Press System

8/3/2019 Press System

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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.

Air heated by

contact with ground

expands; becomes

less dense and rises

Sun heats up ground

Insolation

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 completethe circulation cell


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GLOBAL PRESSURE & WIND

Antarctic circle 66.5S

Arctic circle 66.5N

North Pole 90N

North Pole 90N

Equator 0

Tropic of Cancer23.5N

Tropic of Capricorn23.5N

ZONE of greatest

heating produces

LOW PRESSURE

ZONE of least heating produces

HIGH PRESSURE

ZONE of least heating produces

HIGH PRESSURE

HIGH

HIGH

LOW


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Rising air diverges at the

tropopause, where a

permanent temperatureinversion results in

warmer air above.


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 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.

WIND DIRECTION & STRENGTH


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Strong winds also occur

in low latitudes due tostronger heating and

steeper presure gradients.

Hurricanes and

tornadoes are both

tropical phenomena.

WIND DIRECTION & STRENGTHWind strength depends onthe difference in pressure

between the high and low

pressure systems, and the

distance between them.

This is called the PRESSURE

GRADIENT; it is a similarconcept 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.

Locally, wind is channelled

down streets (wind canyons).

Beach windbreaks reduce windsped

by increasing friction

Strong polar winds due to low

friction

Farmers plant trees to protect orchards,

houses, stock or prevent soil erosion

Hurricane in Florida

A steep pressure gradient results

from a large pressure difference

or short distance between places

and causes strong wind.

Tornado in USA


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In the north,

winds blow anti-

clockwise into a

low pressure

system. In thesouth, they blow

Actual windwhich blows, as

diverted by

Coriolis Force

Theoretical wind

which would result

solely from pressure

gradient

CORIOLIS FORCEHigh

Low

Pressure gradient wind blows from

high presure towards low pressure.

The earths 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 highaltitude where ground level friction

is less significant.

LOW

HIGH

In the north, winds

blow clockwise out

from a high pressure.

(In the south, they blowanti-clockwise).


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INTER-TROPICAL

CONVERGENCE ZONE

-LOW PRESSURETROPICAL HIGH

PRESSURE

TROPICAL HIGH

PRESSURE

POLAR FRONT

MID-LATITUDE

LOW PRESSURE

POLAR FRONT

MID-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. Regional

wind systems (eg the Indian Monsoon) relate to continental heating

effects, and seasonal changes. Local winds relate to smaller scaletemperature contrasts (ie Aspect, Albedo, Altitude etc).


7/12CLEAR SKIES CAUSE FROST

HIGH PRESSUREHigh 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.

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 oftemperature (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.FEW CLOUDS

VISIBILITY ISPOOR

FOG & SMOG ISCOMMON

VISIBLITY REMAINS BETTER INMOUNTAINS - LESS POLLUTION

CLEAR SKIES

LITTLEWIND


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LOW-LATITUDE LOW PRESSURE SYSTEMSLow pressure systems involve air that

tends to rise, thus causing clouds and

precipitation. Those near the equator

tend to be high energy due to strongground 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 ofcloud cover that reducesdiurnal temperature range, strong winds

and high rainfall.

Hurricane offUSA

fuelled by hot

humid air overthe Caribbean

A tornado may

develop from a

cumulo-nimbus cloud

A convectional cumulo-nimbus

cloud results from strong groundheating at the equator


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MID-LATITUDE LOW PRESSURE SYSTEMSMid-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 airfrom 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 overwhich they have passed.

POLARMARITIME

from

Greenland

Cool, humid.

POLAR CONTINENTAL

from E.Europe

Cold, dry in winterWarm, dry in summer.

ARCTIC

MARITIME

from Arctic Ocean

Cold, humid.

TROPICAL

CONTINENTAL

From N.Africa

Hot, dry

TROPICAL

MARITIME

from Atlantic near

tropicWarm, humid

POLAR

MARITIME

RETURN

Coolish, very

humid.

POLAR

FRONT

this shifts

polewards in

summer and

equatorwards in winter,

hence British

seasonal

contrasts.


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MID-LATITUDE LOW PRESSURE SYSTEMSMid-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. 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, theseason and the nature of the surface over which they have passed.

Depressions over

NW Europe

Depressions (L)

over Europe

showing FRONTS

WARMFRONT

WARMFRONT

COLDFRONT

COLDFRONT

OCCLUDED FRONT

Cold & warm fronts meet

COLDFRONT

TROPICAL

MARTIME AIR

POLAR MARITIME

AIR


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1

2

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.

MAINLY STRATUSCLOUDS

POLAR MARITIME AIR

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

COLD FRONTS


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OCCLUSION

Tm air forced up

Pm air

meets at

ground level

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.

As the depression moves

eastwards, the warm

tropical air continues to be

forced upwards by the

colder, denser polar air

mass.

COLD FRONTS

TROPICAL MARITIME

POLAR

MARITIME

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