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GEOG. 1710 EARTH SCIENCE LECTURE 6. ATMOSPHERIC PRESSURE AND WINDS As mentioned earlier, atmospheric pressure is generated by the weight of overlying air; due to compression, the air becomes less dense with increasing height and the rate of pressure reduction decreases with increasing height. Harry Williams, Earth Science 1 Harry Williams, Earth Science 2 Air pressure is usually measured by a barometer - such as the old mercury barometer; air pressure forces the mercury to rise up the tube containing a vacuum. The higher the pressure, the higher the column of mercury supported. Average sea level pressure is about 30 inches of mercury, which is equivalent to about 15 lbs per square inch; however, the most-widely accepted measure is the millibar (a measure of force per unit area) average sea level pressure is about 1013 mb. Harry Williams, Earth Science 3 The pressures are shown by isobars - lines of equal pressure. This is similar to the contour map, except the highs and lows are of air pressure rather than elevation. Note that a linear zone of low pressure is a trough; a linear zone of high pressure is a ridge; the steepness of the pressure slope is a pressure gradient. Harry Williams, Earth Science 4 Harry Williams, Earth Science 5 Harry Williams, Earth Science 6 Wind Air will naturally flow from high pressure to low pressure areas; the driving force is the pressure gradient force (pgf) - the larger the pgf, the faster the air will move. It is this movement of air that we call wind. Factor Controlling Wind Direction 1. PGF - acts perpendicular across the isobars from high to low. 2. The Coriolis effect is caused by the rotation of the Earth; it causes moving objects (including air) to be deflected always to the right in the northern hemisphere and always to the left in the southern hemisphere. The magnitude of the Coriolis effect increases with increasing velocity. Harry Williams, Earth Science 7 Harry Williams, Earth Science 8 In the absence of other forces, for example, in the upper atmosphere above 5000 feet, PGF and Coriolis force balance out and produce a GEOSTROPHIC WIND which blows parallel to the isobars. Harry Williams, Earth Science 9 Below 5000 feet friction becomes a factor - it slows down the wind. This doesn’t affect PGF but it does reduce Coriolis force - the result is a wind blowing across the isobars towards the low Harry Williams, Earth Science 10 If the air is converging onto the low pressure center, where does it all go? The answer is "up". The air converges and rises in a low and descends and diverges in a high. The correct terminology for the pressure centers - a low is referred to as a CYCLONE with CYCLONIC CIRCULATION; a high is referred to as an ANTICYCLONE with ANTICYCLONIC CIRCULATION. Harry Williams, Earth Science 11 Harry Williams, Earth Science 12 General Circulation of the Atmosphere The principles outlined above can be used to understand the large scale global circulation of winds - both horizontally and vertically. The Intertropical Convergence Zone (ITC) Imagine vertical circulation on a non-rotating Earth; a simple convection cell might result between the hot low at the equator and the cold high at the pole. However, the Earth is rotating and this simple cell breaks down by about 30 degrees north and south of the equator, due to the deflection of the upper air flow by Coriolis and radiative colling of the upper air flow causing it to become denser and sink down through the atmosphere. The result is the HADLEY CELLS - the equatorial component of global circulation. Harry Williams, Earth Science 13 Harry Williams, Earth Science 14 Associated features: 1. The INTERTROPICAL CONVERGENCE ZONE between them, marked by low pressure, rising air and cloud development (explained later). 2. The SUBTROPICAL HIGHS (STHs) around 30 degrees north and south, where the air descends causing high pressure and clear skies. 3. The surface TRADE WINDS, blowing from the STHs towards the ITC. Note that the trade winds are deflected by Coriolis, forming the Northeast trades in the northern hemisphere and the southeast trades in the southern hemisphere. The ITC and STHs also shift north and south with the seasons. 4. The surface WESTERLIES, blowing poleward from the STHs across the mid-latitudes and deflected from west to east by Coriolis. Harry Williams, Earth Science 15 The S.E. Asia Monsoon, caused by the shifting trade winds and ITCZ. Harry Williams, Earth Science 16 At the pole, cold sinks down forming the POLAR HIGH. Air diverging from this high is deflected to the west by Coriolis forming the POLAR EASTERLIES, which blow to about 60 degrees latitude. The POLAR EASTERLIES meet the WESTERLIES at about 60 degrees latitude; this converging air rises, forming the POLAR FRONT or SUBPOLAR LOW. Harry Williams, Earth Science 17 Harry Williams, Earth Science 18 Jet Streams And Rossby Waves There are two places in the upper troposphere where air converges strongly and is being deflected from west to east by Coriolis - above the Tropics and above the Polar Front; these zones of high velocity winds are subtropical and polar front JET STREAMS - velocities can exceed 200 miles per hour. Harry Williams, Earth Science 19 ROSSBY WAVES The polar front jet is particularly important, firstly because it tends to "guide" upper and lower air flow in the mid-latitudes, and secondly, because it often undulates (due to mountain barriers and migrating pressure system) producing large waves known as ROSSBY WAVES. As these develop, lobes of cold polar air can push down into mid-latitudes and lobes of warm subtropical air can push poleward - this makes mid-latitude weather the most variable on Earth. Harry Williams, Earth Science 20 Harry Williams, Earth Science 21 Harry Williams, Earth Science 22 Harry Williams, Earth Science 23 Ocean Currents Much of the world's oceanic circulation simply reflects the movement of ocean water by the global wind patterns Note particularly the effect of the subtropical highs and southern hemisphere westerlies. Global Heat Transfer: the westerlies and ocean currents transfer surplus heat from the tropics towards the poles. Harry Williams, Earth Science 24 Harry Williams, Earth Science 25 Harry Williams, Earth Science 26