1. 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:
A_____ M_______.
P____ C__________.
T_______ C__________.
T_______ M_______.
P____ M_______.

2. 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:
Warm air constantly moves out of tropical areas, towards the poles. A warm front is the leading edge of a warm air mass.
Cold air moves out of polar regions, towards the tropics. A cold front is the leading edge of a cold air mass.
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).
The rising warm air means that atmospheric pressure is reduced.
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.
Cold fronts move more quickly than warm fronts, so the cold front catches up with the warm front.
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.

3. 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:
Ahead of the warm front, it’s cool because cold air is overhead. Thin clouds form high up as warm air is pushed upwards.
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.
When the warm air is overhead, it’s warm. There’s no cloud because warm air holds a lot of water vapour.
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.
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.

4. Cold air overhead
As the cold front passes
Warm air overhead
As the warm front passes
Ahead of the warm front
Rain
Showers
Heavy showers
None
Sustained drizzle
Clouds
High broken
Towering, thick
Low, thick
High, thin
Pressure
Rising
Suddenly rising
Steady
Falling
Temperature
Cold
Warm
Cool
Wind speed
Decreasing
Strong
Increasing
Wind direction NW
SW to NW SW
SE to SW SE

6. Anticyclones are High Pressure Weather Systems
The UK also experiences anticyclones – areas of high atmospheric pressure caused by a large mass of falling air.
The air falls from the upper atmosphere, and warms on its way down. This causes humidity to decrease because air masses can hold more moisture as they get warmer. This means clouds don’t develop, and conditions are very dry.
In anticyclones there’s not much difference in air pressure between the centre and edges. Because the pressure is similar, the winds are weak, and flow gently outwards. In the UK, the winds flow clockwise around the centre.
Anticyclones cause different weather in winter than in summer:
WINTER WEATHER
Low temperatures during the day (from below freezing to 5°C), because the sun is at a low angle.
Extremely cold overnight temperatures (below freezing) with frosts, because clear skies allow loss of heat through radiation.
Low level cloud and radiation fogs – radiation fog forms overnight. As the ground cools, moisture in the air close to the ground condenses, forming fog.
High levels of atmospheric pollution in urban areas – pollutants are trapped by temperature inversions (when the air at higher altitudes is warmer than the air at lower levels). A lack of wind means pollutants aren’t dispersed.
SUMMER WEATHER
The absence of clouds leads to intense isolation, which means it’s hot, sunny and there’s no rain.
Rapid radiation at night can cause temperature inversions, dew and morning mist.
Coastal areas may get fogs and strong breezes. Highlands may experience strong winds due to heating of valley sides, which causes a pressure gradient.
After several days there’s a risk of thunderstorms, due to large amounts of rapidly rising warm air.
Some anticyclones are described as ‘blocking’. Blocking anticyclones can sit over the UK and remain there for many days. Depressions that would normally travel across Britain are forced around the upper edge of the anticyclone. Extreme weather conditions are likely, e.g. heatwaves in summer and dry, freezing weather in winter.

7. 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 the isobars show increasing pressure towards the centre.
An occluded front is shown by the line with alternative triangles and semi-circles in purple.
A depression is a low pressure system so the isobars show decreasing pressure towards the centre.
A warm front (the leading edge of a mass of warm air) is shown by the line with red semi-circles.
A cold front (the leading edge of a mass of cold air) is shown by 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.

8. Exam Question
Describe, and give reasons for, the changes in weather as a depression passes over the UK. (8 marks)