science and study of the atmosphere and atmospheric phenomena.
Various areas of meteorology include agricultural, applied,
astrometerology, aviation, dynamic, hydrometeorology, operational, and
synoptic, to name a few. A scientist who studies the atmosphere and
atmospheric phenomena.
Microburst
A severe localized wind blasting down from a thunderstorm. It covers an
area less than 2.5 miles (4 kilometers) in diameter and is of short
duration, usually less than 5 minutes.
Microscale
The smallest scale of meteorological phenomena that range in size from a
few centimeters to a few kilometers. Larger phenomena are classified as
mesoscale. It also refers to small scale meteorological phenomena with
life spans of less than a few minutes that affect very small areas and
are strongly influenced by local conditions of temperature and terrain.
Middle Clouds
A term used to signify clouds with bases between 6,000 and 18,000 feet.
At the higher altitudes, they may also have some ice crystals, but they
are composed mainly of water droplets. Altocumulus, altostratus, and
nimbostratus are the main types of middle clouds. This altitude applies
to the temperate zone. In the polar regions, these clouds may be found
at lower altitudes. In the tropics, the defining altitudes for cloud
types are generally higher.
Millibar (MB)
The standard unit of measurement for atmospheric pressure used by the
National Weather Service. One millibar is equivalent to 100 newtons per
square meter. Standard surface pressure is 1,013.2 millibars.
Mixed Precipitation
Any of the following combinations of freezing and frozen precipitation:
snow and sleet, snow and freezing rain, or sleet alone. Rain may also be
present.
Moisture
Refers to the water vapor content in the atmosphere, or the total water,
liquid, solid or vapor, in a given volume of air.
Monsoon
The seasonal shift of winds created by the great annual temperature
variation that occurs over large areas in contrast with associated ocean
surfaces. The monsoon is associated primarily with the moisture and
copious rains that arrive with the southwest flow across southern India
. The name is derived from the word mausim, Arabic for season. This
pattern is most evident on the southern and eastern sides of Asia,
although it does occur elsewhere, such as in the southwestern United
States .
Multicell Storm
A thunderstorm made up of two or more single-cell storms.
Multiple Vortex Tornado
A tornado which has two or more condensation funnels or debris clouds,
often rotating around a common center.
National Center for
Atmospheric Research (NCAR)
A division of the University Corporation for Atmospheric Research, the
Center plans, organizes, and conducts atmospheric and related research
programs in collaboration with universities. For further information,
contact NCAR, located in Boulder , Colorado .
National Centers for Environmental Prediction (NCEP)
As part of the National Weather Service, the centers provide timely,
accurate, and continually improving worldwide forecast guidance
products. Some of the centers include the Aviation Weather Center , the
Climate Prediction Center , the Storm Prediction Center , and the
Tropical Prediction Center . Formerly known as NMC. For further
information, contact the NCEP, with central offices located in Silver
Spring , Maryland .
National Climatic Data Center (NCDC)
The agency that archives climatic data from the National Oceanic and
Atmospheric Administration as well as other climatological
organizations. For further information, contact the NCDC, located in
Asheville , North Carolina .
National Hurricane Center (NHC)
A branch of the Tropical Prediction Center , it is the office of the
National Weather Service that is responsible for tracking and
forecasting tropical cyclones over the North Atlantic, Caribbean, Gulf
of Mexico , and the Eastern Pacific. For further information, contact
the NHC, located in Miami , Florida .
National Meteorological Center (NMC)
Now incorporated into the National Centers for Environmental Prediction,
it was the division of the National Weather Service that produced,
processed, handled, and distributed meteorological and oceanographic
information to users throughout the Northern Hemisphere, specifically
U.S. governmental organizations.
National Oceanic and Atmospheric Administration (NOAA)
A branch of the U.S. Department of Commerce, it is the parent
organization of the National Weather Service. It promotes global
environmental stewardship, emphasizing atmospheric and marine resources.
For further information, contact NOAA, located in Silver Spring ,
Maryland .
National Severe Storms Forecast Center (NSSFC)
As of October 1995, the responsibilities of this Center were divided
into two branches, the Storm Prediction Center and the Aviation Weather
Center .
National Severe Storms Laboratory (NSSL)
A branch of the National Oceanic and Atmospheric Administration, it
provides accurate and timely forecasts and warnings of hazardous weather
events, especially flash floods, hail, lightning, tornadoes, and other
severe wind storms. For further information, contact the NSSL,
headquartered in Norman , Oklahoma .
National Weather Association (NWA)
An organization whose membership promotes excellence in operational
meteorology and related activities, recognizing the professional as well
as the volunteer. For further information, contact the NWA.
National Weather Service (NWS)
A primary branch of the National Oceanic and Atmospheric Administration,
it is responsible for all aspects of observing and forecasting
atmospheric conditions and their consequences, including severe weather
and flood warnings. For further information, contact the NWS.
Nimbostratus
This cloud exhibits a combination of rain or snow, and sometimes the
base of the cloud cannot be seen because of the heaviness of
precipitation. They are generally associated with fall and winter
conditions, but can occur during any season.
Nitrogen
A colorless, tasteless, odorless gas that is the most abundant
constituent of dry air. It comprises 78.09%.
Nor'easter
A cyclonic storm occurring off the east coast of North America . These
winter weather events are notorious for producing heavy snow, rain, and
tremendous waves that crash onto Atlantic beaches, often causing beach
erosion and structural damage. Wind gusts associated with these storms
can exceed hurricane force in intensity. A nor'easter gets its name from
the continuously strong northeasterly winds blowing in from the ocean
ahead of the storm and over the coastal areas.
Normal
The recognized standard value of a meteorological element as it has been
averaged in a given location over a fixed number of years. Normals are
concerned with the distribution of data within limits of common
occurrence. The parameters may include temperatures (high, low, and
deviation), pressure, precipitation (rain, snow, etc.), winds (speed and
direction), thunderstorms, amount of clouds, percent relative humidity,
etc.
Numerical Forecasting
The use of numerical models, such as the fundamental equations of
hydrodynamics subjected to observed initial conditions, to forecast the
weather.
Observation
In meterology, the evaluation of one or more meteorological elements,
like temperature, pressure or wind that describe the state of the
atmosphere at a given time. A trained observer is one who records the
evaluations of the meteorological records.
Oceanography
The study of the ocean, embracing and integrating all knowledge
pertaining to the ocean's physical boundaries, the chemisty and physics
of sea water, and marine biology.
Omega Block
A warm high aloft which has become displaced and is on the polarward
side of the jetstream. It frequently occurs in the late winter and early
spring in the Northern Hemisphere. The name comes from its resemblance
to the Greek letter, Omega, when analyzed on upper air charts.
Outflow
Also referred to as an outflow boundary. It is the outward flow of air
from a system such as a thunderstorm. It is the result of cold
downdrafts and its passage includes a wind shift and most often a
temperature drop. Outflow boundaries sometimes help produce
thunderstorms as they move into regions of instability.
Overcast
When the sky is completely covered by clouds.
Overrunning
This occurs when a relatively warm air mass is forced above a cooler air
mass of greater density. Weather generally associated with this event
include cloudiness, cool tempertures and steady precipitation.
Ozone Layer
An atmospheric layer that contains a high proportion of oxygen that
exists as ozone. It acts as a filtering mechanism against incoming
ultaviolet radiation. It is located between the troposphere and the
stratosphere between 9.5 and 12.5 miles above the Earth's surface. Ozone
at the surface is not healthy for humans to breathe.
Palmer Drought Index
A long-term meteorological drought severity index produced by the
NOAA/USDA Joint Agricultural Weather Facility. The index depicts
prolonged times, in months or years, of abnormal dryness or wetness. It
responds slowly. Changing little from week to week, it reflects
long-term moisture runoff, recharge and deep percolation, as well as
evapotranspiration.
Partly Cloudy
The state of the weather whene clouds are conspicuously present, but do
not completely cover the sky at a given time. Sometimes interchanged
with mostly sunny.
Pilot Report
A report of in-flight weather by an aircrat pilot or crew member. Often
referred to as a PIREP.
Polar Front
A semi-continuous, semi-permanent boundary between polar airmasses and
tropical air masses. An integral part of an early meteoroligical theory
known as the Polar Front Theory.
Polar-Orbiting Satellite
A satellite whose orbit passes over both of the Earth's poles gathering
cloud and temperature data.
Precipitation
Any and all forms of water, liquid or solid, that falls from clouds and
reaches the ground. This includes, drizzle, freezing drizzle, freezing
rain, hail, ice crystals, ice pellets, rain, snow, snow pellets, and
snow grains.
Pressure
The force per unit area exerted by the weight of the atmosphere above a
point on or above the earth's surface.
Pressure Gradient
The amount of pressure change that occurs over a fixed distance at a
fixed altitude.
Prevailing Wind
A wind that blows from one direction more frequently than any other
during a given period, such as a day, month, season, or year.
Profiler
A type of doppler radar that typically measures both wind speed and
direction from the surface to 55,000 feet in the atmosphere. Used to
monitor winds with height in determining severe weather threat.
Psychrometer
An instrument used to measure the water vapor content of the atmosphere.
It consists of two thermometers, a wet bulb and dry bulb. May also be
referred to as a sling psychrometer.
Quasi-stationary Front
A front which is nearly stationary and moves very little since the last
synoptic position. Also known as a stationary front.
Radar
Acronym for RAdio Detection And Ranging. An electronic instrument used
to detect distant objects and measure their range by how they scatter or
reflect radio energy. Precipitation and clouds are detected by measuring
the strength of the electromagnetic signal reflected back. Related
Terms: Doppler Radar and NEXRAD
Radiation
The process by which energy is propagated through any medium by virtue
of the wave motion of that medium. Electromagnetic radiation, which
emits heat and light, is one form. Sound waves are another.
Radiation Fog
Fog that is created when radiational cooling at the earth's surface
lowers the temperature of the air near the ground to or below its dew
point. Formation is best when there is a shallow surface layer of
relatively moist air beneath a drier layer, clear skies, and light
surface winds. This primarily occurs during the night or early morning.
Related
ground fog.
Radiosonde
An instrument attached to a weather balloon used to measure pressure,
temperature, humidity, and winds aloft. Observations are made when the
radiosonde is aloft and emits radio signals as it ascends. May be
referred to as a RAOB, an acronym for RAdiosonde OBservation.
Rainbow
A luminous arc featuring all colors of the visible light spectrum (red,
orange, yellow, green, blue, indigo, and violet). It is created by
refraction, total reflection, and the dispersion of light. It is visible
when the sun is shining through air containing water spray or raindrops,
which occurs during or immediately after a rain shower. The bow is
always observed in the opposite side of the sky from the sun.
Reflectivity
A measure of the process by which a surface can turn back a portion of
incident radiation into the medium through which the radiation
approached. It also refers to the degree by which precipitation is able
to reflect a radar beam. Related albedo.
Relative Humidity
A type of humidity that considers the ratio of the actual vapor pressure
of the air to the saturation vapor pressure. It is usually expressed in
percentage.
Retrogression
In meteorology, it is the movement of a weather system in a direction
opposite to the direction of the basic flow in which it is embedded.
Rime
The rapid freezing of supercooled water droplets as they touch an
exposed object, forming a white opaque granular deposit of ice. It is
one of the results of an ice storm, and when formed on aircraft it is
called rime icing. Related glaze
Roll Cloud
A relatively rare, low-level, horizontal, tube-shaped cloud. Although
they are associated with a thunderstorm, they are completely detached
from the base of the cumulonimbus cloud.
Rossby Waves
The movement of ridges and troughs in the upper wind patterns, primarily
the jet stream, circling the earth. Named for Carl-Gustaf Rossby, a U.S.
Weather Bureau (NWS) employee, who first theorized about the existence
of the jet stream in 1939.
Saffir-Simpson Damage-Potential Scale
Developed in the early 1970s by Herbert Saffir, a consulting engineer,
and Robert Simpson, then Director of the National Hurricane Center , it
is a measure of hurricane intensity on a scale of 1 to 5. The scale
categorizes potential damage based on barometric pressure, wind speeds,
and surge. Related Saffir Simpson Scale
Salinity
A measure of the quantity of dissolved salts in sea water. The total
amount of dissolved solids in sea water in parts per thousand by weight.
Santa Ana Winds
The hot, dry winds, generally from the east, that funnel through the
Santa Ana river valley south of the San Gabriel and San Bernadino
Mountains in southern California , including the Los Angeles basin.
Classified as katabatic, it occurs most often during the winter and it
is an example of a foehn wind.
Satellite
Any object that orbits a celestial body, such as a moon. However, the
term is often used in reference to the manufactured objects that orbit
the earth, either in a geostationary or a polar manner. Some of the
information that is gathered by weather satellites, such as GOES9,
includes upper air temperatures and humidity, recording the temperatures
of cloud tops, land, and ocean, monitoring the movement of clouds to
determine upper level wind speeds, tracing the movement of water vapor,
monitoring the sun and solar activity, and relaying data from weather
instruments around the world.
Saturation Point
The point when the water vapor n the atmosphere is at its maximum level
for the existing temperature.
Scattering
The process by which small particles suspended in the air diffuse a
portion of the incident radiation in all directions. This is a primary
reason for colors, such as blue skies, rainbows, and orange sunsets.
Scud
Low fragments of clouds, usually stratus fractus, that are unattached
and below a layer of higher clouds, either nimbostratus or cumulonimbus.
They are often along and behind cold fronts and gust fronts, being
associated with cool moist air, such as an outflow from a thunderstorm.
When observed from a distance, they are sometimes mistaken for
tornadoes.
Sea Breeze
A diurnal coastal breeze that blows onshore, from the sea to the land.
It is caused by the temperature difference when the surface of the land
is warmer than the adjacent body of water. Predominate during the day,
it reaches its maximum early to mid afternoon. It blows in the opposite
direction of a land breeze.
Sea Breeze Front
A coastal phenomena, it is restricted to large bodies of water and their
immediate coast lines. This is usually the landward extent of the sea
breeze. Due to the imbalance of heating between land and water, a region
of maximum upward motion or convergence occurs by mid-afternoon in the
summer some 10 to 15 miles inland.
Sea Fog
A type of advection fog which forms in warm moist air cooled to
saturation as the air moves across cold water. Related Arctic Sea Smoke
Sea Level
The height or level of the sea surface at any time. It is used as a
reference for elevations above and below. Related mean sea level
Sea Level Pressure
The atmospheric pressure at mean sea level, usually determined from the
observed station pressure.
Severe Thunderstorm
A thunderstorm with winds measuring 50 knots (58 mph) or greater, 3/4
inch hail or larger, or tornadoes. Severe thunderstorms may also produce
torrential rain and frequent lightning. Related supercell
Severe Weather
Generally, any destructive weather event, but usually applies to
localized storms, such as blizzards, intense thunderstorms, or
tornadoes.
Shear
It is the rate of change over a short duration. In wind shear, it can
refer to the frequent change in wind speed within a short distance. It
can occur vertically or horizontally. Directional shear is a frequent
change in direction within a short distance, which can also occur
vertically or horizontally. When used in reference to Doppler radar, it
describes the change in radial velocity over short distances
horizontally.
Short Wave
A progressive wave of smaller amplitude, wave length, and duration than
a long wave. It moves in the same direction as the basic current in
which it is embedded and may induce upward vertical motion ahead of it.
They are more numerous than long waves and often disappear with height
in the amtosphere.
Skew T-Log P Diagram
A thermodynamic diagram, using the temperature and the logarithm of
pressure as coordinates. It is used to evaluate and forecast air parcel
properties. Some values that can be determined are the Convective
Condensation Level (CCL), the Lifting Condensation Level (LCL), and the
Level of Free Convection (LFC).
Sky Cover
The amount of the celestial dome that is hidden by clouds and/or
obscurations.
Sleet
Also known as ice pellets, it is winter precipitation in the form of
small bits or pellets of ice that rebound after striking the ground or
any other hard surface.
Small Craft Advisory
An advisory issued for marine interests, especially for operators of
small boats or other vessels. Conditions include wind speeds between 20
knots (23 mph) and 34 knots (39 mph).
Snow
Frozen precipitation in the form of white or translucent ice crystals in
complex branched hexagonal form. It most often falls from stratiform
clouds, but can fall as snow showers from cumuliform ones. It usually
appears clustered into snowflakes.
snow flakes
An ice crystal or an aggregate of ice crystals which fall from clouds.
Snow Flurry/Flurries
Light showers of snow, generally very brief without any measurable
accumulation.
Snow Flurry/Flurries
Light showers of snow, generally very brief without any measurable
accumulation.
Snow Grains
Frozen precipitation in the form of very small, white, opaque grains of
ice. The solid equivalent of drizzle.
Snow Grains
Frozen precipitation in the form of very small, white, opaque grains of
ice. The solid equivalent of drizzle.
Snow Pellets
Frozen precipitation in the form of white, round or conical opaque
grains of ice. Their diameter ranges from 0.08 to 0.2 inch (2 to 5 mm).
They are easily crushed and generally break up after rebounding from a
hard surface, unlike hail. Sometimes it is called small or soft hail.
Snow Pellets
Frozen precipitation in the form of white, round or conical opaque
grains of ice. Their diameter ranges from 0.08 to 0.2 inch (2 to 5 mm).
They are easily crushed and generally break up after rebounding from a
hard surface, unlike hail. Sometimes it is called small or soft hail.
Snowflakes
An ice crystal or an aggregate of ice crystals which fall from clouds.
Solar Eclipse
An eclipse of the sun occurs when the moon is in a direct line between
the sun and the earth, casting some of the earth's surface in its
shadow. The moon's disk shaped outline appears to cover the sun's
brighter surface, or photosphere. That part of the earth that is
directly in the moon's shadow will see a total eclipse of the sun, while
the areas around it will see a partial eclipse.
Solar Radiation
"Current Solar Radiation" is technically know as Global Solar Radiation, a
measure of the intensity of the sun's radiation reacing a horizontal
surface. The irradiance includes both the direct component from the sun
and the reflected component from the rest of the sky. The solar
radiation reading gives a measure of the amount of solar radiation
hitting the solar radiation sensor at any given time, express in
Watts/sq. m (W/m2). This is useful in determining current cloud cover
conditions (more clouds = lower W/m2) as well as for use in trend
analysis of the Earth's atmosphere.
Solstice
The point at which the sun is the furthest on the ecliptic from the
celestial equator. The point at which sun is at maximum distance from
the equator and days and nights are most unequal in duration. The Tropic
of Cancer and the Tropic of Capricorn are those parallels of latitude
which lies directly beneath a solstice. In the Northern Hemisphere, the
winter solstice falls on or about December 21 and the summer solstice on
or about June 21.
Southern Oscillation
A periodic reversal of the pressure pattern across the tropical Pacific
Ocean during El Nino events. It represents the distribution of
temperature and pressure over an oceanic area.
Spring
The season of the year which occurs as the sun approaches the summer
solstice, and characterized by increasing temperatures in the
mid-latitudes. Customarily, this refers to the months of March, April,
and May in the Northern Hemisphere, and the months of September,
October, and November in the Southern Hemisphere. Astronomically, this
is the period between the vernal equinox and the summer solstice.
Squall
A sudden onset of strong winds with speeds increasing to at least 16
knots (18 miles per hour) and sustained at 22 or more knots (25 miles
per hour) for at least one minute. The intensity and duration is longer
than that of a gust.
Squall Line
A narrow band of line of active thunderstorms that is not associated
with a cold front. It may form from an outflow boundary or the leading
edge of a mesohigh.
Station Pressure
The atmospheric pressure with respect to the station elevation.
Stationary Front
A front which is nearly stationary or moves very little since the last
synoptic position. May be known as quasi-stationary front.
Storm Prediction Center (SPC)
A branch of the National Centers for Environmental Prediction, the
Center monitors and forecasts severe and non-severe thunderstorms,
tornadoes, and other hazardous weather phenomena across the United
States. Formerly known as the Severe Local Storms (SELS) unit of the
National Severe Storms Forecast Center .
Storm Winds
On the Beaufort Wind Scale, a wind with speeds from 56 to 63 knots (64
to 72 miles per hour).
Straight-Line Winds
Any surface wind that is not associated with rotation. An example is the
first gust from a thunderstorm, as opposed to tornadic winds.
Stratiform
Clouds composed of water droplets that exhibit no or have very little
vertical development. The density of the droplets often blocks sunlight,
casting shadows on the earth's surface. Bases of these clouds are
generally no more than 6,000 feet above the ground. They are classified
as low clouds, and include all varieties of stratus and stratocumulus.
Stratocumulus
A low cloud composed of layers or patches of cloud elements. It can form
from cumulus clouds becoming more stratiformed and often appears as
regularly arranged elements that may be tessellated, rounded, or
roll-shaped with relatively flat tops and bases. It is light or dark
gray in color, depending on the size of the water droplets and the
amount of sunlight that is passing through them.
Stratopause
The boundary zone or transition layer between the stratosphere and the
mesosphere. Characterized by a decrease in temperature with increasing
altitude.
Stratosphere
The layer of the atmosphere located between the troposphere and the
mesophere, characterized by a slight temperature increase and absence of
clouds. It extends between 11 and 31 miles (17 to 50 kilometers) above
the earth's surface. It is the location of the earth's ozone layer.
Stratus
One of the three basic cloud forms (the others are cirrus and cumulus).
It is also one of the two low cloud types. It is a sheetlike cloud that
does not exhibit individual elements, and is, perhaps, the most common
of all low clouds. Thick and gray, it is seen in low, uniform layers and
rarely extends higher than 5,000 feet above the earth's surface.
Sublimation
The process of a solid (ice) changing directly into a gas (water vapor),
or water vapor changing directly into ice, at the same temperature,
without ever going through the liquid state (water).
Subsidence
A sinking or downward motion of air, often seen in anticyclones. It is
most prevalent when there is colder, denser air aloft. It is often used
to imply the opposite of atmospheric convection.
Subtropical
The region between the tropical and temperate regions, an area between
35 and 40 degrees North and South latitude. This is generally an area of
semi-permanent high pressure that exists and is where the Azores and
North Pacific Highs may be found.
Subtropical Jet
Marked by a concentration of isotherms and vertical shear, this jet is
the boundary between the subtropical air and the tropical air. It is
found approximately between 25 and 35 degrees North latitude and usually
above an altitude of 40,000 feet. Its position tends to migrate south in
the Northern Hemispheric winter and north in the summer.
Summer
Astronomically, this is the period between the summer solstice and the
autumnal equinox. It is characterized as having the warmest temperatures
of the year, except in some tropical regions. Customarily, this refers
to the months of June, July, and August in the North Hemisphere, and the
months of December, January, and February in the South Hemisphere.
Supercell
A severe thunderstorm characterized by a rotating, long-lived, intense
updraft. Although not very common, they produce a relatively large
amount of severe weather, in particular, extremely large hail, damaging
straight-line winds, and practically all violent tornadoes.
Supercooling
The reduction of the temperature of any liquid below the melting point
of that substance's solid phase. Cooling a substance beyond its nominal
freezing point. Supercooled water is water that remains in a liquid
state when it is at a temperature that is well below freezing. The
smaller and purer the water droplets, the more likely they can become
supercooled.
Surface Boundary Layer
The lowest layer of the earth's atmosphere, usually up to 3,300 feet, or
one kilometer, from the earth's surface, where the wind is influenced by
the friction of the earth's surface and the objects on it.
Surge
The increase in sea water height from the level that would normally
occur were there no storm. Although the most dramatic surges are
associated with hurricanes, even smaller low pressure systems can cause
a slight increase in the sea level if the wind and fetch is just right.
It is estimated by subtracting the normal astronomic tide from the
observed storm tide.
Swell
Ocean waves that have traveled out of their generating area. Swell
characteristically exhibits a more regular and longer period and has
flatter wave crests than waves within their fetch.
Synoptic Chart
Any map or chart that depicts meteorological or atmospheric conditions
over a large area at any given time.
Synoptic Scale
The size of migratory high and low pressure systems in the lower
troposphere that cover a horizontal area of several hundred miles or
more.
Temperature
The measure of molecular motion or the degree of heat of a substance.
Terminal Doppler Weather Radar (TDWR)
Doppler radar installed at major airports throughout the United States
to detect microbursts.
Terrestrial Radiation
Long wave radiation that is emitted by the earth back into the
atmosphere. Most of it is absorbed by the water vapor in the atmosphere,
while less than ten percent is radiated directly into space.
Thermal Low
Also known as heat low, it is an area of low pressure due to the high
temperatures caused by intensive heating at the surface. It tends to
remain stationary over its source area, with weak cyclonic circulation.
Thermometer
An instrument used for measuring temperature. The different scales used
in meteorology are Celsius, Fahrenheit, and Kelvin or Absolute.
Thermosphere
A thermal classification, it is the layer of the atmosphere located
between the mesophere and out space. It is a region of steadily
increasing temperature with altitude.
THSW (Temperature/Humidity/Sun/Wind) Index
Temperature measurement that uses humidity and temperature like the Heat
Index, but also includes the heating effects of sunshine (solar
radiation) and the cooling effects of wind (like wind chill) to
calculate an apparent temperature of what it “feels” like out in the
sun.
Thunder
The sound emitted by rapidly expanding gases along the channel of a
lightning discharge. Over three-quarters of lightning's electrical
discharge is used in heating the gases in the atmosphere in and
immediately around the visible channel. Temperatures can rise to over
10,000 degrees Celsius in microseconds, resulting in a violent pressure
wave, composed of compression and rarefaction. The rumble of thunder is
created as one's ear catches other parts of the discharge, the part of
the lightning flash nearest registering first, then the parts further
away.
Thunder Snow
A wintertime thunderstorm from which falls snow instead of rain.
Thunderstorm
Produced by a cumulonimbus cloud, it is a microscale event of relatively
short duration characterized by thunder, lightning, gusty surface winds,
turbulence, hail, icing, precipitation, moderate to extreme up and
downdrafts, and under the most severe conditions, tornadoes.
Tide
The periodic rising and falling of the earth's oceans and atmosphere. It
is the result of the tide-producing forces of the moon and the sun
acting on the rotating earth. This propagates a wave throught the
atmosphere and along the surface of the earth's waters.
Tornado
A violently rotating column of air in contact with and extending between
a convective cloud and the surface of the earth. It is the most
destructive of all storm-scale atmospheric phenomena. They can occur
anywhere in the world given the right conditions, but are most frequent
in the United States in an area bounded by the Rockies on the west and
the Appalachians in the east.
Tornado Alley
A geographic corridor in the United States which stretches north from
Texas to Nebraska and Iowa . In terms of sheer numbers, this section of
the United States receives more tornadoes than any other.
Towering Cumulus
Another name for cumulus congestus, it is a rapidly growing cumulus or
an individual dome-shaped clouds whose height exceeds its width. Its
distinctive cauliflower top often mean showers below, but lacking the
characteristic anvil of a cumulonimbus, it is not a thunderstorm.
Trace
Generally, an unmeasurable or insignificant quantity. A precipitation
amount of less than 0.005 inch.
Trade Winds
Two belts of prevailing winds that blow easterly from the subtropical
high pressure centers towards the equatorial trough. Primarily lower
level winds, they are characterized by their great consistency of
direction. In the Northern Hemisphere, the trades blow from the
northeast, and in the Southern Hemisphere, the trades blow from the
southeast.
Transpiration
The process by which water in plants is transferred as water vapor to
the atmosphere.
Triple Point
The point at which any three atmospheric boundaries meet. It is most
often used to refer to the point of occlusion of an extratropical
cyclone where the cold, warm, and occluded fronts meet. Cyclogenesis may
occur at a triple point. It is also the condition of temperature and
pressure under which the gaseous, liquid, and solid forms of a substance
can exist in equilibrium.
Tropic of Cancer
The most northern point on the earth where the sun is directly overhead,
located at approximately 23.5 degrees North latitude.
Tropic of Capricorn
The most southern point on the earth where the sun is directly overhead,
located at approximately 23.5 degrees South latitude.
Tropical Air Mass
An air mass that forms in the tropics or subtropics over the low
latitudes. Maritime tropical air is produced over oceans and is warm and
humid, while continental tropical air is formed over arid regions and is
very hot and dry.
Tropical Cyclone
A warm core low pressure system which develops over tropical, and
sometimes subtropical, waters, and has an organized cirulation.
Depending on sustained surface winds, the system is classified as a
tropical disturbance, a tropical depression, a tropical storm, or a
hurricane or typhoon.
Tropical Depression
A tropical cyclone in which the maximum sustained surface winds are 38
miles per hour (33 knots) or less. Characteristically having one or more
closed isobars, it may form slowly from a tropical disturbance or an
easterly wave which has continued to organize.
Tropical Disturbance
An area of organized convection, originating in the tropics and
occasionally the subtropics, that maintains its identity for 24 hours or
more. It is often the first developmental stage of any subsequent
tropical depression, tropical storm, or hurricane.
Tropical Prediction Center (TPC)
A division of the National Centers for Environmental Prediction, the
Center issues watches, warnings, forecasts, and analyses of hazardous
weather conditions in the tropics for both domestic and international
communities. The National Hurricane Center is a branch. For further
information, contact the TCP, located in Miami , Florida .
Tropical Storm
A tropical cyclone in which the maximum sustained surface winds are from
39 miles per hour (34 knots) to 73 miles per hour (63 knots). At this
point, the system is given a name to identify and track it.
Tropical Wave
Another name for an easterly wave, it is an area of relatively low
pressure moving westward through the trade wind easterlies. Generally,
it is associated with extensive cloudiness and showers, and may be
associated with possible tropical cyclone development.
Tropics/Tropical
The region of the earth located between the Tropic of Cancer, at 23.5
degrees North latitude, and the Tropic of Capricorn, at 23.5 degrees
South latitude. It encompasses the equatorial region, an area of high
temperatures and considerable precipitation during part of the year.
Tropopause
The boundary zone or transition layer between the troposphere and the
stratosphere. This is characterized by little or no increase or decrease
in temperature or change in lapse rate with increasing altitude.
Troposphere
The lowest layer of the atmosphere located between the earth's surface
to approximately 11 miles (17 kilometers) into the atmosphere.
Characterized by clouds and weather, temperature generally decreases
with increasing altitude.
Trough
An elongated area of low atmospheric pressure that is associated with an
area of minimum cyclonic circulation. The opposite of a ridge.
Tsunami
An ocean wave with a long period that is formed by an underwater
earthquake or landslide, or volcanic eruption. It may travel unnoticed
across the ocean for thousands of miles from its point of origin and
builds up to great heights over shallower water. Also known as a seismic
sea wave, and incorrectly, as a tidal wave.
Turbulence
The irregular and instantaneous motions of air which is made up of a
number of small eddies that travel in the general air current.
Atmospheric turbulence is caused by random fluctuations in the wind
flow. It can be caused by thermal or convective currents, differences in
terrain and wind speed, along a frontal zone, or variation in
temperature and pressure.
Twilight
Often called dusk, it is the evening period of waning light from the
time of sunset to dark.
Twister
A slang term used in the United States for a tornado.
Typhoon
The name for a tropical cyclone with sustained winds of 74 miles per
hour (65 knots) or greater in the western North Pacific Ocean . This
same tropical cyclone is known as a hurricane in the eastern North
Pacific and North Atlantic Ocean, and as a cyclone in the Indian Ocean .
Ultraviolet
Electromagnetic radiation that has a wavelength shorter than visible
light and longer than x-rays. Although it accounts for only 4 to 5
percent of the total energy of insolation, it is responsible for many
complex photochemical reactions, such as fluorescence and the formation
of ozone.
United States Weather Bureau
The official name of the National Weather Service prior to 1970.
Universal Time Coordinate
One of several names for the twenty-four hour time which is used
throughout the scientific and military communities.
Unstable/Instability
Occurs when a rising air parcel becomes less dense than the surrounding
air. Since its temperature will not cool as rapidly as the surrounding
environment, it will continue to rise on its own.
Updraft
A small scale current of air with vertical motion. If there is enough
moisture, then it may condense, forming a cumulus cloud, the first step
towards thunderstorm development.
Upper Air/Upper Level
The portion of the atmosphere which is above the lower troposphere. It
is generally applied to the levels above 850 millibars. Therefore, upper
level lows and highs, troughs, winds, observations, and charts all apply
to atmospheric phenomena above the surface.
Upslope Effect
The cooling of an air flow as it ascends a hill or mountain slope. If
there is enough moisture and the air is stable, stratiform clouds and
precipitation may form. If the air is unstable, there might be an
increased chance of thunderstorm development.
UV Index
An international standard measurement of how strong the ultraviolet (UV)
radiation from the sun is at a particular place on a particular day. It
is a scale primarily used in daily forecasts aimed at the general
public. Its purpose is to help people to effectively protect themselves
from UV light, of which excessive exposure causes sunburns, eye damage
such as cataracts, skin aging, and skin cancer (see the section health
effects of ultraviolet light). Public-health organizations recommend
that people protect themselves (for example, by applying sunscreen to
the skin and wearing a hat) when the UV index is 3 or higher; see the
table below for complete recommendations.
Recommendations for protection when the day's predicted UV index is at various values are:
| UV Index | Description | Media Graphic Color | Recommended Protection |
| 0–2 | Low danger to the average person | Green | Wear sunglasses; use sunscreen if there is snow on the ground, which reflects UV radiation, or if you have particularly fair skin. |
| 3–5 | Moderate risk of harm from unprotected sun exposure | Yellow | Wear sunglasses and use sunscreen, cover the body with clothing and a hat, and seek shade around midday when the sun is most intense. |
| 6–7 | High risk of harm from unprotected sun exposure | Orange | Wear sunglasses and use sunscreen having SPF 15 or higher, cover the body with sun protective clothing and a wide-brim hat, and reduce time in the sun from two hours before to three hours after solar noon (roughly 11:00 AM to 4:00 PM during summer in zones that observe daylight saving time). |
| 8–10 | Very high risk of harm from unprotected sun exposure | Reddish-purple | Same precautions as above, but take extra care — unprotected skin can burn quickly. |
| 11+ | Extreme risk of harm from unprotected sun exposure | Violet | Take all precautions, including: wear sunglasses and use sunscreen, cover the body with a long-sleeve shirt and pants, wear a broad hat, and avoid the sun from two hours before to three hours after solar noon. |
Vapor Trail
A cloudlike streamer or trail often seen behind aircraft flying in
clear, cold, humid air. A vapor trail is created when the water vapor
from the engine exhaust gases are added to the atmosphere. Also called a
contrail, for condensation trail.
Vernal Equinox
Taking place in the Northern Hemispheric spring, it is the point at
which the ecliptic intersects the celestial equator. Days and nights are
most nearly equal in duration. It falls on or about March 20 and is
considered the beginning of spring in the Northern Hemisphere and autumn
in the Southern Hemisphere.
Vertical Temperature Profile
A series of temperature measurements taken at various levels in the
atmosphere that show the thermal structure of the atmosphere over a
specific location. Obtained through a rawinsonde sounding or comparable
method, and exhibited in a skew t-log p diagram.
Vertical Wind Profile
A series of wind direction and wind speed measurements taken at various
levels in the atmosphere that show the wind structure of the atmosphere
ove a specific location. Obtained through a rawinsonde sounding or
comparable method, and exhibited in a skew t-log p diagram.
Virga
Streaks or wisps of precipitation, such as water or ice particles, that
fall from clouds but evaporate before reaching the ground.
Visibility
A measure of the opacity of the atmosphere, and therefore, the greatest
distance one can see prominent objects with normal eyesight.
Visible Light
The portion of the electromagnetic spectrum that can be detected by the
human eye. It travels at the same speed as all other radiation, that is
at 186,000 miles per second. It has a wave length longer than
ultraviolet light and shorter than x-rays.
Visual Flight Rules (VFR)
Refers to the general weather conditions pilots can expect at the
surface. VFR criteria means a ceiling greater than 3,000 feet and
greater than 5 miles.
Vortex
Any circular or rotary flow in the atmosphere that possesses vorticity.
Vorticity
The measurement of the rotation of a small air parcel. It has vorticity
when the parcel spins as it moves along its path. Although the axis of
the rotation can extend in any direction, meteorologists are primarily
concerned with the rotational motion about an axis that is perpendicular
to the earth's surface. If it does not spin, it is said to have zero
vorticity. In the Northern Hemisphere, the vorticity is positive when
the parcel has a counterclockwise, or cyclonic, rotation. It is negative
when the parcel has clockwise, or anticyclonic, rotation.
Vorticity Maximum
A center of vorticity, or the maximum of the vorticity field fo a fluid.
Wall Cloud
An abrupt lowering of a cloud from its parent cloud base, a cumulonimbus
or supercell, with no visible precipitation underneath. Forming in the
area of a thunderstorm updraft, or inflow area, it exhibits rapid upward
movement and cyclonic rotation. It often develops before strong or
violent tornadoes.
Warm Advection
The horizontal movement of warmer air into a location.
Warm Front
The leading edge of an advancing warm air mass that is replacing a
retreating relatively colder air mass. Generally, with the passage of a
warm front, the temperature and humidity increase, the pressure rises,
and although the wind shifts (usually from the southwest to the
northwest in the Northern Hemisphere), it is not as pronounced as with a
cold frontal passage.
Warm High
A high pressure system that has its warmest temperatures at or near the
center of circulation.
Warm Low
A low pressure system that has its warmest temperatures at or near the
center of circulation. Also referred to as a warm core low.
Warning
A forecast issued when severe weather has developed, is already
occurring and reported, or is detected on radar. Warnings state a
particular hazard or imminent danger, such as tornadoes, severe
thunderstorms, flash and river floods, winter storms, heavy snows, etc.
Watch
A forecast issued well in advance of a severe weather event to alert the
public of the possibility of a particular hazard, such as
tornadoes, severe thunderstorms, flash and river floods, winter storms,
or heavy snows.
Waterspout
A small, weak tornado, which is not formed by a storm-scale rotation. It
is generally weaker than a supercell tornado and is not associated with
a wall cloud or mesocyclone. It may be observed beneath cumulonimbus or
towering cumulus clouds and is the water equivalent of a landspout.
Weather
The state of the atmosphere at a specific time and with respect to its
effect on life and human activities. It is the short term variations of
the atmosphere, as opposed to the long term, or climatic, changes.
Weather Surveillance Radar (WSR-88D)
The newest generation of Doppler radars, the 1988 Doppler weather radar.
The radar units, with help from a set of computers, show very detailed
images of precipitation and other phenomena, including air motions
within a storm.
Westerlies
Usually applied to the broad patterns of persistent winds with a
westerly component. It is the dominant persistent atmospheric motion,
centered over the midlatitudes of each hemisphere.
Wet Bulb
The wet bulb temperature is the temperature at which no more evaporation
will occur, and thus no further decrease in the temperature. The air
will continue to cool until the air can evaporate no more moisture. The
temperature, when the cooling continues until the evaporation stops and
the air becomes saturated, is the wet bulb temperature.
Wet Bulb Depression
Dependent on the temperature and the humidity of the air, it is the
difference between the dry bulb and the wet bulb readings.
Wet Bulb Thermometer
A thermometer used to measure the lowest temperature in the ambient
atmosphere in its natural state by evaporating water from a wet
muslin-covered bulb of a thermometer. The wet bulb temperature is used
to compute dew point and relative humidity. One of the two thermometers
that make up a psychrometer.
Whiteout
When visibility is near zero due to blizzard conditions or occurs on
sunless days when clouds and surface snow seem to blend, erasing the
horizon and creating a completely white vista.
Wind
Air that flows in relation to the earth's surface, generally
horizontally. There are four areas of wind that are measured: direction,
speed, character (gusts and squalls), and shifts. Surface winds are
measured by wind vanes and anemometers, while upper level winds are
detected through pilot balloons, rawin, or aircraft reports.
Wind Chill Index
The calculation of temperature that takes into consideration the effects
of wind and temperature on the human body. Describes the average loss of
body heat and how the temperature feels. This is not the actual air
temperature.
Wind Direction
The direction from which the wind is blowing.
Wind Run
A measurement of how much wind has passed a given point in a period of
time. For example, a wind blowing at three miles per hour for an entire
hour would give a wind run of three miles. Used to calculate
evapotranspiration as well as sea wave height.
Wind Shear
The rate of wind speed or direction change with distance. Vertical wind
shear is the rate of change of the wind with respect to altitude.
Horizontal wind shear is the rate of change on a horizontal plane.
Wind Vane
An instrument that indicates the wind direction. The end of the vane
which offers the greatest resistance to the motion of the air moves to
the downwind position.
Windward
The direction from which the wind is blowing. Also the upwind side of an
object. The opposite of the downwind or leeward side.
Winter
Astronomically, this is the period between the winter solstice and the
vernal equinox. It is characterized as having the coldest temperatures
of the year, when the sun is primarily over the opposite hemisphere.
World Meteorological Organization (WMO)
From weather prediction to air pollution research, climate change
related activities, ozone layer depletion studies and tropical storm
forecasting, the World Meteorological Organization coordinates global
scientific activity to allow increasingly prompt and accurate weather
information and other services for public, private and commercial use,
including international airline and shipping industries. Established by
the United Nations in 1951, it is composed of 184 members.
X-rays
The portion of the electromagnetic spectrum that has a very short wave
length. It has a wave length longer than gamma rays, yet shorter than
visible light. X-rays can penetrate various thicknesses of all solids,
and when absorbed by a gas, can result in ionization.
Zonal Flow
The flow of air along a latitudinal component of existing flow, normally
from west to east.
One of several names for the twenty-four hour time which is used throughout the scientific and military communities.