Transcript Pressure, Wind and Weather Systems

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
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
Sun heats up ground
Denser air drawn in at low
level to replace rising, less
dense air
What are winds?
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.
�
GLOBAL PRESSURE & WIND
North Pole 90°N
Arctic circle 66.5°N
HIGH
Tropic of Cancer
23.5°N
Equator 0°
LOW
Tropic of Capricorn
23.5°N
Antarctic circle 66.5°S
HIGH
North Pole 90°N
ZONE of least heating produces
HIGH PRESSURE
ZONE of greatest
heating produces
LOW PRESSURE
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 midlatitides where it meets cold, dense polar air blown out from the polar HIGH PRESSURE.
POLAR HIGH
POLAR FRONT (LOW PRESSURE)
TROPICAL HIGH
EQUATORIAL (Inter-tropical convergence zone - ITCZ) LOW
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.
POLAR HIGH
POLAR FRONT (LOW PRESSURE)
TROPICAL HIGH
EQUATORIAL (Inter-tropical convergence zone - ITCZ) LOW
�
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
POLAR HIGH PRESSURE
POLAR FRONT
MID-LATITUDE
LOW PRESSURE
TROPICAL HIGH
PRESSURE
-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.
HIGH PRESSURE
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.
LITTLE
WIND
CLEAR SKIES
CLEAR SKIES CAUSE FROST
VISIBLITY REMAINS BETTER IN
MOUNTAINS - LESS POLLUTION
FOG & SMOG IS
COMMON
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,
�
LITTLE
WIND
CLEAR SKIES
FEW CLOUDS
CLEAR SKIES CAUSE FROST
VISIBLITY REMAINS BETTER IN
MOUNTAINS - LESS POLLUTION
FOG & SMOG IS
COMMON
MID-LATITUDE LOW PRESSURE SYSTEMS
ARCTIC MARITIME
from Arctic Ocean
Cold, humid.
POLAR MARITIME
from Greenland
Cool, humid.
POLAR MARITIME
RETURN
Coolish, very humid.
TROPICAL MARITIME
from Atlantic near tropic
Warm, humid
POLAR CONTINENTAL
from E.Europe
Cold, dry in winter
Warm, dry in summer.
POLAR FRONT
this shifts polewards in summer
and equatorwards in winter,
hence Irish seasonal contrasts.
TROPICAL CONTINENTAL
From N.Africa
Hot, dry
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
OCCLUDED FRONT
Cold & warm fronts meet
COLD
FRONT
POLAR MARITIME AIR
WARM
FRONT
COLD
FRONT
COLD
FRONT
WARM
FRONT
TROPICAL
MARTIME AIR
WARM FRONTS
A FRONT is the boundary betwen two air masses. A depression
has two, a warm (the front of the warm air) and a cold.
1
TROPICAL MARITIME AIR
POLAR 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.
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
POLAR
MARITIME
TROPICAL MARITIME
As the depression moves eastwards, the warm tropical air
continues to be forced upwards by the colder, denser polar air
mass.
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.
�