Pressure, Wind and Weather Systems 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 Pressure, Wind and Weather Systems 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. Insolation Air heated by contact with ground expands; becomes less dense and rises Denser air drawn in at low level to replace rising, less dense air LOW PRESSURE Denser air drawn in at low level to replace rising, less dense air Sun heats up ground

GLOBAL PRESSURE & WIND ZONE of least heating produces HIGH PRESSURE 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 least heating produces HIGH PRESSURE HIGH ZONE of greatest heating produces LOW PRESSURE LOW HIGH ZONE of least heating produces HIGH PRESSURE

GLOBAL PRESSURE & WIND 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 at the POLAR FRONT LOW PRESSURE BELT, 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. Rising air diverges at the tropopause, where a permanent temperature inversion results in warmer air above. POLAR HIGH POLAR FRONT (LOW PRESSURE) TROPICAL HIGH EQUATORIAL (Inter-tropical convergence zone - ITCZ) LOW

WIND DIRECTION & STRENGTH Wind strength depends on the difference in pressure between the high and low pressure systems, and the distance between them. This is called the PRESSURE GRADIENT; it is a similar concept to the physical slope between two places, shown on a contour map. Pressure is shown by ISOBARS on a weather map. Pressure difference essentially depends on the temperature difference between the two places. Farmers plant trees to protect orchards, houses, stock or prevent soil erosion Locally, wind is channelled down streets (wind canyons). Strong polar winds due to low friction Strong winds also occur in low latitudes due to stronger heating and steeper presure gradients. Hurricanes and tornadoes are both tropical phenomena. A steep pressure gradient results from a large pressure difference or short distance between places and causes strong wind. Tornado in USA Beach windbreaks reduce windsped by increasing friction Hurricane in Florida

Theoretical wind which would result solely from pressure gradient CORIOLIS FORCE High Pressure gradient wind blows from high presure towards low pressure. The earth’s rotation diverts this wind direction laterally. This force is called the CORIOLIS FORCE. The Coriolis force diverts wind the the right in the northern hemisphere; to the left in the south. The effect is stronger at high altitude where ground level friction is less significant. Theoretical wind which would result solely from pressure gradient Actual wind which blows, as diverted by Coriolis Force Low LOW In the north, winds blow anti-clockwise into a low pressure system. In the south, they blow clockwise. In the north, winds blow clockwise out from a high pressure. (In the south, they blow anti-clockwise). HIGH

GLOBAL PRESSURE & WIND POLAR HIGH PRESSURE POLAR FRONT MID-LATITUDE LOW PRESSURE TROPICAL HIGH PRESSURE INTER-TROPICAL CONVERGENCE ZONE -LOW PRESSURE TROPICAL HIGH 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. Regional wind systems (eg the Indian Monsoon) relate to continental heating effects, and seasonal changes. Local winds relate to smaller scale temperature contrasts (ie Aspect, Albedo, Altitude etc).

HIGH PRESSURE CLEAR SKIES LITTLE WIND 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. FEW CLOUDS In Britain, 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, resulting in a high diurnal ranges of temperature (hot days, cold nights). Due to the trapped dust particles and cold nights, dew, frost, fog or smog are common. Air quality is low as all forms of pollution are retained in the lower atmosphere. VISIBILITY IS POOR FOG & SMOG IS COMMON CLEAR SKIES CAUSE FROST VISIBLITY REMAINS BETTER IN MOUNTAINS - LESS POLLUTION

LOW-LATITUDE LOW PRESSURE SYSTEMS Low pressure systems involve air that tends to rise, thus causing clouds and precipitation. Those near the equator tend to be high energy due to strong ground heating (convectional). Low pressure systems may develop tornadoes and sometimes develop into hurricanes, fuelled by warm, very humid air evaporated from tropical oceans in summer. They tend to be fast moving, with plenty of cloud cover that reduces diurnal temperature range, strong winds and high rainfall. A convectional cumulo-nimbus cloud results from strong ground heating at the equator A tornado may develop from a cumulo-nimbus cloud Hurricane off USA fuelled by hot humid air over the Caribbean

MID-LATITUDE LOW PRESSURE SYSTEMS Mid-latitude low pressure systems are called depressions in Britain. They also involve rising air, clouds, strong winds and rainfall and are fast moving. ARCTIC MARITIME from Arctic Ocean Cold, humid. POLAR CONTINENTAL from E.Europe Cold, dry in winter Warm, dry in summer. POLAR MARITIME from Greenland Cool, humid. Depressions result from the convergence of warm air from the tropical high pressure belt with cold air from the poles along the Polar Front. The energy of the depresion is a result of the difference in temperature and humidity between the two air masses. This contrast varies with the exact origin of the air mass, the season and the nature of the surface over which they have passed. POLAR FRONT this shifts polewards in summer and equatorwards in winter, hence British seasonal contrasts. POLAR MARITIME RETURN Coolish, very humid. TROPICAL MARITIME from Atlantic near tropic Warm, humid TROPICAL CONTINENTAL From N.Africa Hot, dry

MID-LATITUDE LOW PRESSURE SYSTEMS Mid-latitude low pressure systems are called depressions in Britain. They also involve rising air, clouds, strong winds and rainfall and are fast moving. OCCLUDED FRONT Cold & warm fronts meet Depressions (L) over Europe showing FRONTS Depressions over NW Europe COLD FRONT POLAR MARITIME AIR WARM FRONT WARM FRONT COLD FRONT TROPICAL MARTIME AIR COLD FRONT Depressions result from the convergence of warm air from the tropical high pressure belt with cold air from the poles along the Polar Front. The systems move rapidly across the Atlantic before filling and drifting north-eastwards to Scandinavia from Britain. The energy of the depresion is a result of the difference in temperature and humidity between the two air masses. This contrast varies with the exact origin of the air mass, the season and the nature of the surface over which they have passed.

A FRONT is the boundary betwen two air masses 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 1 The warm front is angled gently due to ground level friction which slows the air at low level as the whole system moves eastwards. TROPICAL MARITIME AIR POLAR MARITIME AIR 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

Pm air meets at ground level POLAR MARITIME 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 The cold front is steeper, also due to ground level friction slowing the lower air, so uplift is more rapid than along the warm front. 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. OCCLUSION Tm air forced up Pm air meets at ground level