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Temperature

A measure of the warmth of the ambient air measured by a suitable instrument such as a thermometer.

Dew Point (Dew-Point Temperature)

A measure of atmospheric moisture. The temperature to which air must be cooled, at constant pressure and moisture content, in order for saturation to occur. The higher the dew point, the greater amount of water vapor in the air mass.

Wind

Sustained Wind Speed	Descriptive Term
40 mph or greater	Strong, dangerous, or damaging
30-40 mph	Very Windy
20-30 mph	Windy
15-25 mph	Breezy, Brisk, or Blustery
5-15 mph or 10-20 mph	None
0-5 mph	Light or light and variable wind

 

The horizontal motion of the air past a given point. Winds begin with differences in air pressures. Pressure that's higher at one place than another sets up a force pushing from the high toward the low pressure. The greater the difference in pressures, the stronger the force. The distance between the area of high pressure and the area of low pressure also determines how fast the moving air is accelerated. Meteorologists refer to the force that starts the wind flowing as the "pressure gradient force."

High and low pressure are relative. There's no set number that divides high and low pressure. Wind is used to describe the prevailing direction from which the wind is blowing with the speed given usually in miles per hour or knots. The following table gives descriptions of winds used in National Weather Service forecasts.

Visibility

The greatest distance an observer can see and identify prominent objects.

Relative Humidity

A dimensionless ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated. Since the latter amount is dependent on temperature, relative humidity is a function of both moisture content and temperature. As such, relative humidity by itself does not directly indicate the actual amount of atmospheric moisture present.

Heat Index

The Heat Index (HI) or the "Apparent Temperature" is an accurate measure of how hot it really feels when the Relative Humidity (RH) is added to the actual air temperature. To find the Heat Index (HI), look at the Heat Index (HI) Chart (right).

As an example, if the air temperature is 90°F (found at the left side of the table) and the Relative Humidity (RH) is 70% (found at the top of the table), the Heat Index (HI)--or how hot it actually feels--is 106°F. This is at the intersection of the row 90°F and the 70% column.

This index was devised for shady, light wind conditions. Exposure to full sunshine can increase Heat Index (HI) values by up to 15°F. Also strong winds, particularly with very hot, dry air, can be extremely dangerous.

Any value Heat Index (HI) greater than 105°F is in the Danger Category. When the Heat Index is between 105-115°F for 3 hours or more, a Heat Advisory will be issued by the local National Weather Service Forecast Office.

Category	Classification	Heat Index/Apparent Temperature (°F)	General Affect on People in High Risk Groups
IV	Extremely Hot	130°F or Higher	Heat/Sunstroke HIGHLY LIKELY with continued exposure
III	Very Hot	105°F - 130°F	Sunstroke, heat cramps, or heat exhaustion LIKELY, and heatstroke POSSIBLE with prolonged exposure and/or physical activity
II	Hot	90°F - 105°F	Sunstroke, heat cramps, or heat exhaustion POSSIBLE with prolonged exposure and/or physical activity
I	Very Warm	80°F - 90°F	Fatigue POSSIBLE with prolonged exposure and/or physical activity

Wind Chill

The wind chill is the effect of the wind on people and animals. The wind chill temperature is based on the rate of heat loss from exposed skin caused by wind and cold and is to give you an approximation of how cold the air feels on your body.

As the wind increases, it removes heat from the body, driving down skin temperature and eventually the internal body temperature. Therefore, the wind makes it FEEL much colder. If the temperature is 0°F and the wind is blowing at 15 mph, the wind chill temperature is -19°F. At this level, exposed skin can freeze in just a few minutes.

The only effect wind chill has on inanimate objects, such as car radiators and water pipes, is to shorten the amount of time for the object to cool. The inanimate object will not cool below the actual air temperature. For example, if the temperature outside is -5°F and the wind chill temperature is -31°F, then your car's radiator temperature will be no lower than the air temperature of -5°F.

Convective Available Potential Energy (CAPE)

It defines the vertically integrated positive buoyancy of an adiabatically rising air parcel on a sounding. This is proportional to the amount kinetic energy that the air parcel gains while it is warmer that its surrounding environment. As a result, CAPE provides the best measure of the potential instability available in the atmosphere. Increasing values of CAPE generally lead to progressively vigorous convection. However, severe thunderstorms can form in environments showing weak to moderate CAPE, especially if the Storm Relative Helicity values are high.

CAPE = 0 to 1000 > marginally unstable
CAPE = 1000 to 2500 > moderately unstable
CAPE = 2500 to 3500 > very unstable
CAPE = 3500 or greater > extremely unstable

Lifted Index (LI)

Lifted Index (LI)	Thunderstorm Indication
< -5	Very Unstable, Heavy/strong thunderstorm potential
-3 to -5	Unstable, Thunderstorms probable
0 to -2	Marginally Unstable, Thunderstorms possible

 

It is a stability index used to determine thunderstorm potential. The LI is calculated by taking a representative low level air parcel and lifting it adiabatically to 500 mb. The algebraic difference between this air parcel and the sounding temperature at 500 mb (around 18,000 feet) denotes the LI. Since the LI accounts for moisture below 850 mb, it provides more reliable stability information than the Showalter Index (SWI). The greater negative values of LI indicate energy available for parcel ascent.

K-Index

K-index value	Thunderstorm Probability
Above 35	Numerous thunderstorms
31 to 35	Scattered thunderstorms
26 to 30	Widely scattered thunderstorms
20 to 25	Isolated thunderstorms
< 20	None

 

It is a measure of the thunderstorm potential based on vertical temperature lapse rate, moisture content of the lower atmosphere, and the vertical extent of the moist layer. The temperature difference between 850 mb and 500 mb is used to parameterize the vertical temperature lapse rate. The 850 dew point provides information on the moisture content of the lower atmosphere. The vertical extent of the moist layer is represented by the difference of the 700 mb temperature and 700 mb dewpoint. This is called the 700 mb temperature-dew point depression.

The index is derived arithmetically and does not require a plotted sounding. K-index = [(850 mb temperature - 500 mb temperature) + 850 mb dew point - 700 dew point depression]. The K-index favors non-severe convection, especially heavy rain producing convection. Threshold values vary with season, location, and synoptic situation.

K-indices are also used to determine the potential of flooding. When your K-index is high (above 35), it means that you will likely see numerous thunderstorms develop. If these thunderstorms track across the same area, you may have a various serious flooding situation on your hands.

Showalter Index (SWI)

Showalter Index	Thunderstorm Consideration
< -6	Extremely unstable - good strong thunderstorm potential
-4 to -6	Very unstable - good heavy thunderstorm potential
0 to -3	Unstable - thunderstorms probable
3 to 1	Thunderstorm possible - strong trigger needed

 

It is a stability index used to determine thunderstorm potential. The SWI is calculated by lifting an air parcel adiabatically from 850 mb to 500 mb. The algebraic difference between the air parcel and the environmental temperature at 500 mb represents the SWI. It is especially useful when you have a shallow cool airmass below 850 mb concealing greater convective potential aloft. However, the SWI will underestimate the convective potential for cool layers extending above 850 mb. It also does not take in account diurnal heating or moisture below 850 mb. As a result, one must be very careful when using this index.

Total Totals (TT)

This index estimates the potential for severe convection. It combines the effects of vertical temperature lapse rate (Vertical Totals or VT) and low level moisture (Cross Totals or CT) in a given environment. The following formulas are used to create the Total Totals index:

• Vertical Totals (VT) = 850 mb temperature - 500 mb temperature
• Cross Totals (CT) = 850 mb dew point - 500 mb temperature

When the two are combined, you have the following formula: Total Totals (TT) = Vertical Totals (VT) + Cross Totals (CT). The following table shows what these relationships typically mean east of the Rockies:

 

Cross Totals	Vertical Totals	Total Totals	Forecast
30	26 or more	56	Numerous thunderstorms, scattered severe, scattered tornadoes
26-29	26 or more	52	Scattered to numerous thunderstorms, few to scattered severe, few tornadoes
24-25	26 or more	50	Scattered thunderstorms, few severe, isolated tornadoes
22-23	26 or more	48	Scattered thunderstorms, isolated severe
20-21	26 or more	46	Scattered thunderstorms
18-19	26 or more	44	Isolated or few thunderstorms

 

High lapse rates or a source of low level moisture will yield large values of TT. However, high lapse rates can produce large TT, with little supporting low level moisture. The sounding must be examined carefully to ascertain the validity of the TT for a given environment. Also as with any index, you must carefully examine your environment.

Sky Cover

Used in a forecast to describes the predominant/average sky condition based upon percent (%) of the sky covered by opaque (not transparent) clouds.

 

Sky Condition	Cloud Coverage
90-100%	Cloudy
70-90%	Mostly Cloudy or Considerable Cloudiness
40-70%	Partly Cloudy or Partly Sunny
20-40%	Mostly Clear or Mostly Sunny
0-20%	Clear or Sunny

Pressure

The force exerted by the weight of the atmosphere, also known as atmospheric pressure. When measured on a barometer, it is referred to as barometric pressure and it is expressed in inches of mercury, millibars, or kiloPascals.
 

Descriptions and script by the NWS Office  in Burlington, Vermont