The air with distinctive characteristics in terms of temperature and humidity is called an airmass. It is defined as a large body of air having little horizontal variation in temperature and moisture. The homogenous surfaces, over which air masses form, are called the source regions. The homogenous surfaces, over which air masses form, are called the source regions.

The air masses are classified according to the source regions. There are five major source regions. These are:

(i) Warm tropical and subtropical oceans

(ii) The subtropical hot deserts

(iii) The relatively cold high latitude oceans

(iv) The very cold snow covered continents in high latitudes

(v) Permanently ice covered continents in the Arctic and Antarctica.

Accordingly, following types of airmasses are recognised:

(i) Maritime tropical (mT)

(ii) Continental tropical (cT)

(iii) Maritime polar (mP)

(iv) Continental polar (cP)

(v) Continental arctic (cA)

Tropical air masses are warm and polar air masses are cold.

What are air-fronts? What are the different types of fronts?

When two different air masses meet, the boundary zone between them is called a front. The process of formation of the fronts is known as frontogenesis.

There are four types of fronts:

(a) Cold

(b) Warm

(c) Stationary

(d) Occluded

When the front remains stationary, it is called a stationary front.

When the cold air moves towards the warm air mass, its contact zone is called the cold front, whereas if the warm air mass moves towards the cold air mass, the contact zone is a warm front.

If an air mass is fully lifted above the land surface, it is called the occluded front.

The fronts occur in middle latitudes and are characterised by steep gradient in temperature and pressure. They bring abrupt changes in temperature and cause the air to rise to form clouds and cause precipitation.

What are the middle latitudes?

The middle latitudes (mid-latitudes, sometimes midlatitudes) are between 23°26’22” North and 66°33’39” North, and between 23°26’22” South and 66°33’39” South latitude, or, the Earth’s temperate zones between the tropics and the Arctic and Antarctic polar regions.