What are Air Masses?

What are air masses and how do they affect the weather in the UK?

Air masses are parcels of air that bring distinctive weather features to the UK. An air mass is a body or 'mass' of air in which changes in temperature and humidity within them are relatively slight. This is to say that the air making up the mass is very uniform. An air mass is separated from an adjacent body of air by a weather front. An air mass may cover several millions of square kilometres and extend vertically through the troposphere.