Content:

Air pressure
Apparent temperature
Beaufort
Dew point
Evapotranspiration
Glaze
Growing degree-days
Heat transfer
Humidity
Radiation
Solar radiation
UV dose
Wind chill
Wind run



Air pressure:
Using the measurement of the height of a column of mercury to indicate barometric pressure dates back to 1643 and the invention of the mercury barometer. The greater the air pressure, the higher the mercury rises in the tube. The rise and fall is measured in inches or millimeters.
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Apparent temperature:
Apparent temperature is 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.
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Beaufort
The Beaufort wind scale was 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.
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Dew point:
While dew point is given in terms of temperature, it is actually a measure of humidity. The dew point is the temperature at wich air would be saturated and dew would begin to form if the amount of water vapor in the air were held constant.
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Evapotranspiration:
Evapotranspiration is 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.
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Glaze:
Freezing rain is made of supercooled raindrops. The rain falls in liquid form, but freezes when it hits the ground or an exposed object, creating a couting of ice known as glaze.
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Growing degree-days:
Growing degree-days are also known as GDU's (growing degree-units) or heat units.
A degree day is 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.
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Heat transfer:
Heat is transferred by three means: conduction, convection, and radiation. A radiation shield prevents unwanted radiation from biasing the measurement.
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Humidity:
Humidity (more precisely called "relative humidity") is 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.
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Radiation:
Radiation that strikes a surface at an oblique angle is spread over a larger area than radiation that strikes perpendicularly. The absorbtion of the radiation is proportional to the cosine of the angle between the perpendicular and the oblique rays.
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Solar radiation:
Solar radiation (more precisely called solar irradiance or incident solar radiation) is measured in watts per square meter. One watt is equal to 0.143 Kg-calories/minute or 1 volt-ampere.
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UV dose:
UV dose is measured in MED's (Minimal Erythermal Dose). Erythema is the medical term for the reddening of the skin due to capillary congestion. Sunburn is among the most common forms of erythema.
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Wind chill:
Wind chill is 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.
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Wind run:
Wind run is calculated by multiplying the wind speed by the measurement period and summing the results over time. If the wind speed was a constant 10 km per hour for three hours, the wind run would be equal 10 km.
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