Air mass:-

1. An air mass is a large volume of air in the atmosphere that is mostly uniform in temperature and moisture. Air masses can extend thousands of kilometres across the surface of the Earth and can reach from ground level to the Stratosphere -16 kilometres into the atmosphere.
2. Air masses form over large surfaces with uniform temperatures and humidity, called source region
3. They acquire a distinct identity by their humidity, orgin and movement.
4. They are classified based on:-

• Latitudinal origination – Arctic, Tropical, Equatorial, Antarctic,etc.
• Continental air mass/ Oceanic air mass

An air mass may sit over its source region for long periods of time, or it may migrate. An air mass on the move begins to transform as it passes over new landscapes, while at the same time retaining enough of its original conditions to alter local weather.

Conditions for the formation of Air masses:-

• Source region should be extensive with gentle, divergent air circulation(slightly at high pressure).
• Areas with high pressure but little pressure differenceor pressure gradient are ideal source regions.
• There are no major source regions in the mid-latitudes as these regions are dominated by cyclonic and other disturbances

Role in macro climatic changes:-

Most of the migratory atmospheric disturbances such as cyclones and storms originate at the contact zone between different air masses and the weather associated with these disturbances is determined by characteristics of the air masses involved.

Low windspeeds let air remain stationary long enough to take on the features of the source region, such as heat or cold. When winds move air masses, they carry their weather conditions (heat or cold, dry or moist) from the source region to a new region. When the air mass reaches a new region, it might clash with another air mass that has a different temperature and humidity. This can create a severe storm.

The properties of an air mass which influence the accompanying weather are vertical distribution temperature(indicating its stability and coldness or warmness) and the moisture content.

The air masses carry atmospheric moisture from oceans to continents and cause precipitation over landmasses.

• Frontal Precipitation – when warn air mass and cold air mass come in contact frontal precipitation occurs. It is widely witnessed in temperate region.
• The Air masses when pass through warm water or currents acquire their moisture and cause rainfall in coastal regions.
• The climates of most regions worldwide are affected by air masses. For example, maritime-tropical air sourced over warm waters of the Atlantic Ocean, Caribbean Sea and Gulf of Mexico, primarily between 10 and 30 degrees north of latitude, is the main contributor of precipitation for much of North America east of the Rocky Mountains.

They transport latent heat, thus removing the latitudinal heat balance.

Desertification:
They cause arid conditions when dry air mass is present in a region. Sahel region of Africa is impacted by this.

Cyclonic and Anticyclone conditions:

• When these continental air masses move towards pole side and polar air masses move towards equatorial side both of them form fronts. These cyclonic fronts are responsible for cyclonic storms at temperate regions
• Stormy cyclones form near the air-mass fronts.
• The temperate cyclones occur in the mid latitude of both the hemisphere. These cyclones are born along the polar front, particularly in the region of Icelandic and Aleutian sub –polar low pressure areas in the northern hemisphere.

A continental polar air mass originating from the tundra of northern Canada may push southward during the winter.

• It brings frigid temperatures to the central United States, even as it warms up somewhat on its journey across lower latitudes.
• While dry in its source region, such an air mass often picks up substantial moisture during an early-winter transit of the Great Lakes, allowing it to dump so-called lake effect snow on leeward coasts

Also helps in creation of an occluded front