Temperature: the air temperature measured using a shaded thermistor sensor located about 6 feet above the roof. Note that this is not an accurate measurement of the temperature near the ground (where you would most likely be located) although the daily minimum and maximum values measured by this thermometer seem to track the values from the official BYU station south of campus quite well.
Humidity: the relative humidity about 6 feet above the roof.
Pressure: the atmospheric pressure. There are two possible pressures that can be displayed on the monitor. The Absolute Pressure is the actual air pressure or the station pressure at the elevation of the tower (4720' or 1439 m elevation). The (Mean Sea Level) Pressure is the pressure corrected to sea level for comparison purposes. The correction we are currently using is to multiply the absolute pressure by exp(0.03416*Alt/Tm) where "Alt" is the altitude in meters and "Tm" is the average of the current temperature and the temperature 12 hours ago in Kelvin. Only history graphs showing the Mean Sea Level Pressure are available.
Wind Speed: the wind speed about 25' above the roof of the Eyring Science Center. Note that wind from the southeast and from the northwest may be modified by the presence of the Widtsoe Building and the Kimball Tower respectively (both buildings are higher than our wind instruments).
Wind Direction: the direction (on the monitor screen in both alphabetic directions and in degrees clockwise from north; on the graphs just in degrees) at 25' above the roof of the Eyring Science Center. As with the Wind Speed, the Wind Direction may be modified by the presence of neighboring buildings.
Solar Irradiance: the rate at which energy is deposited by the sun shining on a horizontal surface. This value varies due to both sky conditions (clouds, haze, etc.) and to the angle of the sun with respect to the horizontal surface.
Rainfall: both the current daily value (amount that has fallen since the previous midnight) and the monthly value (amount that has fallen since midnight on the first day of the current month) are available on the monitor screen. History graphs will generally only show the daily amount. Note that this rainfall gauge is not heated so snowfall will generally be registered slowly as the snow in the catch bucket melts naturally and is counted.
Heat Index: if the temperature is above 80° F the heat index will be shown on the monitor screen. This values combines the effect of the temperature and the relative humidity to arrive at an apparent temperature indicating how hot it "feels" due to those conditions. A large relative humidity will make the temperature appear to be hotter than it really is. On the history graphs this value is always shown but if the air temperature is below 70° F the heat index is arbitrarily set equal to the air temperature because the formulas to calculate the heat index generate very poor values (i.e., abnormally large) at lower temperatures.
Wind Chill: if the temperature is below 45° F the wind chill will be shown on the monitor screen. The "Wind Chill" is an effective temperature that indicates how cold it "feels" due to the combined effects of the air temperature and the wind speed. The data reported after November 1, 2001, use the new wind chill chart which more accurately reflects the apparent temperature.
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