# 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. Chilling Hours The number of hours of temperature below a threshold temperature. Chilling Hours Calculator Tool to calculate number of chilling hours for a period specified by user and compared to previous years. Degree Day Difference between the mean temperature of a particular day and a pre-determined reference temperature. GDD are calculated by subtracting a base temperature from the average daily temperature (°F). If the resulting value is less than 0, then it is set to 0. Thus GDD are always 0 or greater. GDD = (Tmax+Tmin)/2 - Tbase Degree Day Model 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 The amount of water required to grow a crop consists of transpiration by the plant (T) due to water uptake and the water evaporated from the soil surface (E). Combined these two are called evapotranspiration (ET) and is also referred to as crop water use. ET is measured in inches (or mm) of water used per day. ETr is the estimated evapotranspiration of a reference surface of fully grown alfalfa crop and is calculated from measured weather parameters. These include solar radiation, air temperatures, relative humidity, and wind speed. The reference ET for alfalfa (ETr) is calculated with the ASCE standardized Penman-Monteith Equation (ASCE - EWRI, 2005) Heat Index An index that combines air temperature and relative humidity to determine apparent temperature—how it feels. Humidity Water vapor content of air. 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. 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 Wind Air motion relative to the Earth's surface. Unless otherwise specified, only the horizontal component is considered. Wind Chill The combined effect of wind and temperature on exposed human flesh.
Washington State University