Calculates potential evapotranspiration using the Thornthwaite method from monthly mean temperature and latitude-adjusted daylight hours. Thornthwaite PET is temperature-only — it underestimates PET in windy, dry climates; the Penman-Monteith equation is preferred where radiation and humidity data are available.
Inputs
T Mean C
Thermal state of the substance. Check whether the formula needs Celsius, Fahrenheit, or Kelvin (K = °C + 273.15).
I Heat Index
Reference formula or conversion factor shown for context.
N Daylight Hr
Arithmetic average. Sensitive to outliers — if your data has extreme values, the median may be more representative.
Month Days
Reference formula or conversion factor shown for context.