For a country ranging from 50° to 60° North the dominant feature of Britain's climate is its mildness, and the quality that characterizes its weather is its variability. For the wind brings the weather and Britain is visited, at times, by air-masses varying from polar to tropical, blowing sometimes over thousands of miles of ocean, sometimes, though much more rarely, from the heart of the European landmass. But even the continental air, hot and dry in summer, cold and dry in winter, must cross the narrow seas and its extremes are more or less tempered in its passage. Britain is a battleground invaded from time to time by one of these distinctive air-masses, soon to be reconquered by another. Each brings its own type of weather and the battleground is not infrequently a 'front' with a sequence of wet and sometimes stormy weather changes that follow one of a number of patterns. The four chief masses are Tropical maritime, Tropical continental, Polar maritime and Polar continental.
The essential properties of each of these air-masses can be deduced from their source regions, in anticyclones, either permanent or temporary, and the regions over which they pass on their way. Broadly speaking Tropical maritime air comes from the west, south-west or south over the warm waters of the Atlantic Ocean. It is warm, damp and stable, for the lower layers are being slowly cooled. It brings stratus cloud and, especially in summer, sea fog. Rising up windward slopes it makes hill fog and if forced to rise suddenly over mountains or by strong convection, and particularly over a cold air-mass along a warm front, it may cause heavy rain. Polar maritime air comes mainly from the north-west or north, though occasionally from the south-west (Polar maritime air returning). Initially cold, its lowest layers become warmed over the warm waters of the North Atlantic Drift and it becomes unstable, causing cumulus cloud, sometimes growing to cumulo-nimbus which brings sudden heavy showers, but between the showers are bright intervals of clear blue sky. In the spring the showers are often of sudden hail.
Polar continental air affects Britain only in the winter months, coming from the continental high pressure to the east or north-east. It is a cold, dry, biting wind and brings black frosts, sometimes severe and prolonged. Being very cold it keeps to the ground giving rise to an inversion of temperature, beneath which fog in the country and smog in industrial towns are trapped. But if the cold air is deep the sky is clear, blue and hard.
Tropical continental air comes from the south or south-east in summer only. It is hot, dry and stable, giving a low pearly haze. In summer anticyclones the upper (superior) air is descending and, being further heated by compression, gives some of Britain's rather rare heat-waves.
At no season of the year is all Britain without risk of invasion by one of these air-masses, but clearly the climate depends on their frequency. Thus, in winter, when the Icelandic low is at its deepest and the continental high is at its strongest, the winds blow mostly from the south-west bringing rain, especially to the hilly west, and mild, cloudy, damp weather. Frontal depressions are never far away and, especially in early winter, they may cross the islands in procession bringing an alternation of rain followed by bright fine weather for a day or so, or there may be a strong surge of Arctic air behind the cold front. But towards the end of winter the continental high often sends a wave of cold dry polar continental air. Between the polar air, affecting the eastern counties mostly, and the damp oceanic air to the west, snow may fall, sometimes thick and soft, occasionally as a fierce blizzard building drifts. February and March are the months with most snow, though it can fall as late as June in the Scottish Highlands.
Spring is the driest season; the weather is often anticyclonic, with cold, dry easterly winds or with calm and clear skies and strong radiation at night, giving damaging frosts into May. But it may be otherwise, with depressions crossing the islands, bringing south-west winds, mild temperatures and rain.
By early summer the wind is more westerly and sometimes the Azores High sends fine sunny weather with blue skies in which cumulus clouds build up by day and die away at night. Or they may, especially if encouraged by a shallow depression coming from the
south over France, build up into great thunder-heads. Thunderstorms are most frequent now and the eastern counties especially get their highest rainfall in this manner at this season. But still the rain is heavier in the hills and it is generally here that occasional cloudbursts cause great floods, the run-off concentrated, of course, in narrow valleys where it does the worst damage.
Calm anticyclonic conditions often occur into early autumn, especially after a fine summer, but the air is damp and as the sun sinks lower, the temperature falls and mist and valley fog form in the evening after a fine day. At first they melt in the warmth of the early morning sun but as the days get shorter they persist later and form earlier until they last throughout the day. At other times strong winds, associated with depressions, bring stormy weather with heavy rain and gales out of the south-west, usually backing to west and north-west as the colder air succeeds the warmer air when the cold front passes through.
Thus the fluctuations of the weather, related to the air-masses that bring it, or to the passage of the fronts that separate them, blur the passage of the seasons. They bring back winter cold when spring seems already to be here, or halcyon summery days return in midOctober (St Luke's summer), gilding the autumn leaves already sealed for their winter rest.