Class 16: Atmospheric Conditions and Weather

Atmospheric Pressure and Winds

Global air circulation: The circulation of air over the Earth’s surface is complex. If the Earth were not rotating, things would be much simpler. The temperature gradient between the equator and the poles would cause the formation of a large convection cell in each hemisphere; warm equatorial air would rise and flow toward the poles, become cold, then sink and flow at the surface back toward the equator. However, because the Earth is rotating there is a more complex wind system.
Because of input of significant solar energy, air warms and rises at the equator. This creates a low pressure area sometimes called the doldrums. As the air rises, it cools and sinks back toward the surface 30 degrees north and south of the equator. The descending air produces a high pressure area. Some of this sinking air will flow back toward the equator and some will flow toward the pole. The air flowing toward the equator will be deflected to the west by the Coriolis force. This air forms the westward-flowing wind belts we call the trade winds. The air flowing toward the poles will be deflected to the east. These east-flowing winds are called westerlies (because they come from the west). As these winds reach 60 degrees latitude, they meet air flowing from the poles, which causes the air to rise creating a low pressure area. At the poles, a high pressure area exists because cold air is sinking and flowing along the surface toward the equator. Thus, going from the equator towards each pole, there are three convection cells that describe atmospheric circulation; these cells are called Hadley cells, Ferrel cells, and polar cells. Winds that are part of the global air circulation and result from pressure-gradient flow modified by the Coriolis effect are often called geostrophic winds.
Visit the sites below for information and diagrams regarding global air circulation:
	McGraw-Hill Companies site on global air circulation. Good, concise explanation of insolation, Hadley, Ferrel, and Polar cells, and the Coriolis effect.
	Comprehensive site from the Okanagan University College (Canada) on 1) solar insolation, 2) atmospheric temperatures and thermal convections, 3) global air circulation, including Hadley, Ferrel, and Polar cells, 4) trade winds and other wind systems, and 5) some of the effects of air circulation on climate. Many parts of the chapter should be useful, but definitely click on the parts that deal with winds and global air circulation: sections n and p.
Question: Where are most deserts located on the earth and why?
Air circulation around low pressure centers: The general term cyclone is used for an area of low pressure in the atmosphere that displays circular inward movement of air. In the northern hemisphere, circulation of a cyclone is counterclockwise, whereas southern hemisphere cyclones have clockwise wind patterns. Tropical storms or cyclones in the northern hemisphere usually are called hurricanes, and in the southern hemisphere they are called typhoons.
	The following site describes air flow around low pressure areas and fronts. There is a good explanation, with diagrams, as to why cyclones rotate counterclockwise in the northern hemisphere. John Denker at AT&T Bell Laboratories describes air circulation around low pressure centers and fronts. The site is a good overview, written from the perspective of an airplane pilot. Read only through section 20.2.1.
	The websites below describe the Coriolis force in more detail and explain how it influences world weather patterns. This is an interesting phenomenon first described by the French mathematician Gaspard Coriolis in the early 19th century. An object that moves in a straight line above the surface of the Earth (and not parallel to the equator) will appear to curve because the Earth is turning under it. This Coriolis effect must be taken into account in determining the trajectory of an object launched into space.
	This website from The Ohio State University provides a teacher's viewpoint on explaining the Coriolis force.
Local wind systems: Global winds usually are important in determining the prevailing winds in a given area, but local conditions (terrain, etc.) sometimes determine the most common wind directions.
The coupled sea and land breeze is typically found along coastlines adjacent to large bodies of water and results from differences between the heating or cooling of the water and the adjacent land. Water has a higher heat capacity (i.e., more heat is required far a given temperature change) than the materials on the land, and this means that solar radiation heats the land more than the water. As a result, the land transfers more heat to its overlying air mass and thus induces a circulation cell with surface winds moving from the water toward the land (sea breeze). At night, the land cools more rapidly than the water. Thus, the cooler landmass causes a circulation cell with air movement opposite to that during the day. This air flow from land to water is a land breeze.
Some local winds are caused by the presence of mountains or valleys. Mountain winds or breezes are the result of differential heating or cooling along mountain slopes. During the day, heating of sunlit slopes causes the overlying air to move upslope. At night, as the slopes cool, the air motion is reversed.
Foehn winds (also called chinook winds east of the Rocky Mountains, and Santa Ana winds in southern California), are induced by adiabatic temperature changes as air moves up and over a mountain. Adiabatic temperature changes occur without the external addition or subtraction of heat when air moves vertically. When air rises, it expands because pressure is lower. This expansion is accompanied by a reduction of temperature called adiabatic cooling. When air sinks, it is compressed and undergoes adiabatic warming.
Katabatic wind is the local flow of cold, dense air. This can occur when a cold plateau is adjacent to a relatively warm region at lower elevation. Air over the cold plateau cools and flows down to the lower elevations. Two varieties of katabatic wind are well known in Europe: One is the bora, which blows from the highlands of Croatia, Bosnia and Hercegovina, and Montenegro to the Adriatic Sea; the other is the mistral, which blows out of central and southern France to the Mediterranean.
	The following site from Okanagan University College (Canada) is on local and regional wind systems, global air circulation, fronts, etc. and is optional: