Wind Directions and Airmasses
Aristotle was among the first to recognise the eight principal wind
directions, and the Babylonians were aware of the different weather types
associated with them. Modern day meteorologists speak about 'air masses' and
land and sea tracks (which modify them), but essentially the day to day
weather, wherever you are on the planet, is affected by the direction of the
wind.
Deep in the continental interiors, the effect of a change in wind direction may be less immediate than in the maritime British Isles or on the western seaboards of the great continents, but the effects can still be measured.
Often, the immediate effects of a wind change are due to local factors like topography or 'fetch,' but larger-scale differences are the result of airmass change and 'track'.
Remember, wind direction is the direction the wind comes from, not the direction it's moving towards.
In the British Isles, we are subject to four (or five) distinct air masses. The most common wind direction, planet-wide, is westerly, and in our part of the world that corresponds to either a Tropical Maritime or a Polar Maritime airmass. Either way, the airmass is modified, and temperatures tempered, by its long sea track. Summer temperatures are held back (especially in western coastal areas) by the relatively cool sea, while winter temperatures are mollified by the heat-retaining qualities of the same stretch of water.
Westerlies, therefore, are rarely bringers of severe or extreme weather (except for gales, which are most likely to be westerly, and some heavy and prolonged rain events). Tropical Maritime air originates in the mid Atlantic, flowing around the almost permanent high pressure area known as the Azores Anticyclone.
A northerly wind is most likely to be Polar Maritime too, but in winter it sometimes brings even colder,
Arctic air, when it becomes our fifth airmass, Arctic Maritime. Of course, it's always a cold or cool direction, even in summer, originating in the fairly stagnant and cold arctic regions. Northerlies often bring showers, wintry in winter, thundery and heavy at times in spring and summer. Sheltered southern parts may well stay mainly fine in winter northerlies, with showers confined to the sea and windward coasts.
In winter also, Polar Continental air may feed across from mainland Europe on an easterly, southeasterly or northeasterly flow. At this time of year, the continent may be as calm, stagnant and almost as cold as the Arctic, and air flowing from there is responsible for some of our lowest recorded temperatures. Though often dry, moisture picked up from the North Sea, or moister, milder air from the west being undercut by the continental air, can produce significant snowfall. The memorable snow events of 1947 and 1962 were essentially the result of bitterly cold Polar Continental episodes.
In summer, our highest temperatures are also the consequence of a continental flow, from the east, southeast or south. Tropical Continental air can bring temperatures as high as those of southern and central Europe. Again, it's often dry, but some of our most violent thunderstorms and heavy rain can happen during Tropical Continental spells, though they're most common in southern and southeastern areas. Very often, these 'thundery breakdowns' mark the end of a Tropical Continental spell, although the hot air may soon return and repel the westerly 'invasion.'
To summarise:
· Tropical Maritime is a mild or warm, often humid airmass, reaching us from the west or southwest. It's our most common airmass, although often alternating with Polar Maritime during less settled westerly spells. It produces variable cloud, often thicker and more widespread on windward coasts, where there may be drizzle, but is associated with mainly dry weather elsewhere.
· Polar Maritime is common, too. Often bringing showers, it's cool in summer and quite cold in winter. It has a sub-type, known as Returning Polar Maritime, where the air has travelled some way south over the Atlantic before returning northeastwards over us. Then, it will be less cold but often more moist and so more likely to be showery, even thundery.
· Arctic Maritime really only occurs in winter and early spring. It's colder than Polar Maritime, and almost always brings snow showers to northern parts, sometimes more widespread, though southern areas may escape with clear but bitterly cold conditions.
· Tropical Continental air only reaches us during summer and autumn. The first incursion of this air often feels like the first day of summer, when temperatures can rise very quickly from those of a day or so earlier. The last Tropical Continental air of the autumn will often bring 'Indian Summer' conditions and a welcome chance to prolong our outdoor activities while it lasts.
· Polar Continental air is a winter and spring phenomenon. Our lowest temperatures are associated with it and it can be persistent. Cold, snowy Easters are often the result of this airmass' reluctance to give way. Eventually, by late spring, the air over Europe has warmed up though, and the supply of cold air is cut off.
Weather forecasts make a lot more sense when you understand more about the science behind them!
You can measure wind direction and strength, air temperature and humidity, and by observing current weather and cloud you can usually assess what airmass or type you're in, armed with only that basic information. Portable instruments and weather stations to measure and record these parameters and more are available from Novanna Measurement Systems at www.novanna.co.uk/weather-stations
