When an air mass stays over one surface for a long period, it begins to take on the characteristics of that surface. We categorise these air masses primarily by two main features – whether it is in the polar regions or tropical regions and whether it is over land or over sea.
We therefore categorise air masses into 4 types:
The Polar Maritime air mass is cold and moist since it comes from the north-west over the ocean. As it moves further south, the lower layers are warmed by the increasing ground temperature, but air is a poor conductor of heat so the higher layers remain cold. This creates an unstable atmosphere (the temperature drops rapidly as altitude increases), which combined with high moisture content, leads to cumuliform clouds, rain, and high chances of thunderstorms.
The Polar Continental air mass is a cold and dry air mass that arrives from over the Scandinavian or Russian land to the north-east of the UK. The lower layers of the polar continental air mass will heat up as it moves south over the warmer ground, again creating an unstable atmosphere. Since the air mass has lower humidity it can bring clear skies, but when the air mass takes a route over more of the water between the UK & Scandinavia it can bring frost and showers (mostly on the east coast).
The Tropical Maritime air mass arrives from the south-west so is warm and moist. As it moves north over colder ground, the lower layers cool but since air is a poor conductor of heat the upper layers of the air mass remain warm. This creates a stable atmosphere (little decrease in temperate as altitude increases). Tropical maritime air often brings pleasant weather but the moist, humid air sometimes creates stratus or nimbostratus (with drizzle), and sea fog.
The Tropical Continental air mass arrives from the south-east over continental Europe. This is a warm and dry air mass with the lower layers cooling as it travels over colder ground to the north, but since air is a poor conductor of heat the upper layers of the air mass remain warm. This again creates a stable atmosphere (little decrease in temperate as altitude increases). The tropical continental air mass often picks up dust, smoke, and other particles that get trapped in the lower layers as it travels over the land, creating haze.
As a result of differences in air pressure, temperature, humidity (and much more!), the air masses around the world are constantly moving. As air masses move, they frequently encounter another air mass with different characteristics. Since the two air masses have different temperatures, and therefore different densities, they do not mix well (like oil and water). The colder, more dense air mass stays close to the ground and forces the warmer, less dense air mass to rise over it.
The region where two air masses meet is known as a front.