By Philip Eden
For the first time this summer the forecasts indicate an extended spell beginning Monday or Tuesday (21st or 22nd July) with "maritime tropical" air in charge - maritime because it travels to us via a long route across the Atlantic Ocean, and tropical because it originates in sub-tropical latitudes. (Air from within the tropics rarely if ever affects the British Isles.)
True, there were brief warm spells earlier in the summer, May 7-12 and June 8-10, and also an isolated day on July 1, when the temperature reached or exceeded 80°F (27°C) but on these occasions the high temperatures were associated with air of continental origin, or at least the maritime air was dried out by a lengthy land-track across Spain and France before it reached us.
One of the features of tropical maritime air-masses at all times of the year is that they generally arrive here with so much moisture in their lowest layers that the sky is clogged with clouds. The clouds are usually grey and featureless, or arranged in large soft rolls or patches, but they are not normally rain clouds. These stratus and stratocumulus decks usually occupy a shallow layer of atmosphere insufficient to generate anything heavier than a little drizzle.
During the winter half of the year the cloud cover usually lasts all day, but in summer the strength of the sun is enough to burn off the cloud in inland areas. The warmer air is, the greater capacity it has for holding moisture, and air which is completely saturated at 60°F (16°C) has a relative humidity of 59 per cent at 70°F (21°C), and 32 per cent at 80°F (27°C). So, as the temperature of the air climbs on a summer's morning the cloud droplets gradually evaporate and the clouds seem to vanish into thin air. Once that has happened the temperature races away and the rest of the day is sunny and hot.
On windward coasts, however, there is an unending stream of cool, moist air supplied by the breeze blowing in from the sea, so the weather does not get a chance to warm up. Thus the temperature remains subdued throughout the day, the air remains saturated, and the weather stays grey and damp and surprisingly chilly.
© Philip Eden