3 What we need to know for today Pressure gradient force: from High to Low pressureCoriolis force (effect):Results from the rotation of the planet.Maximum at the poles and no effect at the equator.Acts perpendicular to the direction of motion: changes the direction of the wind but not the wind magnitude.In the NH deflects the wind to the right.In the SH deflects the wind to the left.Winds aloftBalance between the pressure force and the Coriolis force.The wind is parallel to the isobars.Surface windsBalance between the pressure gradient force, the Coriolis force and the air friction.The wind crosses the isobars (from High to Low pressure).
4 Average Wind Structure The direction and the magnitude of the winds at a given location can vary significantly during the day, and from day to day.The general circulation (GC) refers to the average (the prevailing) winds on a global scale (around the world).The GC of the atmosphere is the result of the uneven heating of the Earth’s surface.It is impacted by the Earths rotation.The GC transports and redistributes energy from one region to another (warm air towards the poles and cold air towards the equator).
5 The Single Cell ModelThis is a very simplified model based on the following three assumptions:The Earth’s surface is uniformly covered with water (no differential heating of the land and the oceans)The sun is always directly over the equator (no seasonal variations of the winds).The Earth does not rotate.No Coriolis effect.The only active force is the pressure gradient force.
6 Thermal circulations Due to uneven heating of the surface. Example: South area heats up, North area coolsWarmer southern air aloft moves north towards low pressureIt then cools and sinksSurface pressure to the North increasesSurface wind from N to SThe surface air warms up and rises.The process continues
7 The Hadley CellIt is driven by the uneven heating of the Earth’s surface by the sun - thermally direct cell: warm air rises, cold air sinks.One Hadley cell in each hemisphere.The equator is warmer than the poles.Warm moist air at the equator rises upwardsIt expands, cools, and saturates, the water vapor condenses and forms clouds.It creates low surface pressurein the tropics.At the poles we have cool, dry,sinking air that creates high surfacepressure in the polar region.The PGF (pressure gradient force) drivesthe surface winds from the poles towardsthe equator.The winds aloft close the cell by blowingfrom the equator towards the poles.
8 The one cell model does not work! It is obviously wrong: predicts northern prevailing winds everywhere in the NHWhat is wrong with the model? It is too simple!The rotation of the Earth will deflect the winds to the right in the Northern hemisphere and to the left in the Southern hemisphere.This will result in surface winds blowing:From the East (easterlies) in the NHFrom the East (easterlies) in the SHThis will result in winds aloft blowing:From the West (westerlies) in the NHFrom the West (westerlies) in the SH
9 Observing global winds Intertropical convergence zoneObserving global windsfrom space
10 Winds AloftWarm air above the equator and cold air above the polar regionsHigher pressure at the equator, lower pressure both to the north and to the south of the equatorThe pressure gradient force is towards the poles, sets the airin motionThe Coriolis forceNH: to the rightSH: to the leftThe wind turns right in the NH and left in the SH, becomes parallel to the isobarsWesterly winds aloft in both the NH and SH.Easterly winds at the surface in both the NH and SH.
11 The Three Cell Model Keep two of the assumptions, relax the third: The Earth is covered with a continuous oceanThe sun is always directly over the equatorThe Earth rotates -> Coriolis force!
12 Three cell model: the Hadley cell (0-30 deg) Thermally direct cell: warm air rises, cool air sinksIntertropical Convergence Zone (ITCZ)A.k.a. equatorial doldrumsWarm air, weak PGF, light winds, cumulus clouds and thunderstormsAir rises up to the tropopause, then laterally toward the polesDeflected east due to the CFWinds aloft in NH: from southwestSubtropical highs (anticyclones)Equatorial air cools, sinks, warms up, clear skies -> major desertsAir converges (follow the meridians on a globe) – high surface pressureHorse latitudes: small PG, weak horizontal winds -> sailors get stuckSurface winds in NH: from the northeast (Trade winds)
13 Three cell model: the Ferrel cell (30-60 deg) Thermally indirect cell: cool air rises and warm air sinksSome of the sinking air in the horse latitudes heads toward the poleDeflected east by the CFSurface winds in NH: from the southwest (westerlies)At the polar front the westerlies encounter cold air moving down from the polesAir is forced to rise, some of it returns to the horse latitudes, completing the Ferrel cell, the rest heads for the poleUpper air winds in the Ferrel cell: from the northeast.William FerrelWilliam Ferrell
14 Three cell model: the polar cell (60-90 deg) It is a Hadley type of circulation.Surface winds: from the north east (polar easterlies)Upper winds in NH: from the southwestSummary: two major areas of Low pressure (ITCZ and subpolar low), and two of High pressure (poles and subtropical highs)
15 The converging/diverging regions ITCZ (Intertropical Convergence Zone ) - EquatorLow surface pressure with small PG and weak horizontal winds.Upward motion of warm moist air. Results in convective cloud towersSubtropical highs (the horse latitudes) – 30N; 30SHigh surface pressureThe upper air is sinking, warms up and the relative humidity is very low.Weak winds, clear sky, dry climate – large deserts at these latitudes.Subpolar lows (polar front) – 60N, 60SA converging zone at the surface. Air moves up and results in strong storms.Weak windsPolar highs – 90N, 90S
16 Winds and pressure in the real world Semi-permanent highs and lows: persist throughout the year, correspond to converging/diverging upper air masses.Bermuda, Pacific highs; Icelandic, Aleutian lowsSeasonal highs and lows (continents heat/cool faster)Winter: Siberian high, Canadian highSummer (thermal lows): Southwest US, IranJanuaryJulySubtropical highsSubtropical highs
17 The General Circulation and Precipitation Paterns Converging surface flows:Low surface pressureUprising airHeavy precipitationDiverging surface flows:High surface pressureSinking airDry climate
18 Winds and Pressure Systems Aloft The wind system aloft differs from the surface wind system. It is close to a geostrophic flow.There is no significant friction with the ground.The three cell model does not work that well in the middle latitudes.The winds aloft are stronger than on the ground.In the winter the gradients are bigger -> the winds are stronger.JulyJanuary
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