What is the impact of tropical air masses on the UK?

An air mass is a large body of air that has similar temperature and humidity levels. The UK is affected by two 'tropical' air-masses: tropical-continental and tropical-maritime. The tropical-continental air mass begins in northern Africa and southern Europe; it brings warm, dry conditions to the UK during summer. The tropical-maritime air mass begins in the south Atlantic and the Azores; as it has to travel across the Atlantic ocean it picks up a large amount of moisture, and thus brings warm, moist, unsettled and often cloudy conditions to the UK in both summer and winter.