Global Air Circulation PDF

Global Air Circulation The pressure difference causes cold, dry surface winds to flow from polar high regions to the equatorial low pressure. At the equator warm moist air rises. This warm air diverges in the upper atmosphere and flows towards the poles. Over the polar regions, the air c...

Global Air Circulation The pressure difference causes cold, dry surface winds to flow from polar high regions to the equatorial low pressure. At the equator warm moist air rises. This warm air diverges in the upper atmosphere and flows towards the poles. Over the polar regions, the air cools, becomes more dense and sinks back down to the equator. Southern Hemisphere Northern Hemisphere High Pressure (anticyclones) Air cool, dry and dense HP Air sinks and diverges HP Clockwise in NH Anticlockwise in SH Low Pressure (cyclones) Air warm, moist and less LP dense LP Air rises and converges Anticlockwise in NH Clockwise in SH  90° Tropics HP Subtropical regions 60° Subpolar Low Pressure Belt LP Polar regions 30° Subtropical High Pressure Belt HP Horse latitudes 0° Equatorial Low Pressure Belt LP ITCZ (doldrums) 30° Subtropical High Pressure Belt HP Horse latitudes 60° Subpolar Low Pressure Belt LP 90° HP HORIZONTAL WIND VERTICAL CELLS Tropical easterlies (trade winds) Hadley cell Subtropical westerlies Ferrell cell Polar easterlies Polar cell BELTS AND CELLS Equatorial Low-pressure Belt – in the equatorial region and composed of warm, light, ascending and converging air. Subtropical High-Pressure Belt – a zone of hot dry air that forms as the warm air descending from the tropics heats adiabatically. Subpolar low-pressure Belt – a zone of cool wet weather caused by the meeting of cold air masses from higher latitudes and warm air masses from lower latitudes Polar High-Pressure Belt – an area of high pressure located at 90° N/S, the air is extremely cold and dry Hadley Cell – atmospheric circulation cell between the equator and 30° latitude. Ferrell Cell – atmospheric circulation cell between 30° latitude and the polar front. Polar Cell – atmospheric circulation cell between the polar front and the poles Intertropical Convergence Zone (ITCZ) – equatorial low pressure belt along which the trade winds of the two hemispheres converge. WINDS Polar Easterly – dry, cold prevailing winds that blow from the high-pressure areas near the north and south poles towards the low-pressure areas. Subtropical westerly – warm, prevailing winds in the middle latitudes that blow from the subtropical high-pressure belt towards the subpolar low-pressure belt. These blow from the west to the east. Tropical Easterly (trade winds) – warm, moist winds that blow from the east to the west. They originate from the subtropical high-pressure belt and flow towards the equatorial low-pressure belt. The Coriolis Force The earth's rotation on its axis causes surface winds to be deflected off their normal North and South directions. Coriolis Force – a force which causes a body that moves freely with respect to the rotating earth to veer to the right in the northern hemisphere and to the left in the southern hemisphere. This force is not found between 0° — 5°N & S This force strengthens when wind is blowing faster Ferrell’s law dictates what direction wind will be deflected in. Ferrell’s law – due to the Coriolis force winds are deflected to the right in the Northern hemisphere and to the left in the Southern hemisphere. (Buy’s Ballot law) Geostrophic Flow This is a theoretical wind that is formed when there is a balance between the pressure gradient force and the Coriolis force. This balance occurs in ocean currents and winds when there is a geostrophic balance. The geostrophic wind blows parallel to the isobars. This wind often occurs in the upper atmosphere where surface friction is eliminated, enabling geostrophic balance to occur. Nearer to the surface, friction weakens the Coriolis force and so geostrophic balance cannot be reached. Because there is no Coriolis force between 0°— 5°, there is no geostrophic flow here. Jet streams – strong geostrophic winds blowing from west to east in the upper atmosphere. Adiabatic cooling – the process of decreasing heat through a change in air pressure caused by an air mass expanding. Adiabatic heating – the process of increasing heat through a change in air pressure caused by an air mass compressing. Norther Hemisphere (coriolis force is deflected to the right) Low pressure PGF Air PGF flow Coriolis force High pressure  Global Pressure Belts EQUATORIAL LOW PRESSURE BELT Near the equator, warm air expands and rises in huge convection currents (A) In the upper atmosphere, the air is the same temperature as the surrounding particles, so it stops rising. SUBTROPICAL HIGH PRESSURE BELT Airs starts to cool and diverges in the upper atmosphere towards the poles. (B) The cool air sinks. (C) While sinking, the air is compresses and heated adiabatically. (D) SUBPOLAR LOW PRESSURE BELT Cool polar air converges with warm surface air. (E) This creates a front and the warm air rises. It diverges in the upper atmosphere towards the poles. POLAR F E HIGH PRESSURE C BELT D The cold B polar air sinks. A This creates a high B pressure on D the surface. C (F) E F  Cells HADLEY CELL Air rises, creating a LP at the surface. Rising air cools in the atmosphere and condenses at DPT. Cumulonimbus clouds form with thunderstorms. In the troposphere, the air diverges and sinks at 30°. As the air sinks, it heats adiabatically creating warm and dry conditions at the subtropical HP belt. On the surface, air converges towards the ELPB at the ITCZ – these surface winds are known as tropical easterly winds (trade winds). FERREL CELL Air arrives from the subtropical HP belt as warm westerly winds and from the subpolar HP belt as cold polar easterly winds Two planetary winds converge at the polar front Cooler air squeezes beneath warm air, forcing it to rise dramatically. Frontal rain forms POLAR CELL Air sinks on the Polar HP belt and runs along the surface towards the subpolar LP belt. Surface air is called polar easterly winds.  Cell Profiles Hadley Cell Ferrel Cell 30°N 0° 30°S 60°N 30°N 0° 30°S 60°S Polar Cell 90°N 60°N 30°N 0° 30°S 60°S 90°S Air Masses An airmass is a large volume of air with similar characteristics (in terms of temperature, atmospheric pressure and humidity) to the area that it covers. Air mass can be stable or unstable. Stable air à subsiding (sinking), heavy air that is associated with high pressure and thus no rain. Unstable air à rising condensing air that is associated with low pressure and can cause rain. AIR MASS TYPE TEMPERATURES MOISTURE Equatorial air masses High (hot) Both = extremely Maritime (mE) humid (very Continental (cE) unstable) because of tropical oceans & forests Tropical Air masses Both are warm as mT à very humid Maritime (mT) they are formed in the (unstable) Continental (cT) lower tropics cT à hot + dry (stable) Polar air masses mP à very cool mP à moderately Maritime (mP) (not cold due to moist (slightly Continental (cP) warm currents unstable) moderating cP à very dry temperatures) (stable) cP à very cold on land in sub polar regions Artic & Antarctic air Extremely cold Extremely dry masses because of extremely Continental artic cold and frozen (CA) waters (stable) Continental Antarctic (CAA)