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Lecture 2

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University of Toronto Scarborough
Environmental Science
Tanzina Mohsin

Lecture 2 What is wind?  Movement of air in an ordered fashion  Difference in pressure forms a pressure gradient  Air flows from high pressure to low pressure called pressure gradient force Coriolis force  Fictitious force due to rotation of the earth  In northern Hemisphere, causes a deflection to the right of the motion, to the left in the Southern Hemisphere  Formulated mathematically by Gaspard Gustave de Coriolis in 1835; described by George Hadley a century before  Is related to the distance from the equator  Acts at right angle to the wind affecting the direction Summary facts  PGF is always directed from higher pressure towards the lower pressure  Steep PG (closely spaced isobars) indicate strong PGF and high winds  When the wind starts to blow, the Coriolis force causes it to bend to the right of its intended path in the Northern Hemisphere and to the left of its intended path in the Southern hemisphere Geostrophic Wind  Balance between PGF and Coriolis:  Blows parallel to isobars  Typical 1km or so above the Earth’s surface Surface Winds  Below 1km, the wind is influenced by friction Global Circulation  3 broad categories of atmospheric circulation: o Global: 10000 of km o Synoptic: 100 to 1000 km o Small scales: <100 km  There is more energy released in the Polar Regions than is received from the sun. The reverse is true for the equatorial region One Cell Theory  One large overturning atmospheric cell, where air rises at the equator, moves pole ward and then sinks at the pole. Also called one large Hadley cell after George Hadley  Energy is transported as sensible (heat you feel) and latent heat (due to the change in phase) from equator to pole. Latent heat transport occurs when moist equatorial air moves poleward, cools, condenses and releases latent heat  The theory works fine if the earth did not rotate. However, there is til and land/water contrast, which also play a role for the weather system on Earth o Rotation introduces the Coriolis effect o Tilt causes seasonality o Continentality causes land/ sea variation due to varying thermal inertia Three Cell Theory  On the global scale, there are three main circulation cells per hemisphere which consists of vertical and horizontal winds  Hadley Cell o Characterized by air rising in the equatorial region, driven by warm surface conditions o In the upper troposphere, this air starts to move horizontally towards the poles o o o At 30 N and 30 S, air descends to the surface where the air returns to the equatorial region to complete the circulation and this circulation occurs in both the northern and southern hemispheres  Ferrel Cell o Shares the descending branch of air at 30 N and 30 S with Hadley cell 0 o At the surface, the air moves poleward until about 60 N and S and then ascends o Circulation is completed by a return flow equatorward in the upper troposphere  Polar Cell o Shares the ascending branch with Ferrel cell at 60 N and S. o In the upper troposphere, the air from this cell flows poleward and then descends at the pole. o o To complete circulation, air flow equatorward at the surface to 60 N and S Definitions  Intertropical Convergence Zone (ITCZ) – area of rising air at or near the equator, heavy precipitation  Trade Winds – winds that blow to the south west, surfa
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