'Maritime Tropical' air

East-west divide
by Philip Eden

For the first time this summer the UK has experienced a spell of what meteorologists called maritime tropical air - 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 hot spells earlier in the summer, on May 15-17, June 1-2, June 16-17, and July 14-15 when the temperature has reached the high-20s C but on these occasions the heat was 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 the UK.

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 15C has a relative humidity of 62 per cent at 20C, and 37 per cent at 25C. 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. The last few days have provided a perfect illustration of this. On Friday the 26th of July 2002, for instance, London enjoyed 9 hours' sunshine and a high of 28C but Newquay on the Cornish coast had negligible sunshine and a high of 18C.

Related features:
Air-masses and their sources