Air Masses

● When the air remains over a homogenous area for a sufficiently longer time, it acquires the characteristics of the area.

● The air with distinctive characteristics in terms of temperature and humidity is called an air mass. It is defined as a large body of air having little horizontal variation in temperature and moisture.

● The homogenous regions can be the vast ocean surface or vast plains. The homogeneous 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 air masses are recognized:

(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.

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

(d) Occluded

● 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.

● When the front remains stationary, it is called a stationary 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 characterized by steep gradients in temperature and pressure. They bring abrupt changes in temperature and cause the air to rise to form clouds and cause precipitation.

Jet Streams

● Jet streams have been termed as the high-altitude river of air.

● These are high velocity winds blowing at high altitudes.

● These are tubular shaped meandering flow of the wind moving from west to east located at altitudes of 5 to 12 KM.

● They are more pronounced in the Southern Hemisphere.

● The jet stream winds have velocities of 300 to 500 km per hour.

● The greatest velocities generally occur over the eastern parts of the continents and the western sides of the ocean basins. The lowest velocities occur in the longitudinal or west-east sectors near the surface subtropical high-pressure cells.

● Velocities change seasonally. The winter velocities are double those of the summer.

● The highest speeds in the winter occur along the Asian Coast, over South-eastern U.S.A., and in the region between North Africa and the Indian Ocean.