Winter Air Masses
It has been said by visitors to the UK that it is possible to experience all four seasons in one day here. Perhaps that variability is at the root of our national interest in all things weather and climate-related, especially the desire to work out what is going to happen next. Fortunately, there is a way any of us can look at the weather to make a reasonable first guess at the forecast. That is by noticing from which direction the wind is blowing; known more formally as Air Mass analysis.
In this blog, we look at why this can work and what weather conditions are often seen when a particular Air Mass affects the UK; that is to say when the wind is blowing from a particular direction. For now, we will consider the wintertime situation; summer Air Masses will be discussed in a future article.
Image 1: Air Masses affecting the UK in winter
Image 1 shows the five different Air Masses that characterise much of the weather over the UK in winter. These are split up according to two criteria:
1. Where did the air begin its journey to the UK?
2. What sort of surface did it predominantly travel across on the way here?
Essentially, the answer to question one shows whether the air is initially warm or cold, whilst the answer to question two shows whether it becomes warmer or cooler and wetter or drier than at first.
Polar Maritime Air – West to north westerly winds
- starts its journey in high latitudes and is therefore relatively cold
- travels south-east over increasingly warm water so that its temperature and humidity both increase
- because it is being heated from below, shower clouds (cumulus and cumulonimbus) are created, and the air is described as unstable
- associated weather consists of sunny spells and heavy showers, mainly over north and west facing coasts and high ground. These showers will fall as sleet or snow on the coldest days, especially in the north, but overall, rain with a risk of hail and thunder is most likely
- further south-east, when the air has had a chance to travel overland for some distance, the sky is generally much clearer and showers are far less frequent and intense, especially at night
Arctic Maritime Air – Northerly winds
- a colder version of Polar Maritime air, travelling directly from the north and generating showers over some eastern coastal districts of Scotland and England as well as northern Scotland, Northern Ireland and north Wales
- the showers are often snow showers and, because the convection generated by heating from below is far stronger, small scale weather features called Polar Lows can develop, bringing very heavy and prolonged snowfall, even well inland and over low ground
- much of central, southern England will have clear skies under this Air Mass
Polar Continental Air – Easterly winds
- originating in the strong winter Siberian anticyclone, where it can remain for several weeks, this air stream is the coldest of all the air masses
- very dry as it will have travelled almost exclusively over cold land on its journey to the UK
- cloud free unless it crosses the much warmer North Sea further to the north when it will have sufficient time to become unstable and pick up moisture similarly to Polar Maritime Air
- so, whilst Kent may experience lots of sunshine but bitter cold, East Anglia, north east England and eastern Scotland may have frequent and prolonged snow showers, leading to a significant build-up of snow on east facing coasts and hills
- very occasionally there will be persistent snow across Kent and Sussex if the wind direction is more north-easterly or over Hampshire, Dorset and Devon if the air travels along the English Channel for long enough before making landfall
Tropical Maritime Air – South westerly winds
- the so-called ‘Azores High’ over the tropical Atlantic is the origin of this Air Mass, making it both warm and humid
- it then travels north east over the sea picking up further moisture
- is simultaneously cooled from below as it meets ever lower sea temperatures
- this makes the air stable, and the result is a largely cloudy and mild air stream, which is moist enough for drizzle and hill/coastal fog over western and south-western areas
- to the north east of high ground, it is generally much drier and sometimes sunny with notably warm temperatures; Aberdeen and the Moray Firth can have the highest temperatures in the UK on such days
Returning Polar Maritime Air – West to south westerly winds
- was originally Polar Maritime air heading south over the central Atlantic rather than straight towards the UK. But...
- it swings round a trough to the west of the British Isles, arriving at these shores as a westerly air stream
- with this prolonged sea track, it becomes considerably warmer and moister than it was at the start of its journey, but although unstable at first, rather less so later as it turns northwards towards the UK and crosses cooler seas
- the weather is generally less showery than Polar Maritime Air, with plenty of sunny intervals, although the fact that it is forced to rise over coasts and high ground may be sufficient to generate some more significant downpours of rain with isolated hail and thunder occasionally possible
It should be noted that significant weather features such as fronts, depressions and anticyclones will dominate the weather when they are in the vicinity, so just looking at the wind direction and deciding on the Air Mass classification will not be enough to make an accurate forecast every time. Nevertheless, consideration of where the air has come from and what might have happened to it on the way, can provide a good start when considering the prediction for the day!