Download L4 UK Weather Systems

UK Weather Systems
Aim:
To look at the Low and High pressure
systems of the UK
Starter:
What are the Five Main Air Masses that
Affect the UK:
1. A_____ M_______.
2. P____ C__________.
3. T_______ C__________.
4. T_______ M_______.
5. P____ M_______.
Depressions are Low Pressure Weather Systems
Depressions form over the Atlantic Ocean, then move east over the UK. They usually form when
the tropical maritime and polar maritime air masses meet. Here’s how:
1. Warm air constantly moves out of tropical areas, towards the poles.
A warm front is the leading edge of a warm air mass.
2. Cold air moves out of polar regions, towards the tropics. A cold
front is the leading edge of a cold air mass.
3. Where the air masses meet, the warm air heading towards the pole
rises above the cold air heading towards the tropics (because warm
air is less dense than cold air).
4. The rising warm air means that atmospheric pressure is reduced.
5. Strong winds blow from surrounding areas of high pressure towards
this area of low pressure and the whole system rotates as air
continues to rise.
6. Cold fronts move more quickly than warm fronts, so the cold front
catches up with the warm front.
7. When this happens the warm air behind the warm front is undercut
by the incoming cold front and is lifted away from the ground
entirely. It now sits above the cold air – this is an occluded front.
The warm air rises into the upper atmosphere and the depression
dissolves.
Depressions bring Depressing Weather
As depressions pass over a location they bring a distinct pattern of weather, which
usually lasts for two or three days:
1. Ahead of the warm front, it’s cool because cold air is overhead. Thin clouds form high
up as warm air is pushed upwards.
2. As the warm front passes, conditions on the ground get warmer. The warm air is pushed
up over the cold air and condenses, forming low, thick clouds and sustained drizzle.
3. When the warm air is overhead, it’s warm. There’s no cloud because warm air holds a lot
of water vapour.
4. As the cold front passes, cool air brings the temperature down again. The cold dense
air behind the cold front undercuts the warm air mass in front. This forces the warm
air to rise rapidly and condense, forming clouds and heavy showers. Rapidly rising air
makes it very windy at the surface.
5. When the cold front catches up with the warm front, all of the warm air has been
squeezed upwards so there’s no warm air left at the bottom. There’s less rain because
the uplift of air has decreased. There is little condensation, so cloud cover decreases.
Wind speed also decreases, but air pressure rises as the dense cold air replaces the
uplifted air and the depression dies out.
Cold air
overhead
As the cold Warm air
front passes overhead
As the warm Ahead of
front passes the warm
front
Rain
Showers
Heavy
showers
None
Sustained
drizzle
None
Clouds
High
broken
Towering,
thick
None
Low, thick
High, thin
Pressure
Rising
Suddenly
rising
Steady
Falling
Falling
Temperature
Cold
Falling
Warm
Rising
Cool
Wind speed
Decreasing
Strong
Decreasing
Strong
Increasing
SW to NW
SW
SE to SW
SE
Wind direction NW
Anticyclones are High Pressure Weather Systems
1. The UK also experiences anticyclones – areas of high atmospheric pressure caused by a
WINTER WEATHER
SUMMER WEATHER
large mass of falling air.
• Low temperatures during the day (from
• The
of This
clouds
leads humidity
to
2. The air falls from the upper atmosphere, and warms
on itsabsence
way down.
causes
below freezing to 5°C), because the sun
intense
isolation,
which This
means
it’s
to decrease because air masses can hold more moisture
as they
get warmer.
means
is at a low angle.
hot, sunny and there’s no rain.
clouds don’t develop, and conditions are very dry.
• Extremely cold overnight temperatures
Rapid radiation
night
can and
cause
3. In anticyclones there’s not much difference in air •pressure
betweenat
the
centre
edges.
(below freezing) with frosts, because
temperature
inversions,
and UK,
Because the pressure is similar, the winds are weak, and
flow gently
outwards.dew
In the
clear skies allow loss of heat through
morning mist.
the winds flow clockwise around the centre.
radiation.
• Coastal
areas may get fogs and
4. Anticyclones cause different weather in winter than
in summer:
• Low level cloud and radiation fogs –
strong breezes. Highlands may
radiation fog forms overnight. As the
experience strong winds due to
ground cools, moisture in the air close to
heating of valley sides, which causes
the ground condenses, forming fog.
a pressure gradient.
• High levels of atmospheric pollution in
• After several days there’s a risk of
urban areas – pollutants are trapped by
thunderstorms, due to large
temperature inversions (when the air at
amounts of rapidly rising warm air.
higher altitudes is warmer than the air
5. Some
anticyclones
described
as ‘blocking’. Blocking anticyclones can sit over the UK
at lower
levels). Aare
lack
of wind means
and
remain there
many days. Depressions that would normally travel across Britain are
pollutants
aren’tfor
dispersed.
forced around the upper edge of the anticyclone. Extreme weather conditions are likely,
e.g. heatwaves in summer and dry, freezing weather in winter.
Make Sure You can Read Synoptic Maps
Synoptic maps have lines called isobars. These link together points of equal atmospheric
pressure, which is measured in millibars (mb). Here are two synoptic charts, one showing a
depression and the other an anticyclone.
An anticyclone is a high
pressure system, so An
theoccluded front is
isobars show increasing
shown by the line with
A depression is a low
pressure towards thealternative
centre. triangles and
pressure system so
semi-circles in purple.
the isobars show
decreasing pressure
towards the centre.
A warm front (the
leading edge of a mass of
A cold front (the
warm air) is shown by the
leading edge of a mass
line with red semiof cold air) is shown by
circles.
the line with blue
triangles.
Winds blow almost parallel to the isobars, not straight across them. This means that winds move
in a spiral fashion into the centre of a depression and out of the centre of an anticyclone.
Exam Question
Describe, and give reasons for, the changes in weather
as a depression passes over the UK. (8 marks)