Illinois Climate Network: Equations / Calculated Parameters
Potential evapotranspiration and dew point temperatures are calculated hourly using data collected at the Illinois Climate Network (ICN) stations.
Since December 1, 2012, potential evapotranspiration has been calculated using the Food and
Agricultural Organization (FAO) of the United Nations Penman‐Monteith equation as outlined in
FAO Irrigation and Drainage Paper No. 56 “Crop Evapotranspiration” by RG Allen, LS Pereira, D
Raes, and M Smith. Additional guidance for the calculation was obtained from The ASCE
Reference Evapotranspiration Equation, edited by RG Allen, IA Walter, RL Elliott, TA Howell, D
Itenfisu, ME Jensen, and RL Snyder which was published in 2005 by the American Society of Civil
Prior to that time, the van Bavel method was used.
Dew Point Temperature
Dew point temperature is calculated using the following equation from FAO Irrigation and
Drainage Paper No. 56 “Crop Evapotranspiration” by RG Allen, LS Pereira, D Raes, and M Smith
Where = dew point temperature,
RH = relative humidity, and
T = air temperature.