The main Air Masses that affect the United Kingdom
Air Masses are defined as a large body of air (covering many thousands of square kilometers) which at any given level has almost uniform temperatures, lapse rates(see topic on this), and humidity.
Their Source Regions are large areas of the earth where air often stagnates for long periods. Examples of these are the Polar Regions, and the sub tropics. Air over any of these regions may stay for long periods and thus picks up the characteristics of the land or sea beneath it.
When, because of a pressure gradient (see another topic on this) an air mass moves from its source, its properties will be modified by the land or sea over which it travels. By following the isobars on a pressure chart we can identify where this air mass has come from.
Air Masses are classified by reference to the area they have come from and their subsequent track. In the UK they are known as either POLAR or TROPICAL, depending on where the airmass originated from, and are then sub divided into MARITIME or CONTINENTAL depending on whether the air has passed over land or sea.
There are four major types that affect the UK
Polar Maritime (Pm)
Tropical Maritime (Tm)
Tropical Continental (Tc)
Polar Continental (Pc)
Below is a diagram showing these four major air masses as they approach the UK. One has not been mentioned so far, that is Returning Polar Maritime (shown as a paler colour and approaching the UK from the sw). This is Polar Maritime air which has dome a long sweep over to the south west and is now returning with Tropical Maritime characteristics at the surface.
Without going into a lot of detail the 4 air mass types can be summarized this way.
Polar Maritime (Pm)
In winter this will give convective type cloud with showers, often on windward coasts and hills. Depending on how cold the air is then snow, soft hail and sleet are likely. The showers can extend well inland by day but as the land cools at night the showers become mostly confined to the coastal belt. If troughs form in this air mass then showers can continue into the night and become much more widespread, especially if a Polar low forms (read up on separate section for Polar lows)
In Summer then it is the inland areas that get most shower activity due to the land heating more than the sea. Coastal strips may be shower free but inland showers or thunderstorms can develop.
Another version of this is called Arctic Maritime (Am)). As the map shows this comes direct from the Arctic and is thus even colder and therefore more showery than the Pm, This may well allow marked troughs to develop as it comes south over warmer seas, possibly even a Polar Low(see article on these)
Tropical Maritime (Tm)
It starts off from its source warm, moist and just a shallow layer. As it moves towards us it picks up more moisture, its cooled from below by the sea becoming colder as it moves north. On arrival here, usually into the southwest it has an overcast layer of Stratus and Stratocumulus (see cloud information section) and gives drizzle with extensive hill fog and sometimes coastal fog also.
In winter these conditions can spread well inland with ground temperatures being low, especially at night. If it is associated with frontal weather then coming over a cold, maybe frozen ground then advection fog (see fog section) can set in with the thaw. It can sometimes to the east of high ground, with a Fohn effect(see section on special winds) give better visibility and higher cloud bases than in the west.
In summer , particularly during the afternoon this Fohn effect combines with the warmer land to produce good cloud breaks well inland and only shallow Cumulus or Stratocumulus.
Tropical Continental (Tc)
Starting out from its source as dry, hot and at times hazy. On arrival in the UK it is stable, usually with no cloud, and is either hazy or becomes hazy if this persists for several days. Often it is the hottest air mass for these islands, especially the south and south east.
In winter if this airmass reaches us then it has the characteristics of a short sea track Polar Continental due to it crossing the cold land mass of France.
Polar Continental (Pc)
This may have started over Russia or Scandinavia. In each case its precise track will have a great bearing on what weather it gives.
Taking the winter situation first.
It sets off very cold and dry. If its track is across Europe and then from the Low Countries into south east England then it picks up very little moisture. The weather is usually dry, hazy and very cold with severe night frosts. If its track takes it across the North Sea (often having originated from Scandinavia) then it picks up moisture off the sea, this is also much warmer than the air moving over it. The net result is that the bottom of the atmosphere becomes what we call unstable (see section on Stability and Instability) and large convection clouds develop (Cumulonimbus or Cumulus=again see section on clouds). This causes showers, usually wintry, even possibly with thunder and can give appreciable snowfall in eastern districts, being especially pronounced on the eastern slopes of hills.
In Summer with a short sea track it is often dry and very warm (due in part to the long hours of daylight in high latitudes. So only limited Cumulus and possibly hazy if the situation lasts for any length of time.
With a long sea track it has become stable due its passage over a relatively cold North Sea, and it gives overcast conditions with possibly drizzle and poor visibility, maybe even fog along the coasts. This can extend well inland during the evening. It has similar characteristics on the eastern side to those of Tropical Maritime along southern and western coasts.
Finally to Returning Polar Maritime (RPm)
It starts life in cold areas, maybe off Greenland is the most likely. It then moves south, possibly south east for a time and then swings in towards the UK from the south west. What happens to it during its journey is quite complex. First, as it heads southwards, it is heated strongly from the increasingly warm sea, this makes it unstable. Its moisture content is increased. Then as it moves north east it is cooled from below, so becoming stable in its lowest layers, and it continues to pick up more moisture.
So when it arrives in the south west (its usual entry point) it has broken or overcast low cloud with hill and coastal fog, along with drizzle.
In summer as it moves inland then the heating effect returns it to its original type (POLAR MARITIME) and convective cloud develops giving showers or thunderstorms.
This topic has several links, some of which are already available on the Net Weather Guides; some are yet to be put together. One topic which follows on from this is the development of Fronts and Frontal Depressions. I hope to be able to post this in the not too distant future.
John Holmes
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