Learning Objectives:
1) To understand the formation and development of a depression
2) To know the sequence of weather associated with a depression
3) To know the characteristics of an anticyclone and the contrasting weather conditions
between winter and summer.
Depressions are areas of low pressure. There is a relatively low air pressure at the
surface because the air is rising. Air, carried by surface winds, is drawn into the
centre of he depression to replace the rising air. In the northern hemisphere, winds
blow in an anti-clockwise direction around a depression, spiralling towards it centre.
In general, depressions bring spells of unsettled weather with plenty of wind, cloud
and rain.
In the UK most of our weather is associated with the passage of a depressions
which, having formed over the Atlantic, pass across the UK from west to east driven
by the prevailing winds.
How are frontal depressions formed?
Frontal depressions are formed when warm air from the tropics meets- and is forced to
rise above- cold air from the poles. The rising creates the centre of low pressure. The line
that separates the two air masses is called a front. As the depression develops and moves
east, driven by the westerly circulation, two fronts can be recognised. The leading front is
the warm front, so called because once the warm front passes over a place, the warm air
of the tropical air mass brings warm weather. The front at the rear of the depression is
the cold front, so called because after it passes of the cold air of the polar air mass brings
colder weather.
In well developed depressions, the cold front catches up with the warm front and forms
an occluded front.
Frontal depressions and satellite photographs
and synoptic charts
In the satellite photo the centre of the low
pressure can be detected by the swirl of cloud.
This is where the wind is being sucked into the
centre from the surrounding areas of high
pressure. The swirl is caused by the rotation of
the Earth. The fronts show up as trailing lines
of cloud on the southern sides of the
depression. It is along the fronts that most
activity is occurring as air is being pushed
upwards, leading to condensation of its
moisture into cloud. The white speckles of
cloud in the area behind the cold front indicate
shower clouds. Cold air is being warmed up by
its journey over the sea, encouraging air to rise
to form cumulo-nimbus clouds shown.
Depressions are isolated storms that form at the boundary of cold polar air moving
south and warm tropical air moving north. This boundary is called the polar front.
Once formed, the
depression intensifies
and starts to rotate, with
air spiralling upwards to
form cloud and rain.
The warm front marks the
front of warmer air and the
cold front represents the
front of colder air. At these
fronts air is forced to rise,
often forming distinct
bands of cloud and rain.
Over time the cold front
catches up with the warm
front to form a single
boundary called an
occluded front. Eventually
the depression fizzles out
and dies.
Depressions often develop to the west of Britain, where tropical and polar air masses
meet, and travel eastwards across the country, blown by the prevailing westerly
winds.
A depression is recognisable on a weather map because of the two fronts, a warm and
a cold front, which meets at an apex. The cold front moves more quickly than the
warm front so the warm sector gradually becomes smaller. As the fronts merge an
occluded front forms, often bringing particularly bad weather conditions. As a
depression crosses over the country it brings changeable weather.
Anticyclones- high pressure systems. These large areas of descending air bring
settled weather that can last for several days or weeks. Pressure in these systems is
usually over 1,000 millibars. If they persist they are called ‘blocking highs’ because
they block out depressions.
Main features of the depression
This depression is centred off Northern Ireland to the west coast of Scotland. The warm
front stretches through the North Sea and the cold front runs through the centre of
England.
Ahead of the warm front, conditions slowly become cloudier before a prolonged period of
steady rain sets. Behind the warm front the temperature increases in the warm sector. It
remains cloudy with patchy rain and drizzle. The cold front brings a short period of heavy rain,
often accompanied by strong and gusty winds. Behind the cold front the temperature drops
sharply. The rain stops but heavy showers may occur. The one above takes 18h to cross the UK.
Centre of low pressure
Cold front
Warm front
Warm
sector
Anti-clockwise
winds
3 (a) (ii)
• Isobars are circular in shape.
• Pressure is low/lowest in the middle (below 984mb)
• Pressure increases from the centre
• Closely spaced isobars
Explain why the weather changes with the passage of a depression (6 marks)
Temperature- cool initially and then getting warmer as the warm front approaches and
takes over from the cold.
Cloud- increases and cloud thickens, layer cloud- stratus and cirrus as warm air rises
over cold, then thins and possible breaks as warm sector passes to be followed by thick,
vertical, cumulus clouds as the cold front approaches and the cold air forces the warm
air to rise quickly.
Precipitation- prolonged, relatively light rain and quite mild as the wider warm front
passes, then drier, possibly drizzle and breaks in rain and as the cold front passes heavy
rain, temperatures fall and possibly snow. Rain is heavier due to the cold air
undercutting the warm air and causes rapid uplift.
Wind increases as warm front approaches and pressure falls as warm front approaches,
steadies as warm sector passes and increases to be at its strongest as cold front passes.
The wind direction will change from SW to NW around the centre of the low.
Pressure starts to fall ahead of the warm front, continuing to do so as the warm front
passes. It then steadies before rising as cold front passes.
1) Describe the climate of York (3 marks)
2) Explain why the climate of Aberystwyth is different to that of Edinburgh (4 marks)
Temperature
- Maximum temperature is 21°C
- Minimum is 6°C
- Temperature range is 15°C
- Peak occurs in June and July
- Falls faster than it rises
Precipitation
- Precipitation fluctuates throughout the year
- No clear change with the seasons
- Wettest in summer (July and August)
- Driest during late winter/ early spring- February to April
Edinburgh is cooler than Aberystwyth whilst Edinburgh is wetter than Edinburgh from
the climate graphs
Aberystwyth is wetter due to the prevailing winds and nearness to the sea, it is on the
coast, in the path of the prevailing winds as they hit the shore.
Areas on the east receive less rainfall, due to rainshadow. Depressions come from the
west so more rainfall is released in the west.
Edinburgh is cooler as it is further north/ at a higher latitude. This means that the sun’s
rays are more concentrated in York as the sun is higher in the sky as insolation from the
sun is higher and there is a smaller area for them to be spread over on the surface.
Aberystwyth is on the west coast and is influenced by the sea and North Atlantic Drift,
so is warmer than Edinburgh in winter on the eastern side of Scotland.
1) Study the climate graph for Tenby. Describe annual temperature range in Tenby
(2 marks)
2) Study the map, which shows
information about the climate at for
weather stations in the UK.
Explain the variation in rainfall shown.
You should refer to reasons such as
altitude, winds and distance from the
sea (8 marks).
Distance from the sea/ the direction of the prevailing winds- places nearer the sea, such as
Tenby and St Mawgan tend to be wetter than those further away, such as Cambridge.
This is because the south westerly winds will have crossed a large expanse of water know
as the Atlantic Ocean and when they blow off the sea onto the land the winds are heavily
laden with moisture nearer the coast than inland. The air will rise or meet air from other
areas and deposit much of their rainfall on places in the west and thus the rain has already
fallen by the time eastern areas like Cambridge is reached.
Altitude exaggerates the effects as the winds blowing from the sea to the land have to rise
more at Princetown, which is higher up and in the west, than at St Mawgan, so the winds
are cooled more and so there is more condensation, more clouds more and greater
precipitation occurs.