Simple Procedures for Extrapolation of Humidity Variables in the Mountainous Western United States

Abstract

A series of simple procedures are presented for extrapolating climatic averages of humidity variables from a reference location with long-term humidity measurements to nearby higher elevation locations. The extrapolation of monthly average dewpoint temperature is accomplished by using an exponential function for the height decrease of water vapor pressure. This procedure results in a lapse rate of dewpoint temperature which is to a first approximation a constant. lie root-mean-square (rms) error in estimating dewpoint temperature at nine higher elevation locations averaged 1.I°C. Summer wet-bulb design temperatures are estimated using a region-wide lapse rate of 4.4°C km?1. The rms error in estimating 1%, 2.5%, and 5% wet-bulb design temperatures at 12 higher elevation locations was 0.9°C. Three-hour monthly average wet-bulb temperatures and relative humidity are estimated from the 3-hour monthly average dewpoint temperature and the 3-hour monthly average air temperature at the reference location with the following procedure. An estimate of the 3-hour monthly average air temperature at the desired location is obtained using an average lapse rate calculated from the monthly mean maximum and minimum temperatures at the two locations. An estimate of 3-hour monthly average dewpoint temperature is obtained using the same procedure that was developed for monthly average dewpoint temperature. Estimates of 3-hour monthly average wet-bulb temperature and relative humidity are then calculated from the estimated air and dewpoint temperatures. A test of this procedure for two station pairs resulted in good agreement for 3-hour monthly average air and wet-bulb temperatures with rms values of 0.8°C and 0.7°C, respectively. The rms error for 3-hour monthly average relative humidity was 5%, however, with some individual errors around 10%.