Most desert regions that cover vast geographic areas inhabit isolated interiors of the continents. Without proximity to rivers, streams, lakes and oceans the available water moisture for evaporation, condensation and precipitation remains sparse to none. Without rising water vapor in the air, the humidity lessons to a point where the air becomes dry and hot, minus the cooling effects brought by high humidity and moisture in the air. Weather frontal systems that produce rain clouds can not exist without the proper water-to air mixture. Most deserts receive less than 1 inch of rainfall per year.
High cliffs and extended mountain ranges form a barrier and cut off needed precipitation to desert regions. Storm fronts that form over the oceans push water-laden clouds against the windward side of mountains, where the warm air rises, cools and condenses to form cumulonimbus rain clouds. The moisture-laden clouds fail to travel over the top of the mountains, trapping the weather on the windward side. The much needed water vapor never arrives on the leeward side of the mountains, producing only wind, which heats up and loses its humidity upon reaching the desert.
Wind that descends the leeward sides of cliffs and mountain ranges has some moisture in it before it reaches the desert pan. It quickly heats up and loses all water moisture, causing extreme drafts that turn into wind storms. Fast-moving wind picks up sand and gravel particles and transports it across the desert pan, sometimes in great force, known as sandstorms. Dust devils and sand twisters result from the agitation of wind currents of unlike temperature, colliding with each other. Great tracks of surface soil and sand experience constant movement, bringing about the formation of large dunes and deeply cut arroyos.
The climate associated with most large desert regions is influenced by the global location of the areas of Earth that face more direct sunlight. The equator receives the most direct solar heat. Of the three zones -- polar, tropical and temperate -- most deserts inhabit the tropical zone. The tropical zone is located 23.5 degrees north latitude to 23.5 degrees south latitude. Most major deserts are located between 15 and 28 degrees south and north of the equator. Fluctuation in the seasons, which results from the Earth's tilt on its axis, accounts for some variations in temperature, but the desert regions stay predominantly warm and dry.
The major ocean currents, including the great conveyor current, produce weather conditions as a result of the raising and lowering of waters that have different temperatures. Warm water from the tropical regions is displaced by colder polar waters, which produces a circulation cycle around the perimeter of the continents. When the salinity (salt) content and freshwater induction into the conveyor current changes, so does the desert climate. Recent issues have been raised about the significance of global warming and the addition of carbon dioxide into the atmosphere. The greenhouse effect exacerbates the already warm-hot conditions of the deserts, by further raising their temperatures.
