What is an Air Mass?

Air masses are parcels of air that bring distinctive weather features to an area. The air making up the mass is very uniform in temperature and humidity.

An air mass is separated from an adjacent body of air by a transition that may be more sharply defined. This transition zone or boundary is called a front. An air mass may cover several millions of square kilometres and extend vertically throughout the troposphere.

Air Mass Sources

The temperature of an air mass will depend largely on its point of origin, and whether it has travelled over the land or sea. This might lead to warming or cooling by the prolonged contact with a warm or cool surface. The processes that warm or cool the air mass take place only slowly. It could take over a week for an air mass to warm up by 10°C right through the troposphere. For this to take place, an air mass must lie virtually in a stagnant state over the influencing region. Hence, those parts of the Earth's surface where air masses can stagnate and gradually attain the properties of the underlying surface are called source regions.

The main source regions are the high pressure belts in the subtropics, which produce tropical air masses, and around the poles, that are the source of polar air masses.

Air Mass Modification

As we have seen, it is in the source regions that the air mass acquires distinctive properties that are the characteristics of the underlying surface. The air mass may be cool or warm, or dry or moist. The stability of the air within the mass is also a result of source regions. Tropical air is unstable because it is heated from below, while polar air is stable because it is cooled from below.

As an air mass moves away from its source region, the air is further modified due to variations in the type or nature of the surface over which it passes. Two processes act independently, or together, to modify an air mass.

An air mass that has a maritime track, i.e. a track predominantly over the sea, will increase its moisture content, particularly in its lower layers. This happens through evaporation of water from the sea surface. An air mass with a long land or continental track will remain dry.

A cold air mass flowing away from its source region over a warmer surface will be warmed from below making the air more unstable in the lowest layers. A warm air mass moving over a cooler surface is cooled from below and becomes stable in the lowest layers.