Air at 30 C 86 F can hold Winds that blow onshore tend to do so across cold currents produced by movement of water from high latitudes poles to low latitudes equator , and associated with the upwelling of cold waters from the ocean's depth. Cold or cool winds have relatively small moisture-bearing capacity and, when warmed during their passage over the land, they become stable and, thereby, reinforce the stability produced by the global stability of these latitudes.
Subtropical highs. This occurs along coastal areas where there are cold coastal seas Baja , CA , and in rain shadows adiabatic heating and cooling. Air moving across the frigid currents is cooled to a low temperature; thus the air holds little moisture when it arrives over land, where it may provide fog or mist, but rarely rain.
Namib and Atacama. Rainshadow effects. Moisture-laden air encounters a mountain mass and is moved upward. The ascending air is cooled and releases moisture on the windward side of the range. Once over the summit, the air descends the lee side of the range, warming as it does so, and hence increasing its evaporative power.
The windward side of a range may support a heavy well-watered forest, while the leeward side and the area far below it, robbed of moisture, is occupied by a desert or steppe plant community. The rain-shadow effect produced by great mountains can create arid areas in the lee of the mountains even when continentality is not particularly marked, such as in Patagonia where the Western Ghats and the Andes intercede.
Rainshadows include adiabatic heating and cooling. All these climatic, desert-producing factors — descending, drying air currents; mountain-produced rainshadows ; distance from oceanic moisture sources; and cold ocean currents — are instrumental, sometimes singly, more often in combination, as primary forces producing arid lands. Why are deserts hotter than tropics? Specific heat capacity.
Sand and rock heat up much more rapidly than water, and they also lose heat more quickly. On a hot summer day in the Sahara , sand and rock can heat up to o F. When a substance is exposed to heat, its surface temperature rises, but different substances heat up at different rates. The amount of heat that must be applied to a substance to make its temperature rise by 1 o is called the specific heat capacity shc of that substance.
This mean it requires more than 5 times more heat energy to raise the temperature of water 1 o than to raise the temperature of rock or sand by the same amount. As the sun climbs higher in the sky, the dry ground heats much faster than water, and much faster than the ground would if it were wet. Its s. Sand and rock heat more quickly than water, and for the same reason, it cools more quickly, also. The ground temperature falls sharply, reaching a minimum shortly before dawn, but the temperature of water falls much more slowly.
Over the ocean in subtropical latitudes, the difference in day and night air temperatures diurnal temperature range is about 0. Over the desert at the same latitude the diurnal temperature range is about 72 F 40C. Sand is a poor conductor of heat many air spaces , as is air.
Radiant heat raises the surface temperature, but heat is not conducted very far below the surface. The upper, heated layer gets continually hotter, while just below the surface the temperature changes very little.
Some heat is conducted below the surface, but at a very slow rate. Heat reaches a certain depth before the daily peak temperature is passed. Beyond this depth, the temperature does not alter. This is called the damping depth. Because heat penetrates so slowly, the peak temperature is reached at the damping depth several hours later than it is reached at the surface.
Conditions climates below ground are very different from those at the surface. A local climate of this type is called a microclimate, within the macroclimate of the region as a whole.
Many living organisms exploit the advantages afforded by microclimates. The subsurface climate is not the only desert microclimate.
Hollows that are shaded most of the time are cooler than exposed, sun-lit surfaces. Shelter from or exposure to prevailing winds can also create microclimates.
Above ground air is much cooler than surfaces of sand or rack. At a height of 6. Fertile islands. Desert Geography. Sahara Desert. It occupies more than 3. The Sahara is bounded:.
At various times what is now desert lay beneath ancient seas. This led to a deposition of sand and calcareous deposits that now form sandstone and limestone. Some ancient dunes are in areas now occupied by tropical rain forests.
The Nebraska Sand Hills is an inactive 57,square kilometer dune field in central Nebraska. The largest sand sea in the Western Hemisphere, it is now stabilized by vegetation and receives about millimeters of rain each year. Dunes in the Sand Hills are up to meters high. This aerial photograph of the Nebraska Sand Hills paleodesert shows a well-preserved crescent-shaped dune or barchan about 60 to 75 meters high.
Photograph by Thomas S. Ahlbrandt A dry community of vegetation grows among the dunes of the Nebraska Sand Hills. Photograph by N. Darton Extraterrestrial deserts Mars is the only other planet on which we have identified wind-shaped eolian features.
Although its surface atmospheric pressure is only about one-hundredth that of Earth, global circulation patterns on Mars have formed a circumpolar sand sea of more than five million square kilometers, an area greater than the Empty Quarter of Saudi Arabia, the largest sand sea on our planet.
Martian sand seas consist predominantly of crescent-shaped dunes on plains near the perennial ice cap of the north polar area. Smaller dune fields occupy the floors of many large craters in the polar regions.
There are few plants to protect the soil from the wind, so the soil is blown away to expose the rocky surface.
Even in such a dry climate, most of the landforms are carved by the rare periods of heavy rainfall that result in flash floods, erosion, and sediment deposition. Hot air rises at the equator, where the land receives the greatest amount of the sun's radiation. Most of the world's deserts are located near 30 degrees north latitude and 30 degrees south latitude, where the heated equatorial air begins to descend.
The descending air is dense and begins to warm again, evaporating large amounts of water from the land surface. These patterns of air circulation are called Hadley cells. When the cool air begins to fall back toward the ground, or descend, it starts to warm up again. This warm, dry air can hold a lot of water, so the air starts to suck up what little water is around. At 30 to 50 degrees north and south of the equator, this falling air makes dry air drier. It also turns the land below it into a desert.
Listen to Joellen Russell discuss Hadley Cells in the audio download here. Oftentimes you will find a desert on one side of a mountain range, but not the other. Why might that be? If we think again about the rising and falling air that helps make deserts, it might make a little more sense. Imagine warm clouds full of water vapor always moving toward a tall mountain range from the same direction.
As the clouds cross the mountain range, the clouds are forced to rise up over the crest. This cools the air and the water being carried as vapor, or water-air, is squeezed out as liquid water.
This action forces most of the water vapor out as rainfall, and this rain falls on that side of the mountain the clouds moved toward, called the windward side. This side of the mountains gets lots of rain. But then, after the clouds have crossed the mountains and have dumped most of their water, they move down on the far side of the mountains, called the lee side.
Wind continues to drive the now cool, dry air, but as that air moves down the lee slope of the mountains, it becomes warmer. This means that the air can again hold lots of water, so it again starts picking up water. This pulls water out of the soil and creates extra dry areas of desert on the lee side of the mountain range—what we call a dry shadow.
While this geyser is found in the forest, it is a good example of water vapor. You may have heard of water beds, but what about water blankets? Not a blanket made of liquid water, but one made of water vapor. Water vapor is essentially water-air, like the vapor that comes off of a boiling pot of water.
Because deserts are so dry, they have very low humidity—the measure of water vapor in the air.
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