3 - Atmospheric Motion#
3.4 - How Do Variations in Temperature and Pressure Cause Local Atmospheric Circulation?#
A sea breeze, or onshore breeze, is a wind that blows from the sea to land during the day.
During daytime hours, the land heats up significantly, since it has a low specific heat.
At the same time, the air over water is chilled by the comparatively cool water and by the cooling associated with evaporation. The cool air sinks, and this creates high-pressure area over the water.
The difference between the high pressure over water and the low pressure over land creates an onshore breeze from sea to land.
The air over land is displaced, rising upward and moving out to sea.
The rising humid air on the coast can form clouds and fog.
A land breeze, or an offshore breeze, is a wind that blows from land to sea during the night.
The land cools at night, causing the relatively cool air over the land to sink, forming a high pressure area.
Due to the water’s high specific heat, it stays relatively warm. Therefore, the air above the water rises, creating low pressure over the water.
The higher pressure air over the land is pushed out to the sea, creating the land breeze.
The air pushed above the water is displaced, sending it to land.
An anabatic wind, or a valley breeze is created when the sun heats the land at lower elevations, causing the air to rise upslope. Air from above the low points sink to replace the air.
A katabatic wind, or a mountain breeze is created at night when the land at higher elevations cools faster than the lower elevations. This causes the air to sink into the valley, cooling it significantly.
Cities tend to be warmer than surrounding areas, creating an Urban Heat Island (UHI).
3.5 - What Are Some Significant Regional Winds?#
A Chinook wind can refer to several different types of wind, but are most commonly used to refer to warm, dry wind that blows down the flanks of a mountain range.
Chinook winds are formed when humid winds are pushed against the windward side of a mountain. When the humid air rises up the mountain, it cools, forming clouds, which releases latent heat.
When the air reaches the leeward side of the moutnain, it continues to dry out and heat up.
Katabatic winds blow down from cold, elevated areas (glaciers in Greenland and Antarctica). They are typically cold, but dry.
3.6 - How Do Variations in Insolation Cause Global Patterns of Air Pressure and Circulation?#
What pressure variations and air motions result from differences in insolation?
The most insolation strikes the Earth at the equator and the tropics. This heats the air, causing it to rise and creating a low-pressure system.
Surface winds flow toward the low-pressure zone to replace the rising tropical air. The air flows away from the equator until it reaches about \(30\degree\) latitude, where it descends and creates a zone of high pressure.
The amount of insolation decreases away from the equator, with a relatively sharp drop-off in the mid-latitudes (\(30 \degree\) to \(60 \degree\)). Descending air and high pressure in the subtropics cause surface air to flow toward higher latitudes.
Air near the poles is very cold and dense due to low insolation, sinking to form a zone of high pressure near the surface. This air flows toward the lower-pressure mid latitudes (until about \(60 \degree\)). In the upper atmosphere, air flows toward the poles to replenish the air that sank.
In equatorial regions, the land warms faster than the sea, so the air pressure is comparatively lower.
In the subtropics, oceans are usually cooler than the surrounding continents, so air pressures are typically higher over the oceans.
Air in the mid-latitudes is forced to ascend by surface air converging from the poles and subtropics. As a result, the oceans are usually warmer than the continents.