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

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

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

ZONE of least heating produces HIGH PRESSURE

HIGH

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

TROPICAL HIGH PRESSURE

POLAR FRONTMID-LATITUDE LOW PRESSURE

POLAR FRONTMID-LATITUDE LOW PRESSURE

POLAR HIGH 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 MARITIMEfrom Greenland

Cool, humid.

POLAR CONTINENTALfrom E.Europe

Cold, dry in winterWarm, dry in summer.

ARCTIC MARITIMEfrom Arctic Ocean

Cold, humid.

TROPICAL CONTINENTALFrom N.Africa

Hot, dry

TROPICAL MARITIMEfrom Atlantic near tropic

Warm, humid

POLAR MARITIME RETURN

Coolish, very humid.

POLAR FRONTthis 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

WARM FRONT

COLD FRONT

COLD FRONT

OCCLUDED FRONTCold & warm fronts meet

COLD FRONT

TROPICAL MARTIME AIR

POLAR MARITIME AIR

1POLAR MARITIME AIRTROPICAL 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 MARITIMEPOLAR 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.