From AMS Glossary
A function of precipitation and temperature designed by W. Gorczyński to represent the relative lack of effective moisture (the aridity) of a place.
It is given by (latitude factor) × (temperature range) × (precipitation ratio). The latitude factor is the cosecant of the latitude (taken as 3.0 for 0°–4°). The temperature range is the difference (°F) between the means of the hottest and coldest months. The precipitation ratio is the difference between the highest and lowest annual totals (adjusted to a 50-year record) divided by the average.
The value of this coefficient is about 100 in the middle of the Sahara; in the United States it ranges from 70 at Bagdad, California, to 2 at Eureka, California.