Atmospheric Movements and Flow

Atmospheric Pressure and Winds

When winds converge at a low pressure center, the air (and water vapor) rises. When winds converge at a high pressure center, the air spreads away from the high and the air above sinks to replace it.A few basic principles go a long way toward explaining how and why air moves: Warm air rising creates a low pressure zone at the ground. Air from the surrounding area is sucked into the space left by the rising air. Air flows horizontally at the top of the troposphere; horizontal flow is called advection. The air cools until it descends. Where it reaches the ground, it creates a high pressure zone. Air flowing horizontally from areas of high pressure to low pressure creates winds. The greater the pressure difference between the pressure zones the faster the wind flow.

Convection in the atmosphere creates the planet’s weather. When warm air rises and cools in a low pressure zone, it may not be able to hold all the water it contains as vapor, since cool air cannot hold as much water vapor as warmer air. Some water vapor may condense to form clouds or precipitation. When cool air descends, it warms. Since it can then hold more moisture, the descending air will evaporate water on the ground. Air moving between large high and low pressure systems creates the global wind belts that profoundly affect regional climate. Smaller pressure systems create localized winds that affect the weather and climate of a local area.

Local Winds

Local winds result from air moving between small low and high pressure systems. High and low pressure cells are created by a variety of conditions. Some local winds have very important effects on the weather and climate of some regions.

Land and Sea Breezes

First warm air over the coast rises from a thermal low. The cooler air over the sea is pulled in to fill the now “empty space.” This causes the coastal warm air to further rise. This becomes a high pressure zone and the air cools, eventually becoming a low pressure zone as the cool air sinks above the sea. Formation of the sea breeze

Since water has a very high specific heat, it maintains its temperature well. So water heats and cools more slowly than land. If there is a large temperature difference between the surface of the sea (or a large lake) and the land next to it, high and low pressure regions form. This creates local winds.

Sea breezes blow from the cooler ocean over the warmer land in summer. Where is the high pressure zone and where is the low pressure zone?

Sea breezes blow at about 10 to 20 km (6 to 12 miles) per hour and lower air temperature much as 5 to 10 degrees C (9 to 18 degrees F).

Land breezes blow from the land to the sea in winter. Where is the high pressure zone and where is the low pressure zone? Some warmer air from the ocean rises and then sinks on land, causing the temperature over the land to become warmer.

Land and sea breezes create the pleasant climate for which Southern California is known. The effect of land and sea breezes are felt only about 50 to 100 km (30 to 60 miles) inland. This same cooling and warming effect occurs to a smaller degree during day and night, because land warms and cools faster than the ocean.

Monsoon Winds

Monsoon winds are larger scale versions of land and sea breezes. By definition, a monsoon is a seasonal reversal of winds, blowing steadily from one direction for part of the year, and then blowing from the opposite direction for the other part of the year. In the summer, the interior of a continent heats up more than the offshore water, creating an area of low pressure inland as the land heats the air above it, and the warm air rises. As a result, the winds blow from the sea onto the land in summer (the wet monsoon), since the air flows from higher to lower pressure. In winter, the air flows from the land out to the sea (the dry monsoon) since the land cools, forming higher pressure inland. Thunderstorms are common during the wet summer monsoons.

The most important monsoon in the world occurs each year over the Indian subcontinent. More than two billion residents of India and southeastern Asia depend on monsoon rains in the summer for their drinking and irrigation water. Back in the days of sailing ships, seasonal shifts in the monsoon winds carried goods back and forth between India and Africa.

Map showing average onset (monsoon arrival) dates and wind directions prevalent during India's southwest summer monsoon. Click for a larger image.

Mountain and Valley Breezes

Temperature differences between mountains and valleys create mountain and valley breezes. During the day, air on mountain slopes is heated more than air at the same elevation over an adjacent valley. As the day progresses, warm air rises and draws the cool air up from the valley, creating a valley breeze. At night the mountain slopes cool more quickly than the nearby valley, which causes a mountain breeze to flow downhill.

Katabatic winds

Katabatic winds move up and down slopes, but they are stronger than mountain and valley breezes. Katabatic winds form over a high land area, like a high plateau. The plateau is usually surrounded on almost all sides by mountains. In winter, the plateau grows cold. The air above the plateau grows cold and sinks down from the plateau through gaps in the mountains. Wind speeds depend on the difference in air pressure over the plateau and over the surroundings. Katabatic winds form over many continental areas. Extremely cold katabatic winds blow over Antarctica and Greenland.

Sketch of the generation of katabatic winds in Antarctica

Chinook Winds

A wind flows over a steep mountain.Chinook winds, also called Foehn winds, develop when air is forced up over a mountain range and then descends on the downwind side. This takes place, for example, when the westerly winds bring air over the Rocky Mountains. As the air rises over the windward side of the mountains (in this case the western side), it expands and cools. If the air is humid enough, it may form clouds and drop rain or snow. When the air sinks on the leeward side of the mountains (in this case the eastern side), it warms by compression. 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. Chinook winds can raise temperatures more than 20 degrees C (36 degrees F) in an hour and they rapidly decrease humidity. Snow on the leeward side of the mountain disappears quickly. This dry, sinking air causes a rainshadow effect on the leeward side of the mountains, which creates many of the world’s deserts.

Santa Ana Winds

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 winds blow across the Southwestern deserts, up and over the Sierra Nevada, and then race downhill and westward toward the ocean. The hot, dry air is forced through canyons, sometimes at very high speeds.

The Santa Ana winds often arrive at the end of California’s long summer drought season. The hot, dry winds dry out the landscape even more. If a fire starts, it can spread quickly, causing large-scale devastation.

This map illustration shows a characteristic high-pressure area centered over the Great Basin, with the clockwise anticyclone wind flow out of the high-pressure center giving rise to a Santa Ana wind event as the airmass flows through the passes and canyons of southern California, manifesting as a dry northeasterly wind.

Desert Winds

A large sandstorm blowing in over a parking lot in Iraq. The sand cloud is three or four times the height of the army trucks parked in the lot.High summer temperatures on the desert can create high winds. Desert winds pick up dust because there is not as much vegetation to hold down the dirt and sand. 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 usually fairly small and short-lived, but larger ones may cause damage

This video shows a giant dust devil:

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