The idea that northerly winds (i.e. winds from the north)
are cold, and southerly winds (those from the south) are
warm (at least in the northern hemisphere) is quite common.
Similarly, air that has travelled over the sea picks up
moisture, while air travelling over the land is relatively
dry. These simple concepts help in the understanding of
air masses. However, as may be expected, there are variations
on this theme.
The air in polar and subtropical regions is often within
large anticyclones (high pressure areas), during which
time it is gradually influenced by the underlying surface
- air at the poles is cooled and air in the tropics is
warmed. The result is a large body of air with little horizontal
variation in temperature and moisture content. Depressions
(low pressure areas) develop most frequently in the more
temperate latitudes between the pole and the sub-tropics.
In doing so, they can cause a large outflow of air from
the anticyclones. The warm, sub-tropical air moves into
the southern part of the depression, while the cold, polar
air, moves into the northern part. These air masses may
approach the British Isles, but on their journey, they
can be modified by contact with the underlying surface.
Air that travels over the sea (maritime air) is moistened,
whereas there is little change in moisture content of air
that travels over the land (continental air), unless it
deposits precipitation (rain or snow). For example, air
that has been trapped in an anticyclone over the Sahara
in June slowly heats up and dries. After a while, the air
moves out of the anticyclone and may head for the British
Isles. On its way it may collect moisture over the Mediterranean
Sea, but the journey over Spain and France has little effect
on its properties. The air then arrives here as a hot,
dry air mass.
Air masses affecting the British Isles can be broadly
categorised in terms of their source and their path. This
leads to four possible types.
Tropical maritime - warm and moist
Tropical continental - warm and dry
Polar maritime - cold and (fairly) moist
Polar continental - cold and dry
To these must be added two further air masses:
Returning polar maritime - which consists of polar
air that has moved southwards over the sea and then
turns northwards and approaches the British Isles from
the south.
Arctic - which consists of air which has travelled
southwards from the arctic.
In reality, the type of air mass affecting the British
Isles only gives an indication of the type of weather that
may occur. The actual weather depends upon the detailed
history of the air, the speed of movement and the surface
over which it flows.
The boundary between two different types of air mass is
referred to as a front. It is common for the British Isles
to be affected by a sequence of fronts; usually separating
polar maritime and tropical maritime air.
Tropical
continental
Tropical continental air usually comes with south-easterly
or southerly airstreams. It originates in north Africa
and often travels over the Mediterranean Sea, Spain and
France before reaching the British Isles. In summer, even
easterly winds from central Europe or the Ukraine could
be included in this category, as the continent becomes
so hot at this time of year. The air picks up some moisture
over the Mediterranean (and perhaps the Bay of Biscay),
but overall the air tends to be quite dry and the skies
are typically cloudless.
Strictly speaking,
an air mass cooled from below on its northward
journey should be stable.
Sometimes, however, moisture may have found
its way to medium levels in the atmosphere. Then,
if there is a layer of unstable air and a trigger
to set off convection, altocumulus castellanus
clouds can develop, looking like turrets. These
are often the forerunner to tremendous thunderstorms,
which can occur by day or night.
The majority of tropical continental airstreams
give a marvellous heatwave (in summer), including
the record breaking temperatures of August 2003,
although plants and animals tend to be less appreciative
of this type of weather.
The lack of moisture usually causes the visibility to
be good. However, there may be desert dust, fine soil or
pollution particles in the air, which can lead to moderate
visibility (often described as 'heat haze'). Also, the
cloudless sky sometimes looks milky because of pollutants.
The synoptic chart below shows a typical synoptic situation
where the British Isles is being affected by a tropical
continental air mass. Both infrared and visible satellite
images are also provided for the same time. Click on the
images to view a larger version.
A polar continental air mass originates in Scandinavia
or Russia, and reaches the British Isles when north-easterly
or easterly winds become established. This tends to occur
when there is a high pressure area somewhere to the north
of the British Isles, often over Scandinavia itself. Polar
continental air masses mainly affect the British Isles
during the winter half of the year.
Temperatures in polar continental air masses are below
average in winter, except perhaps to the lee of mountains.
In summer, however, the temperatures tend to be above average.
The moisture content is low
in these air masses, especially when they take
the short sea track in the Calais/Dover region.
This leads to clouds being generally well broken,
and so the weather is fine and sunny.
Air that has crossed the North Sea between Denmark
and Scotland is said to have taken a long sea
track. It therefore collects more moisture and
clouds tend to form during its journey over the
sea. Consequently, it is cloudy in eastern districts
(with perhaps drizzle or snow flurries), but
further inland there tends to be a mixture of
cloud and sunshine.
Visibility varies, generally being very good when
air comes from Scandinavia, but less good when
the air originates in the industrialised regions
of central or eastern Europe.
Even in April or May, the North Sea is cold and does little
to modify the air mass, apart from adding a little unwelcome
moisture. In winter, southern England is particularly chilled
by polar continental air masses. Further north, the sea
surface makes the air a little less cold and the wind is
often less strong.
The synoptic chart below shows a typical synoptic situation
where the British Isles is being affected by a polar continental
air mass. Both infrared and visible satellite images are
also provided for the same time. Click on the images to
view a larger version.
Tropical maritime air usually approaches the British Isles
from the south-west. Its source region is the subtropical
Atlantic Ocean, typically the Azores area, although occasionally
it may come almost directly from the Tropics. During its
passage across the Atlantic, the air is cooled from below
as it passes over a progressively cooler ocean, and so
it becomes more stable. While it cools down, little of
its moisture is lost. It therefore reaches south-west England
or western Ireland almost saturated, giving dull, warm,
overcast weather.
On the
coasts, sea fog is common in these tropical maritime
south-westerlies. However, if the cloud base of
the stratus or stratocumulus is several hundred
feet, sea-level sites may be saved from the fog,
but on rising ground and hills there may be fog
and drizzle. Bodmin Moor, Dartmoor, south-west
Wales, western Ireland and western Scotland can
be shrouded in mild, damp conditions whether it
be winter or summer.
Further inland, in the summer half of the year
at least, the low stratus may be burnt off by
the sun and it could turn out to be quite warm,
though still humid. In the lee of hills or mountain
ranges, the clouds sometimes break up and there
is a lot of sunshine. Favoured locations like
north Somerset, North Wales, Northumberland and
the Moray Firth can become very warm during summer
and bask in spring-like weather on a January
day.
Sometimes, an anticyclone may build to the west of the
British Isles, keeping the warm, moist air away from western
districts and causing it to affect northern Scotland and,
sometimes, to move southwards down the east coast. This
leads to the formation of haar or fret.
In a tropical maritime air mass, the nights are mild and
damp, especially in mid-winter. In December and January,
the overcast skies result in little variation in temperature
between day and night. However, if there are light winds
and clear skies, fog may form inland overnight.
The synoptic chart below shows a typical synoptic situation
where the British Isles is being affected by a tropical
maritime air mass. Both infra-red and visible satellite
images are also provided for the same time. Click on the
images to view a larger version.
Polar maritime air is the most common type of air mass
affecting the British Isles. The air has its source in
the Canadian Arctic or the Greenland area. It reaches the
British Isles from the west or north-west after having
swung around the western side of a depression. As the cold
air travels over the relatively warm sea, it is warmed
from below and becomes unstable. Unstable airstreams tend
to produce convection, and so cumulus clouds, cumulonimbus
clouds and showers are likely in polar maritime air. Other
characteristics of the air are that it is cool (especially
in summer), fairly moist and associated with good visibility.
In winter, most of the convection
is initiated over the Atlantic, and showers hit
the coasts, spreading inland if the winds are strong.
The Scottish and Welsh mountains often shelter
the eastern side of Britain, although, with a
north-westerly wind, some showers sneak through
the Cheshire Gap to reach Birmingham and perhaps
London.
With a westerly wind, the winter showers can cross
Glasgow and central Scotland to reach Edinburgh
and Fife; others travel up the Bristol Channel
to affect Cardiff and Bristol.
In spring and summer, convection clouds tend to
be set off inland by daytime heating. Now, the
shelter of the western mountains is less important,
and showers or short-lived thunderstorms can occur
almost anywhere. At night the clouds disperse.
The synoptic chart below shows a typical synoptic situation
where the British Isles is being affected by a polar maritime
air mass. Both infrared and visible satellite images are
also provided for the same time. Click on the images to
view a larger version.
Returning polar maritime air, like polar maritime air,
originates in polar regions, but travels southwards before
turning north towards the British Isles. The classic returning
polar maritime airstream occurs when a large depression
is situated somewhere to the north-west of the British
Isles. Normally, once the associated weather fronts have
passed through, the British Isles are left in a north-westerly
polar maritime airstream. However, if the air reaching
the British Isles has travelled around the southern edge
of the depression and the winds are between south and south-west,
the air is designated as returning polar maritime.
The air is originally cold, but as it takes a long sea
track southwards across the Atlantic, the lower layers
become warmer, more moist and more unstable. However, as
it returns northwards, the lower layers are cooled and
become more stable. This mixture of a stable layer near
the surface and an unstable layer aloft can lead to a wide
variety of weather. On exposed coasts and hills, the combination
of high moisture content and low-level stability can lead
to stratus clouds and hill fog. Sometimes, however, the
unstable layer leads to the formation of cumulonimbus clouds
and showers (and occasionally thunderstorms). Further inland
a mixture of weather can occur - stratus lifts and disperses,
allowing heavy showers to form.
South-west England and Wales
usually have the first taste of a returning polar
maritime airstream; such airstreams are especially
common in autumn. Further north and east, with
some shelter from the mountains, conditions tend
to be better.
East coast areas may well be quite warm, with
only broken convection clouds. At night, these
areas are usually clear, dry and cool. Moisture
contents are quite high, especially near southern
coasts, but the clean air usually means good
visibility.
Only if the wind becomes very light can inland
fog form, where evening showers have moistened
the ground.
The synoptic chart below shows a typical synoptic situation
where the British Isles is being affected by a returning
polar maritime air mass. Both infrared and visible satellite
images are also provided for the same time. Click on the
images to view a larger version.
Arctic air rarely occurs outside winter and is colder
and drier than PM, although it picks up sufficient moisture
to produce showers, usually of sleet or snow, on north-facing
coasts and hills. As a rule, these showers don't travel
far inland and many places will be fine and sunny, if rather
cold. Occasionally, they may become organised into lines
of heavy showers and, rarely, into small depressions known
as Polar Lows, which can produce quite heavy falls of snow.
If accompanied by strong winds, blizzard conditions may
develop, usually over the Scottish Highlands.
The synoptic chart below shows a typical synoptic situation
where the British Isles is being affected by an arctic
air mass. Both infrared and visible satellite images are
also provided for the same time. Click on the images to
view a larger version.
