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Atmospheric Circulation Lesson

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Local / Regional Winds

Sea Breezes and Land Breezes

In the morning, the air over land warms quicker than that of the ocean Warm air rises and cooler sea air moves in to form a sea breeze.

The process reverses in the evening The air over the land cools faster than that over the sea. Warmer air over the sea rises, and the cooler air over the land moves in to take its place. This breeze is called a land breeze

Mountain and Valley Breezes

These breezes are similar to the sea and land breezes. In the morning, the air in the mountain tops warms quicker than the air in the valleys. Warm air rises and the cooler valley air moves up the mountain slopes to form a valley breeze.

The process reverses in the evening The air on the mountain tops cools faster than that in the valleys. Warmer air in the valley rises, and the cooler air on the mountain tops moves downslope to take its place. This breeze is called a mountain breeze

Foehn winds

Foehn winds are a type of dry down-slope wind which occurs in the lee (downwind side) of a mountain range. These winds are often called “Rain-shadow” wind. Air rises up the sides of the mountain; as it does, the air cools, expands, and water vapor condenses to form clouds and sometimes precipitation. Then it goes up and over the mountains, and down the other side. As it does, the air compresses, heats up and dries out. Many of the world's deserts are formed this way (the Mojave, the Atacama Desert in Chile, and the Tibetan Plateau in Asia)

Katabatic winds

A katabatic wind carries high density air from a higher elevation down a slope under the force of gravity. Typically, these winds form when a high pressure system parks itself over a mountain range (say, the Rockies). Air flows down out of the high pressure system, compressing, heating up and drying out as it does. Then it flows down the mountain slopes, heating up and drying out even more. Then it crosses the desert, and heats up and dries out even more. Finally, it squeezes though the mountain passes in the Santa Susana, San Gabriel, and San Bernardino mountains. This causes the hot, dry air to speed up, and forms the Santa Ana Winds.

These Katabatic winds are common in other parts of the U.S. and world, each with their own name.


"Monsoon" is a pattern of wind circulation that changes with the season. The regions affected by this wind pattern usually have wet summers and dry winters. While many areas are affected by monsoons, India and SE Asia are the hardest hit.

The genesis for these winds is due to the different specific heats of land and water and the annual north-south movement of the intertropical convergence zone. During the winter, the ITCZ is mostly below the equator. Air blows off the Asian continent towards the Indian Ocean; somewhat like a land breeze on a ginormous scale. In summer the ITCZ moves north of the equator. As it does, the winds shift to blow towards the Asian continent and the Himalayas. This brings warm, humid air to India and SE Asia. The air cools, expands and condenses to form clouds and rain as the air moves up the slopes of the Himalayas. India receives anywhere from 50 - 90% of it's annual rainfall amount during the monsoon season. And how much is that? Around 200 - 300 mm (8 - 12 inches) during the three month monsoon season. Compare that to the 12 inches/year Southern California receives!

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