This takes place, for example, when the westerly winds bring air from the Pacific Ocean over the Sierra Nevada Mountains in California.
As the relatively warm, moist air rises over the windward side of the mountains, it cools and contracts. If the air is humid, it may form clouds and drop rain or snow. When the air sinks on the leeward side of the mountains, it forms a high pressure zone. The windward side of a mountain range is the side that receives the wind; the leeward side is the side where air sinks.
The descending air warms and creates strong, dry winds. Snow on the leeward side of the mountain disappears melts quickly. If precipitation falls as the air rises over the mountains, the air will be dry as it sinks on the leeward size.
As air rises over a mountain it cools and loses moisture, then warms by compression on the leeward side. The resulting warm and dry winds are Chinook winds. The leeward side of the mountain experiences rainshadow effect.
Santa Ana winds are created in the late fall and winter when the Great Basin east of the Sierra Nevada cools, creating a high pressure zone. The high pressure forces winds downhill and in a clockwise direction because of Coriolis. The air pressure rises, so temperature rises and humidity falls. The winds blow across the Southwestern deserts and then race downhill and westward toward the ocean. The winds are especially fast through Santa Ana Canyon, for which they are named.
Santa Ana winds blow dust and smoke westward over the Pacific from Southern California. The hot, dry winds dry out the landscape even more.
If a fire starts, it can spread quickly, causing large-scale devastation Figure below. In October , Santa Ana winds fueled many fires that together burned , acres of wild land and more than 1, homes in Southern California.
High summer temperatures on the desert create high winds, which are often associated with monsoon storms. Desert winds pick up dust because there is not as much vegetation to hold down the dirt and sand.
Figure below. A haboob forms in the downdrafts on the front of a thunderstorm. Dust devils, also called whirlwinds, form as the ground becomes so hot that the air above it heats and rises.
Air flows into the low pressure and begins to spin. Dust devils are small and short-lived but they may cause damage. Because more solar energy hits the equator, the air warms and forms a low pressure zone. At the top of the troposphere, half moves toward the North Pole and half toward the South Pole. As it moves along the top of the troposphere it cools. The cool air is dense and when it reaches a high pressure zone it sinks to the ground. The air is sucked back toward the low pressure at the equator.
This describes the convection cells north and south of the equator. If the Earth did not rotate, there would be one convection cell in the northern hemisphere and one in the southern with the rising air at the equator and the sinking air at each pole.
But because the planet does rotate, the situation is more complicated. Air rises at the equator, but as it moves toward the pole at the top of the troposphere, it deflects to the right. Remember that it just appears to deflect to the right because the ground beneath it moves. This air is cool because it has come from higher latitudes. Both batches of air descend, creating a high pressure zone. Once on the ground, the air returns to the equator.
There are two more convection cells in the Northern Hemisphere. This cell shares its southern, descending side with the Hadley cell to its south. There are three mirror image circulation cells in the Southern Hemisphere. In that hemisphere, the Coriolis Effect makes objects appear to deflect to the left. Global winds blow in belts encircling the planet.
The global wind belts are enormous and the winds are relatively steady Figure below. These winds are the result of air movement at the bottom of the major atmospheric circulation cells, where the air moves horizontally from high to low pressure. The wind belts are named for the directions from which the winds come. The westerly winds, for example, blow from west to east. These names hold for the winds in the wind belts of the Southern Hemisphere as well.
Besides their effect on the global wind belts, the high and low pressure areas created by the six atmospheric circulation cells determine in a general way the amount of precipitation a region receives. In low pressure regions, where air is rising, rain is common. In high pressure areas, the sinking air causes evaporation and the region is usually dry. More specific climate effects will be described in the chapter about climate. The polar front is the junction between the Ferrell and Polar cells.
At this low pressure zone, relatively warm, moist air of the Ferrell Cell runs into relatively cold, dry air of the Polar cell. The weather where these two meet is extremely variable, typical of much of North America and Europe. The polar jet stream is found high up in the atmosphere where the two cells come together. A jet stream is a fast-flowing river of air at the boundary between the troposphere and the stratosphere. Jet streams form where there is a large temperature difference between two air masses.
A cross section of the atmosphere with major circulation cells and jet streams. During the Northern Hemisphere summer, the subtropical zone moves northward to influence the Mediterranean region. Mediterranean climates are characterised by hot dry summers, but much cooler and wetter winters than truly subtropical climates nearer the equator. Between the subtropical and equatorial zones trade winds blow, north-easterly in the Northern Hemisphere and south-easterly in the Southern Hemisphere.
These regions are much drier than the equatorial zone, but receive more rainfall than the desert climates. These regions are often characterised by Savannah , scrub and grassland which blossoms during the rainy season and dies off during the prolonged dry season. These regions are known respectively as the Icelandic and Aleutian Lows.
They are characterized by relatively mild, moist winds that tend to bring frequent cyclonic precipitation rain and snow , particularly along the west-facing side of continents. The precipitation tends to develop along warm and cold fronts, where cold air from the polar easterlies forces the warm, moist air of the westerlies to rise, which, on cooling, releases the moisture as clouds and ultimately rain and snow.
Climate in the mid-latitudes is usually temperate. At the highest latitudes in the polar regions, the cold air sinks producing high atmospheric pressure. The polar climates here are characterised by dry, icy winds that tend to radiate outward from the poles.
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