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PRESSURE & WIND, GENERAL CIRCULATION, JET STREAMS FORCES that move AIR: Winds Aloft: Surface Winds: (top of troposphere) 1. Gravity 1. Gravity 2. Pressure gradient 2. Pressure gradient 3. Coriolis effect 3. Coriolis effect 4. Friction 1. Gravity Earth exerts gravitational force on atmosphere. (This causes pressure and density to be greater closer to earth.) Acceleration due to gravity = 9.8 m/sec2 2. Pressure Gradient a) Vertical (Remember hydrostatic equilibrium) 997 998 999 1000 surface 2. Pressure Gradient b) Horizontal (wind) Pressure Gradient Force (PGF) 1005 1000 995 990 985 PGF is perpendicular to isobars. high wind speed. low wind speed. Wind speed determined by steepness of gradient. Current weather map 3. Coriolis Effect / Force Apparent deflection of moving object due to rotation of earth. Animation Animation Animation Animation Deflection… 1. Is to the right of the path of motion in the northern hemisphere and to the left of the path of motion in the southern hemisphere. 2. Increases with latitude: maximum at poles; zero at equator Plane of deflecting force is parallel to earth’s surface at poles; no component of deflection parallel to surface at equator. Deflection... 3. Increases with wind speed. 4. Increases with mass of object. CE is perpendicular to path of motion. If PG and CE were only forces on atmosphere, wind would blow parallel to isobars. 997 998 999 1000 4. Friction Surface provides friction to atmospheric movement; “slows down” the air. Minimal friction aloft > 3000 ft in troposphere “friction layer” : 0 – 3000 ft Winds aloft blow parallel to isobars: geostrophic wind geostrophic balance “balance between pressure gradient and Coriolis forces acting on a parcel so that the forces are equal in magnitude but in opposite directions” GEOSTROPHIC WIND Northern Hemisphere L H Around and clockwise Around and counterclockwise Southern Hemisphere L H Around and counterclockwise Around and clockwise How do surface winds differ from these upper tropospheric winds? Friction and Surface Winds Drag produced by surface. Frictional force is applied opposite to direction of air motion; causes wind to blow across the isobars. At surface, friction reduces wind speed, which reduces Coriolis effect. Coriolis can not balance PGF so wind crosses isobars. Southern hemisphere PG CE HIGH Resulting wind direction: Southern hemisphere PG Out and counterclockwise HIGH FRIC CE S. hem, HIGH HIGH Southern hemisphere PG CE LOW Southern hemisphere CE PG In and clockwise LOW S. hem, LOW LOW Northern hem, HIGH Out and clockwise HIGH Northern hem, LOW In and counterclockwise LOW General Circulation Global Wind Systems driven by Highs and Lows at surface Where are Highs and Lows? Imagine the earth with no rotation HIGH There would be a single cell of convection in each hemisphere LOW But the earth rotates Coriolis deflection causes air to be deflected from those simple convective pathways Creating 3 cells in each hemisphere and a surface High Pressure in subtropics Let’s look at SURFACE Components of each cell Hadley Cells Strong and persistent Warm air rising at Intertropical Convergence Zone (ITCZ) At top of troposphere, spreads poleward, sinks at Subtropical Highs Blows towards ITCZ at Surface, creating… Trade Winds Between subtropical Highs and ITCZ NE in N. Hem SE in S. Hem Ferrel cells Not as strong, persistent, welldefined Westerlies (surface component of Ferrel cells) 35o - 60o N & S not steady or persistent Polar Front Zone 60o - 65o N & S zone of conflict between differing air masses Polar Easterlies 65o - 80o N & S more prevalent in Southern, variability in Northern Distribution of land masses disturbs this idealized system of Highs, Lows, winds Why? Uneven heating of land and water creates temperature differences and therefore pressure differences over land vs water with seasonal changes Canadian High Siberian High Icelandic Low Aleutian Low Azores Bermuda High Pacific High Pacific High Azores Bermuda High Monsoonal Low Upper Air Movement 500 625 750 875 1000 City HEAT City COOL Isobaric surfaces DECREASED DENSITY INCREASED DENSITY It takes a shorter column of cold air to exert the same surface pressure as a tall column of warm air. 500 625 500 625 750 750 875 875 1000 1000 HEAT COOL 2300 meters 500 Constant Pressure Map (isobaric maps) 625 750 1100 meters 500 625 850 750 850 1000 1000 Constant pressure map shows elevation of a certain pressure. Low heights and troughs represent cold air. High heights and ridges represent warm air. 5520 5400 5580 5640 5700 Currently: Current surface temperature map Current map of heights of 500 mb layer Constant Altitude Map Shows pressure at a given altitude 500 625 550 mb 750 810mb 1000m 1000m 500 625 850 750 850 1000 1000 1000m On a constant altitude map: low pressures indicate Cold Air high pressures indicate Warm Air High heights on a constant pressure surface map are equivalent to high pressures on a constant altitude map Low 500 mb heights are associated with low pressure at any given altitude; High 500 mb heights are associated with high pressure at any given altitude. Therefore, high and low heights tell you where high and low pressures are (for a given altitude) Upper Level Winds Westerly in mid- and high latitudes (20°-90° N & S) Easterly in Tropics (15°N - 15°S) Upper Level Westerlies have ridges and troughs: “Rossby Waves” (Longwaves) Wavelength 3 - 6 loops around earth above = 1000s km 500 mb layer influence surface weather c Converging height lines make wind speeds increase On warm side, pressure drops less rapidly with altitude than on cold side; Note isobaric surfaces slope and slope increases with altitude Therefore, wind speed increases with altitude JET STREAMS : zones of high wind speed (Narrow bands, speed increases toward center (up to 150 mph)) Embedded below Jet in upper level Westerlies tropopause streams are located above strong temperature contrasts at surface Polar Jet Stream Subtropical Jet Stream Polar Front Jet Stream Between midlatitude tropopause and polar tropopause Polar Jet above Polar Front Zone : Where cold dense polar air meets warmer air from mid-latitudes Can see polar jet on 300 mb maps Current 300 mb map Subtropical Jet Stream Between midlatitude tropopause and tropical tropopause Subtropical Jet greatest wind speed at North edge of Hadley cell due to Conservation of Angular Momentum: (smaller radius of rotation, faster the spin) Enhanced warming in Arctic is affecting Rossby waves Highs and Lows move horizontally Highs move towards convergence aloft Surface pressure rises in direction High is moving and falls in its wake Rising barometer means air is being ADDED aloft and sinking air (clear skies) are coming