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Weather Terms


Some of the less common weather parameters calculated by my weather stations are described here.

Further information can be found by following the Wikipedia hyperlinks.

Davis Instruments description of how they are calculated.


Apparent Temperatures



Heat Index Uses temperature and relative humidity to determine how hot the air "feels".
THSW Index Uses Temperature and relative Humidity along with Sun and Wind to determine what it "feels" like in the sun.
Wind Chill Takes into account how the wind speed affects our perception of the air temperature.
Average Wind Direction The Vantage Pro2 calculates a 10-minute average wind direction.
Average Wind Speed The Vantage Pro2 calculates a 10-minute wind speed
Cooling Degree Day Quantitative index designed to reflect the demand for energy needed to cool a home.
Daily Rain Displays the rain accumulated since 12 mid-night.
Degree Day Description of Degree Days from BizEE Software
Dew Point Temperature to which air must be cooled for saturation (100% RH) to occur, providing there is no change in water vapour concentration.
EvapoTranspiration (ET) Combination of the words "evaporation" and "transpiration". Provides a measure of the amount of water vapour returned to the air in a given area. Effectively, ET is the opposite of rainfall and is expressed in the same units (in, mm) and is calculated by measuring Solar Radiation, Wind Speed, Temperature and Humidity.
Heating Degree Day Quantitative index designed to reflect the demand for energy needed to heat a home.
Solar Radiation Displays the current solar radiation in W/m2.
Solar Energy The amount of accumulated solar radiation energy over a period of time is measured in Langleys.

1 Langley  = 11.622 Watt-hours per square meter

= 3.687 BTUs per square foot

= 41.84 kilojoules per square meter

Storm Rain Displays the rain total of the last rain event. The Davis Weather Instrument takes two tips of the rain bucket (02") to begin a storm event and 24 hours without rain to end a storm event.
UV (Ultra Violet) Radiation Exposure to UV rays can cause a range of health problems, such as sunburn, skin cancer, skin aging etc. The Vantage Pro2 displays UV readings in two scales: MEDs and UV Index.
UV MEDs MED (Minimum Erythemal Dose) is defined as the amount of sunlight exposure necessary to induce a barely perceptible redness of the skin within 24 hours after sun exposure. Because different skin types burn at different rates, 1 MED for persons with dark skin is different from 1 MED for persons with very light skin.
Skin Type Skin Colour Tanning & Sunburn History
1 - Never tans, always burns Pale or milky white; alabaster Develops red sunburn; painful swelling, skin peels
2 - Sometimes tans, usually burns Very light brown; sometimes freckles Usually burns, pinkish or red colouring appears; can gradually develop light brown tan
3 - Usually tans, sometimes burns Light tan; brown or olive; distinctly pigmented Rarely burns; shows moderately rapid tanning response
4 - Always tans, rarely burns Brown, dark brown, or black Rarely burns, shows very rapid tanning response
UV Index UV Index assigns a number between 0 and 16 to the current UV intensity. The lower the number, the lower the danger of sunburn.

Index Values

Exposure Category

0 - 2 Low
3 - 4 Moderate
5 - 6 High
7 - 9 Very High
10+ Extreme
Wind Run Wind run is measurement of the "amount" of wind passing the station during a given period of time, expressed in either "miles of wind" or "kilometers of wind". WeatherLink calculates wind run by multiplying the average wind speed for each archive record by the archive interval.

For Example:

Average Wind Speed = 5 mph

Archive Interval = 30 minutes (0.5 hours)

Wind run = 5 mph x 0.5 hours = 2.5 miles of wind



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Page last modified 24/05/2020