Pressure, Wind and Weather Systems Air heated by contact with ground expands; becomes less dense and rises Sun heats up ground LOW PRESSURE Denser air drawn in at low level to replace rising, less dense air Air stops rising when it meets air of equal density, then diverges at high level to produce more wind which eventually sinks elsewhere to complete the circulation cell

� WINDS are horizontal flows of air; winds blow from areas of high pressure to areas of low pressure (nature tries to equalise pressure) � PRESSURE describes the tendency of the air to rise or to sink at any given place or time. � Air tends to rise or sink as a result of its density. � Air density varies with altitude but, at the ground level, air density is governed by its temperature. � Thus, variations in radiation and temperature control pressure and wind. What are winds?

GLOBAL PRESSURE & WIND Antarctic circle 66.5°S Arctic circle 66.5°N North Pole 90°N Equator 0° Tropic of Cancer 23.5°N Tropic of Capricorn 23.5°N ZONE of greatest heating produces LOW PRESSURE ZONE of least heating produces HIGH PRESSURE HIGH LOW

GLOBAL PRESSURE & WIND EQUATORIAL (Inter-tropical convergence zone - ITCZ) LOW POLAR FRONT (LOW PRESSURE) TROPICAL HIGH POLAR HIGH Global circulation depends on differential heating over the globe. The system is driven by strong equatorial heating, causing LOW PRESSURE. Equatorial air rises, diverges and descends over the tropics, where HIGH PRESSURE dominates; where it diverges at ground level. This tropical air blows towards the equator, completing the equatorial cell, or towards the mid- latitides where it meets cold, dense polar air blown out from the polar HIGH PRESSURE.

These contrasting tropical and polar air masses meet where the warmer air is forced upwards by the polar air. At high level, this air again diverges towards the pole or to the tropic. EQUATORIAL (Inter-tropical convergence zone - ITCZ) LOW POLAR FRONT (LOW PRESSURE) TROPICAL HIGH POLAR HIGH

� Wind strength depends on the difference in pressure between the high and low pressure systems, and the distance between them. � Pressure is shown by ISOBARS on a weather map. � Pressure difference essentially depends on the temperature difference between the two places.

GLOBAL PRESSURE & WIND -LOW PRESSURE TROPICAL HIGH PRESSURE POLAR FRONT MID-LATITUDE LOW PRESSURE POLAR FRONT MID-LATITUDE LOW PRESSURE POLAR HIGH PRESSURE GLOBAL WIND BELTS (trade winds) are controlled by the major pressure belts, which relate fundamentally to temperature.

High Pressure means that air tends to sink. Sinking air is compressed, warms up as a result and its relative humidity falls below saturation. Any clouds evaporate. Rainfall is unlikely, apart from occasional short, intense convectional storms due to insolation with lack of clouds in daytime. CLEAR SKIES CAUSE FROST CLEAR SKIES LITTLE WIND HIGH PRESSURE FOG & SMOG IS COMMON VISIBLITY REMAINS BETTER IN MOUNTAINS - LESS POLLUTION

CLEAR SKIES CAUSE FROST HIGH PRESSURE � In Ireland, high pressure systems have clear skies, little or no wind, little rainfall and tend to be stable and slow moving. � Visibility is intially good, but rapidly deteriorates as dust is trapped by sinking air and is not washed out by rainfall. � Cloud cover is slight, FEW CLOUDS FOG & SMOG IS COMMON VISIBLITY REMAINS BETTER IN MOUNTAINS - LESS POLLUTION CLEAR SKIES LITTLE WIND

MID-LATITUDE LOW PRESSURE SYSTEMS POLAR MARITIME from Greenland Cool, humid. POLAR CONTINENTAL from E.Europe Cold, dry in winter Warm, dry in summer. ARCTIC MARITIME from Arctic Ocean Cold, humid. TROPICAL CONTINENTAL From N.Africa Hot, dry TROPICAL MARITIME from Atlantic near tropic Warm, humid POLAR MARITIME RETURN Coolish, very humid. POLAR FRONT this shifts polewards in summer and equatorwards in winter, hence Irish seasonal contrasts.

Mid-latitude low pressure systems are called depressions in Britain. They also involve rising air, clouds, strong winds and rainfall and are fast moving. Depressions result from the convergence of warm air from the tropical high pressure belt with cold air from the poles along the Polar Front.

Depressions over NW Europe

MID-LATITUDE LOW PRESSURE SYSTEMS Depressions (L) over Europe showing FRONTS WARM FRONT COLD FRONT OCCLUDED FRONT Cold & warm fronts meet COLD FRONT TROPICAL MARTIME AIR POLAR MARITIME AIR

1 TROPICAL MARITIME AIR The warm front is angled gently due to ground level friction which slows the air at low level as the whole system moves eastwards. A FRONT is the boundary betwen two air masses. A depression has two, a warm (the front of the warm air) and a cold. WARM FRONTS

As the warm tropical maritime air moves eastwards towards Britain, it is forced upwards by colder, denser polar maritime air. The speed of uplift depends on the relative temperature of the two air masses. Uplift causes expansion, cooling, falling relative humidity until dew point temperature is reached when condensation starts to occur on particles. The amount of precipitation depends on the hunidity and temperature of the warm air mass, and the particles available. 2 MAINLY STRATUS CLOUDS

COLD FRONTS As the depression moves eastwards, the warm tropical air continues to be forced upwards by the colder, denser polar air mass. TROPICAL MARITIME POLAR MARITIME

� The cold front is steeper � This causes cumulo-nimbus clouds and possible thunderstorms rather than thick stratus cloud. � Eventually, the two fronts meet, forcing the warm air off the ground. � This is an OCCLUDED FRONT (occlusion), and happens to all depressions as they ‘fill’. � The whole system takes about 24 hours to pass.