Apparent Temperature. A measure of the health risk due to various combinations of high temperature and humidity. The higher the number, the greater the possibility of heat exhaustion or heat stroke.

Beaufort Wind Scale. First used in the early nineteenth century by the British Navy. For every number on the scale, there is a descriptive term such as light air, strong breeze, fresh gale, etc.

Convection. The circulatory motion that occurs at nonuniform temperatures due to gravity and density variation, resulting in the transfer of heat.

Cosine Response. The output of a solar radiation sensor based on a given input. The absorption of radiation is proportional to the cosine of the angle between an oblique ray and one that strikes perpendicularly.

Degree Day. A measure of the departure of the mean daily temperature above or below a given standard. A ten-degree difference for one day equals ten degree days, as does a one-degree difference for ten days.

Dew Point. A measure of humidity stated in terms of the temperature at which the air would be saturated and dew would begin to form if the amount of water vapor in the air were held constant.

Erythema Action Spectrum. The range of wavelengths of light responsible for erythema, which is the reddening of the skin due to capillary congestion. Sunburn is among the most common forms of erythema.

Evapotranspiration. The amount of water transferred from the earth to the atmosphere due to the combined effects of evaporation and transpiration. Transpiration is the process by which plants release water vapor into the air.

FCC. Federal Communications Commission. In the USA, the FCC develops and implements policy concerning interstate and international communications by radio, television, satellite, and cable.

Freezing Rain. Made of supercooled raindrops. The rain falls in liquid form but freezes when it hits the ground or an exposed object, creating a coating of ice known as glaze.

hPA. Hectopascal, a measurement of barometric pressure. One hPA equals 100 pascals or one millibar. One pascal is equal to a force of one newton over an area of one square meter.

Humidity. The ratio of the actual amount of water vapor in the air to the maximum amount of water vapor the air could hold at that temperature. More precisely called relative humidity.

J/cm². Joules per square centimeter, a measurement of solar energy. A joule is the unit of energy in the meter-kilogram-second system of units, equal to 10⁷ ergs or approximately 0.7375 foot-pounds.

K-Factor. Crop coefficient, available from local agricultural advisory services. To find the evapotranspiration for a given crop, multiply the reference evapotranspiration by the K-factor.

Ly. Langley, a measurement of solar energy. One langley is equal to one gram-calorie per square centimeter. A gram-calorie is the amount of heat required to raise the temperature of one gram of water one degree Celsius.

National Climatic Data Center. Collects and analyzes the weather data recorded by more than 11,500 volunteers across the United States. Located in Asheville, NC.

Mb. Millibar, a unit of pressure equal to one-thousandths of a bar. One bar equals 10⁵ pascals, 10⁵ newtons per square meter, or 10⁶ dynes per square centimeter. One millibar equals one hectopascal.

MED. Minimal erythemal dose, a measurement of UV dose. Erythema is the reddening of the skin due to capillary congestion. Sunburn is among the most common forms of erythema.

NEMA. National Electrical Manufacturers Association. Along with many other activities, they write the standards for electrical enclosures. A NEMA 4X enclosure provides protection against falling rain and splashing water, and is undamaged by the formation of ice.

Solar Energy. The energy transmitted from the sun in form of electromagnetic radiation, measured in langleys (Ly).

Solar Radiation. The electromagnetic radiation emitted by the sun. Solar radiation sensors actually measure incident solar radiation or solar irradiance, which is the amount of radiant power per unit area that flows across or onto a surface.

UV Spectrum. Ultraviolet spectrum. The range of wavelengths from 4 to 400 nanometers, beginning at the limit of visible light and overlapping the wavelengths of long x-rays.

W/m². Watts per square meter, a measurement of solar radiation. One watt is equal to 0.143 Kg-calories/minute or 1 volt-ampere.

Wind Chill. The combination of temperature and wind speed. When the wind is blowing, it carries away the air that has been warmed by your body. It feels as if the temperature is lower than it really is.

Wind Run. Calculated by multiplying the wind speed by the measurement period and summing over time. If the wind speed was a constant 10 miles per hour for three hours, the wind run would equal 30 miles.