Localized winds The Sea Breeze Monsoons Lake Effect Snow
Today we will learn about winds that form in specific geographic environments and play a large role in determining the climatology of many areas around the world.
Localized winds
Geography plays a very important role in determining the nature of the winds in different locations. For instance, the presence of mountain ranges causes winds to lift or descend causing changes in temperature and the development of cloud cover and pr ecipitation. Also, winds moving across different landscapes cause changes in the wind which can lead to localized circulations and swirls.
A localized circulation is called an eddy. Small scale wind shear conditions lead to the formation of eddies. For instance, when a wind moves onto a smoother landscape, the value of friction on the smoother landscape will decrease therefore th e speed of the wind will likely increase. We learned that a change in wind speed across a distance is a wind shear condition. This localized wind shear generates these local eddies.
As air ascends over a mountain or ridge, an ideal situation for the development of eddies is also encountered. As the wind ascends over the mountain, the strongest winds are up at the crest and the wind speed decreases with decreasing elevation. This vertical shear can generate the rotation necessary to form eddies.
Air ascending or descending across higher terrain plays a dramatic role in determining local weather conditions. For instance, consider the Cascade Mountains in Washington State. As air comes off the Pacific and encounters the Cascades, it is forced upward where it cools and its moisture condenses into clouds and precipitation. This is an example of orographic lifting. However, on the other side of the Cascades, the air descends and heats and dries as it does so. As a result, there is a vir tual desert on the eastern side of the Cascades.
A good example of an orographic wind is the Chinook Wind that affects the Great Plains states. Wind descending from the Rocky Mountains compresses and warms and often provides the states of Nebraska, South Dakota, Kansas, and Oklahoma with much above normal temperatures particularly during the springtime. In fact, this wind is often referred to as a snoweater wind because it is very effective at melting snow that has accumulated from the winter.
The Sea Breeze
A very good example of a localized wind event is the Sea Breeze. This event exists because of the different heating rates between land and water. Many who live near the ocean are familiar with the gentle cooling benefits of the Sea Breeze. The Sea Breeze helps make ocean communities pleasant during the summer when inland regions just a small distance from the shore can be scorching in unbearable heat.
During the day, the land heats up much more rapidly than the ocean. Therefore, the land is at a higher temperature and the air above the land begins to rise as its density decreases (the result of a higher temperature - think of the Ideal Gas Law). A ir from the ocean rushes in to replace the air that has risen over the land. This is the basic principle of the Sea Breeze circulation. The leading edge of the Sea Breeze is called the Sea Breeze Front.
During the night however, the situation is reversed. The ocean is warmer as the land will cool off much more quickly than the water. Air over the water will rise (as the warmer water heats the air directly above the water causing a decrease in densit y). Air from the land will rush in to replace the rising air. This is a Land Breeze.
The stronger the temperature gradient between the land and the water, the stronger the Sea Breeze will be. Florida's weather is particularly susceptible to being influenced by the Sea Breeze phenomena, particularly the Central Florida area near Orland o. An eastward propagating Sea Breeze Front from the Gulf of Mexico crosses paths with a westward propagating Sea Breeze Front from the Atlantic over Central Florida. The result is an area of convergence, vertical motion, and frequently, strong thunder storm development. The preponderance and intensity of thunderstorm activity would depend on the stability in the area.
Lake Breezes and even River Breezes are common as well in many parts of the world and can play an equally dramatic role in local weather conditions.
Large Scale Sea Breezes - Monsoons
A monsoon can be thought of as a large scale Sea Breeze phenomena. The Indian Subcontinent is an area where monsoons are prevelant. During the hot Indian Summers, the land heats up and air rises forming a semi-permanent area of low pressure. Air from the Bay of Bengal and the Arabian Sea rushes in to replace the rising air. As the moist air from the sea gets to the region of rising air, it begins to rise itself, leading to very heavy rain and thunderstorms.
The reverse situation sets up during the winter and the Indian Subcontinent dries out.
Santa Ana Winds
Santa Ana Winds plague southern California typically during the wintertime. The winds occur when high pressure regions build into the Rockies. Air flowing out of the high descends down the rockies and down the elevated desert plateau of Nevada into southern California. Since the wind crosses a desert region, it is very dry once it gets to California. Santa Ana winds can promote and enhance fires in this part of California.
The Lake Effect Snow Machine
Lake Effect Snow effects regions that border the Great Lakes. As is the case with the Sea Breeze, this phenomena occurs due to the different rates at which land, air, and water are heated and cooled.
During the winter, the land and air cool off much more quickly than the water of the Great Lakes. When a north to westerly flow of cold Canadian air sets up across the Great Lakes, it is warmed as it passes over the relatively warmer lake waters. This warming allows the air to pick up some moisture from the lakes. The warming of the air directly above the lake creates some instability and thus vertical motion. As the air passes completely across the lakes and back over land again, it encounters cold terrain once again. The moisture that was picked up from the lake is condensed and deposited in the form of Lake Effect Snow.
Lake Effect Snow conditions are most likely to occur when there is the strongest temperature gradient between the land surrounding the lakes and the land itself, and when there is the greatest fetch off of the lake. Fetch refers to the distance the air travels over the surface of the lake. The longer the air is over the lake, the more moisture it will pick up and the more snow will be deposited on the other side.
Lake Effect Snow events can be very localized and very dramatic. One town adjacent to the lake shore can get clobbered with 2 feet of snow in one night. Meanwhile, a neighboring community merely 10 miles away may get no snow at all the same night.
Regions downwind of Lakes Erie and Ontario in Ohio, Pennsylvania, and Upstate New York usually get the worst Lake Effect Snow. The Upper Peninsula of Michigan and the northern Lower Peninsula also get hit hard by Lake Effect Snow from Lake Superior.
Snowfall from severe Lake Effect Snow events is often measured in yards as opposed to inches. Some of the more typically hard-hit cities by Lake Effect Snow include Watertown, NY; Syracuse, NY; Oswego, NY; Buffalo, NY; Erie, PA; Akron, OH; and Alpena, MI.
A similar event occurs in Asia downwind of the Japan Sea. Wind comes off the eastern Russia and blows across the Japan Sea during winter, warming and picking up moisture. Once it reaches northern Japan, it deposits it in the form of sometimes extremely heavy snowfall.