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. In the past, barometers were used and measured how much air pushed on a fluid, such as mercury. When you inflate a balloon, the air molecules inside the balloon get packed more closely together than air molecules outside the balloon.
This means the density of air is high inside the balloon. When the density of air is high, the air pressure is high. The pressure of the air pushes on the balloon from the inside, causing it to inflate. If you heat the balloon, the air pressure gets even higher. Air pressure depends on the temperature of the air and the density of the air molecules. Atmospheric scientists use math equations to describe how pressure, temperature, density, and volume are related to each other. They call these equations the Ideal Gas Law.
In these equations, temperature is measured in Kelvin. This equation helps us explain how weather works, such as what happens in the atmosphere to create warm and cold fronts and storms, such as thunderstorms.
For example, if air pressure increases, the temperature must increase. If air pressure decreases, the temperature decreases. It also explains why air gets colder at higher altitudes, where pressure is lower. Between each of these circulation cells are bands of high and low pressure at the surface.
You can see the results of these circulations on a globe. These areas, especially the west coast of continents, tend to have more precipitation due to more storms moving around the earth at these latitudes. Please Contact Us. Toggle navigation JetStream.
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