What makes a desert a desert? Mostly, it is lack of rainfall. Less than ten inches of rain per year qualifies a region as a desert. Hesperia's average is 7.49 inches, as calculated by me, a Hesperia weather spotter for the NWS, over a 23-year period from 1981 to 2004. Some years the total rainfall was about 3 inches. Some years there was over 13 inches. It depends on the year. The lack of rainfall, however, dictates almost all conditions on the Mojave Desert.


Why are we a desert? There is a "belt" of dry, descending air at between 20 and 40 degrees latitude north and south of the equator. Hesperia (and the whole Victor valley) is located a 34 degrees north latitude. The same approximate latitude as Africa's great Sahara Desert. Deserts occupy about one-tenth of the U.S. The Mojave Desert alone is one-firth of the state of California. And fully one-fifth of the Earth's surface is classified arid or semi-arid.


Individual conditions vary for each desert region. The Mojave Desert is a "rain shadow" desert. The reason being the Sierra Nevada and Tehachapi Mountains to the northwest and the San Gabriel and San Bernardino Mountains to the south of us. But to understand the "rain shadow" effect, it is necessary to ask "why does it rain".


As illustrated in the diagram below, cold fronts (principally in the winter months) and warm fronts (summer) are blocked by the mountains. This is referred to as "orographic" (mountain-influenced) rainfall. Cold fronts generally travel from north-to-south and west-to-east and are blocked by the Sierras (which are principally a north-to-south mountain range. Warm fronts travel from south-to-north and west-to-east and are blocked by the San Gabriel and San Bernardino Mountains (which are mostly east-to-west).


It has been said that the mountains "wring" the moisture out of the air mass. But there is more to it than that. Warm air masses and moist air masses are more buoyant than cold and dry air masses. A "front" is where two unlike (relatively) air masses collide. A wet air mass ascends the slope of a mountain range and cools at the wet "adiabatic" rate of 3.2 degrees Fahrenheit per 1000 feet. A colder air mass cannot contain as much moisture as a warm air mass (relatively) and the moisture is extracted from the air mass and falls as rain or snow. That is "orographic" rainfall. After it reaches the mountain peaks it descends to the leeward and warms at the dry "adiabatic" rate of 5.6 degrees Fahrenheit per 1000 feet. This describes the "rain shadow" effect.


Hesperia has 7.5 inches of rainfall per year, and the mountain resorts average well over 30 inches. This aptly demonstrates the "orographic rainfall" effect. Most areas "down below" are "Mediterranean" climate. Next time we'll all be exploring climate and seasonal weather.