Mapping mean areal precipitation

A method is presented for using point mean precipitation data to estimate areal values in regions of high relief. Variation of precipitation with altitude is determined. Local anomalies from this relationship are mapped, and lines of equal anomaly are drawn. By use of the mean relation corrected for the local anomaly, the mean precipitation at any point can be determined and an isohyetal map drawn. A similar approach can be used to determine mean temperature for studies of snowmelt or of potential evapotranspiration.