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Glossary

Atmospheric Pressure Pressure (force per unit area) exerted by the atmosphere on any surface by virtue of its weight; it is equivalent to the weight of a vertical column of air extending above a surface unit area to the outer limit of the atmosphere.
Average Temperature Calculator Tool to calculate average daily maximum, daily minimum and daily average temperature from period specified by user and compared to past years and climatological averages.
Chilling Hours The number of hours of winter temperature below 45 oF
Chilling Hours Calculator Tool to calculate number of chilling hours for a period specified by user and compared to previous years.
Cooling Degree Day A from of degree day used in estimating the amount of energy necessary to reduce the effective temperature of warm air. Computed by subtracting 72 oF from the specified day's average temperature.
Cooling Degree Day Calculator Tool used to calculate the number of cooling degree days for a period specified by user and compared to previous years and climatological averages.
Degree Day Difference between the mean temperature of a particular day and a pre-determined reference temperature (60 oF).
Degree Day Calculator Tool to calculate number of degree days for period specified by user and compared to previous year
Dew Point Temperature to which a volume of air must be cooled at constant pressure and constant moisture in order to reach saturation; any further cooling causes condensation.
Evapotranspiration Vaporization of water through direct evaporation from wet surfaces and the releases of water vapor by vegetation
Heating Degree Day A form of degree day used as an index for fuel consumption computed by subtracting day's average temperature from 65 oF
Heating Degree Calculator Tool used to calculate number of heating degree days for period specified by user and compared to previous years and climaltological average.
Heat Index An index that combines air temperature and relative humidity to determine apparent temperature—how it feels.
Humidity Water vapor content of air.
Minimum Temperature Estimator The dewpoint is the temperature at which water vapor condenses as dew on grass, the roof top of cars or leaves on a tree. The dewpoint has an effect on the amount and rate of heat lost to the atmosphere, especially noticeable in cold weather. Knowing the dewpoint can give growers a rough estimate of minimum temperature. During a radiation frost, when the dewpoint is reached, the latent heat released as water vapor condenses into water and slows the rate of temperature drop significantly.
Growers can estimate the minimum temperature at their location on a cold night by using the air temperature and dewpoint at sunset. The Brunt equation uses this method. The grower should use the sunset temperature directly in his grove as air temperature can vary wildly depending on elevation and topography. Dewpoint temperatures do not vary greatly over a distance of several miles, therefore the grower can use the calculated sunset dew point from the nearest GAEMN site. While the GAEMN dew point will not be the exact dewpoint of the grove, it should provide a close approximation. This method for estimating the minimum temperature was designed to be used for a stable air mass of uniform moisture. Significant errors can occur if dry air moves in or winds increase.
The GAEMN minimum temperature estimator provides a minimum temperature estimate for each site. If a grower wants an estimation for a particular site, the sunset dewpoint of the closest GAEMN site should be used along with the air temperature of the specified site. These readings can then be inserted into the estimator to calculate the minimum temperature of a specified site. The Burnt estimator calculates minimum temperatures; actual minimum temperature can vary several degrees over short distances depending on nighttime conditions . This Brunt estimator should be one of several sources used to manage a cold protect program.
Rainfall Calculator Tool to calculate total rainfall in inches and number of rainy days for a period specified by the user and compared to past years and climatological averages.
Relative Humidity A ratio, expressed in percent, of the amount of atmospheric moisture present relative to the amount that would be present if the air were saturated.
Soil Moisture Moisture contained in that portion of the soil which lies above the water table, including water vapor contained in the soil pores.
Soil Temperature Temperature at variable depths in the soil.
Soil Temperature Calculator Tool to calculate average daily soil temperature at 2", 4" and 8" for a period specified by user and compared to past years.
Solar Radiation A measure of the intensity of the sun's radiation reaching the earth's surface. Radiation with wavelengths from 0.3 to about 4um
Temperature The degree of hotness or coldness of a substance as measured by a thermometer.
Water Balance The comparison of actual and potential evapotranspiration with amount of precipitation
Water Balance Calculator Tool to calculate difference between precipitation and evapotranspiration for period specified by user and compared to previous users and climatological averages.
WBGT (Wet Bulb Globe Temperature) Index The Wet Bulb Globe Temperature is usually used as guidance for environmental heat stress to help prevent heat stroke while at work or during physical exercise. WBGT determines heat stress in humans on the job in harsh environment.

  • WBGT index of 78 oF (26 oC) 
    Cautions is advised as extreme intense physical exercise could lead to heat stroke or heat exhaustion.

  • WBGT index of 82 oF (28 oC)
    Heavy exercise for unseasoned personnel is cautioned.

  • WBGT index of 85 oF (29 oC)
    During first three weeks of training strenuous exercise for unseasoned personnel should be suspended. After second week of training, training activities may be continued at a reduced scale.

  • WBGT index of 88 oF (31oC)
    For those with less than 12 week of training in hot weather, strenuous exercise should be discontinued. Those, who have been acclimatized each season, can continue limited activity at a WBGT of 88 oF to 90 oF (31 oC - 32 oC) for six hours or less each day.

  • WBGT index of 90 oF (32 oC) +
    Physical training and strenuous exercise should be discontinued for all persons.

The following simplified formula is used:
      WBGT = 0.567 Td + 0.393 e + 3.94
  where:
      WBGT = Wet Bulb Globe Temperature (oC)
      T=  Dry bulb temperature (oC)
      e = Water vapor pressure (hPa)

Wet Bulb Temperature
  1. Isobaric wet-bulb temperature: the temperature an air parcel would have if cooled adiabatically to saturation at constant pressure by evaporation of water into it, all latent heat being supplied by the parcel.
  2. Adiabatic wet-bulb temperature (or pseudo wet-bulb temperature): the temperature an air parcel would have if cooled adiabatically to saturation and then compressed adiabatically to the original pressure in a moist-adiabatic process. This is the wet-bulb temperature as read off the thermodynamic diagram and is always less than the isobaric wet-bulb temperature, usually by a fraction of a degree centigrade.
  3. The temperature read from the wet-bulb thermometer.
We use the following steps and equations to calcuate Wet-bulb temperature:

  1. Compute e as [es(T)*rH/100]
    where es(T) = 0.611*EXP(17.27*T/(T+237.3)) in kPa
    T is drybulb temp in oC

    e = (rH/100)*0.611*EXP(17.27*T/(T+237.3))
    where e is ambient vapor pressure in kPa
     
  2. Compute dewpoint temperature (Td)
    Td = [116.9+237.3ln(e)]/[16.78-ln(e)] in oC
     
  3. Compute wet bulb temperature (Tw)
    Tw = [(GAMMA*T)+(DELTA*Td)]/(GAMMA+DELTA)
    GAMMA = 0.00066*P
    where P is ambient barometric pressure in kPa
    DELTA = 4098*e/(Td+237.3)2

    This method should be close, especially when Tw is close to Td (DELTA should be evaluated at (Tw+Td)/2.
Jensen et al. (1990) ASCE Manual No. 70 (see pages 176 & 177)
 
Wind Air motion relative to the Earth's surface. Unless otherwise specified, only the horizontal component is considered.
Wind gust The maximum recorded wind speed in the most recent averaging period.
Wind Chill The combined effect of wind and temperature on exposed human flesh.