GEOG 1112L 7

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University of North Georgia, Oconee *

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1112L

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Geography

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Feb 20, 2024

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pdf

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Name: Lab day: GEOG 1112L Lab 7: General Circulation The different forces at work in the atmosphere lead to the wind flow that creates the weather that we experience every day. The average wind flow that is the result of the interaction of these forces is known as the General Circulation. This lab will focus on aspects of general circulation and how surface and upper-level wind flows interact with each other. Topics covered will include: o Semi-permanent surface features Impact of semi-permanent surface features Circulation models The impact of ocean-atmosphere interaction The connection between surface and upper-level features General Circulation: The Three-Cell Model Studying general circulation can help us determine the predominant climate conditions at various locations across the globe (see the figure below). | North Pole | G e gl SRy High latitudes Arctic Circle : REG;QN ikl - b Middle latitudes Tropic of Cancer | ”?{’;'_’Efifi‘?~”°“‘- e Equator '@,r‘ TROPICS . ' Low latitudes el e afflhb Tropic of Capricorn e L : : ERATEZONE L s ‘"“"w’z ) Middle latitudes omarclc " POLARREGION st i " High latitudes South Pole The reason we have a general circulation at all is because the Earth’s surface is unequally heated. Think back to Lab 1 and the sun angle calculations that you did. After performing these calculations, you should have found that the city of New Orleans had consistently higher sun angles than those of Helsinki. Different Jatitudes receive different amounts of incoming solar Scanned with CamScanner

radiation, and as a result there is an imbalance of energy across latitudes. To balance this out, the atmosphere will move warmer air towards the poles and colder air towards the tropics. The Three-Cell Model helps to explain this movement. Because the Earth rotates, the best model we can use to describe its general circulation is the Three-Cell Model. This model breaks the vertical circulation of air across latitudes into three cells, all stemming from this assumption: the tropics receive the most radiation and the poles receive the least radiation. The Hadley Cell Because the Earth is tilted, the tropics will receive the most radiation. This means that the tropics will be the warmest region. Because the hottest air is located in the tropics, this air will rise into the atmosphere. This rising motion leads to the movement of mass away from the surface, which means there is less weighing down on the surface. This creates an area of low pressure encircling the tropics. The air from the tropics will continue to rise until it reaches the tropopause, the border between the troposphere and the stratosphere. The tropopause will prevent the air from rising any further, and since the air has to go somewhere, it begins moving poleward. As it does so and approaches the subtropics, it will lose some of its heat and converge with other air masses moving equatorward towards the subtropics. This convergence around 30° leads to sinking motion there, and the air will warm as it sinks. This creates a band of high pressure encircling the area around 30°. This type of cell, with rising warm air and sinking cold air, is called a thermally direct cell. The Ferrel Cell This cell functions as a kind of “transition cell” between the Hadley and the Polar cells. This type of cell is called a thermally indirect cell. Sinking motion occurs around 30°, where air from poleward moving warm air of the Hadley cell meets the equatorward moving air from the midlatitudes. Once the sinking air reaches the surface, it has to go somewhere, so it begins to flow back towards the mid-latitudes. Air begins rising again around 60°. The Polar Cell This cell, extending from 60°-90°, is a key driver of much of the weather in the mid-latitudes. This cell stems from the Polar High, a permanent surface high over the poles. Due to the imbalance in received radiation, the poles are the coldest latitudes of the Earth. To try and correct this imbalance, the cold air at the poles begins to move equatorward along the surface. As it moves, it becomes warmer, but not as warm as the air over the tropics. This warmer air will then converge with poleward moving air from the Ferrel Cell around 60°. This convergence and rising motion leads to another band of surface low pressure, called the polar front. As in the Hadley Cell, this rising air will eventually be stopped by the tropopause and will then begin to move poleward. Air will cool as it moves poleward and then sink once it reaches the poles. Scanned with CamScanner

Horizontal wind in the Three-Cell Model The previous three paragraphs described the vertical motion (rising and sinking) of air in the Three-Cell Model. However, there is also horizontal motion associated with this model as well. Between 30° and the equator (the tropics), air begins to move back towards the equator. Due to the Coriolis Force, it will not move in a straight line directly from one latitude to the other. The deflection from Coriolis Force will cause the wind to become northeasterly (from the northeast) in the Northern Hemisphere, and southeasterly (from the southeast) in the Southern Hemisphere. These winds are called the tradewinds. Near the equator, the north and south trades will converge along a boundary called the Intertropical Convergence Zone (ITCZ). This is a boundary associated with surface convergence of warm air (brought by the trades) and rising motion. Due to the warm, moist nature of the air masses brought by the trades, rain and thunderstorms are very common along the ITCZ. This boundary also follows the warm air, meaning that it will move to warmer areas as seasons change throughout the year. There is also horizontal motion within the Ferrel Cell. As the sinking warm air begins to move back towards the poles, some of this air will also be deflected by the Coriolis Force. Because of this deflection, the wind will now have a westerly component (coming from the west). These winds, prevalent in both hemispheres, are called the westerlies. In the Polar Cell, wind moving equatorward is also deflected by the Coriolis Force. This deflection, similar to what occurs within the Hadley Cell, causes the wind to have an easterly component as it approaches 60°N. These winds are referred to as the Polar Easterlies. 1. Please use the figure below to depict the general circulation (the Three-Cell Model). a) Label latitude lines b) Nllustrate the Hadley, Ferrel, and Polar cells (both hemispheres) ¢) Label with and “L” or an “H” the location of any low- or high-pressure bands d) Label the approximate location of the ITCZ (average location) ¢) Draw and label the predominant wind direction for each latitude band. Scanned with CamScanner

e e P Ce\\ gh laktuaes L VWi d ale \atitunédes oW L Fikundes padale latitu b ey \hqia Latiudes Please answer following questions. 2. At which latitudes does rising motion occur? ® YO 3. At which latitudes does sinking motion occur? =0° 4. At which latitude would you expect the highest amount of precipitation to fall? Why? e NAhesr amopnk of Precip iarion occuvs gx §° Iddivude Vecausr ov W TTCL. Scanned with CamScanner

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