Air Masses and its Classification, Fronts and Frontal Characterstics

Air masses

The air mass is defined as an extremely large body of air whose properties of temperature and moisture content ( humidity), at any given altitude are fairly similar in any horizontal direction.

Trewartha defines air masses as an immense body of air that moves over the earth land-sea surface as a recognizable entity with temperature and humidity characteristics which are reversely uniform in as horizontal direction at the different level. Since it may extend through 20 degrees or more of latitude and covers hundreds of the thousands to millions of the square kilometer, the horizontal homogeneity of a mass is never complete. Since an air mass traverses an area in the several days, the region occupied by it will generally have the same weather conditions expect some day to day variation here and there. But the weather usually conditions in an adjacent air mass are altogether different. Along the margins of the air masses, the weather changes are rather very sharp.

When an air remains in the contact with a large and uniform surface for a couple of days, its temperature and moisture attain equilibrium with the surface. If the underlying surface is warm the overlying air will be warmed and the entire mass of air lying above it will be heated gradually by conduction. If the surface is cold the air above it will be naturally cooled and the heat will be removed from the lowest part of the air. Similarly, the moist surface will impart its moisture to the air above it. Whereas the overlying air will lose its moisture to the underlying dry surface. Under any of the circumstances, the air always tends to reach equilibrium with the surface. It takes times from 2 days to the two-weeks but once the equilibrium is reached it will change only with the time. Such an extensive with the surface by remaining into its contact over a large area is designed as an air mass.

The air masses retains their identity even after they have moved far from their source region. But the original physical properties are conserved only in the upper parts of such air masses. The vertical distribution of the temperature is indicative of the stability of an air mass besides its warmness or coldness. The vertical distribution is closely related to the ascent or descent of air currents within the body of the air mass itself.

According to Trewartha, air masses are an important and inseparable part of the planetary wind system. Therefore, every air mass is related with one or the other permanent wind belt.

When the air masses move out of their source regions, they may be taken to be the streams of the tropical or the polar air on a gigantic scale. Actually, it is to the polar regions. Thus, the air masses play an important role in the removal of the latitudinal imbalance in heat.

An air mass with the temperature that is lower than that of the underlying surface is designated as a cold air mass. On the other hand, a mobile air mass which is warmer than that of the underlying surface is known as the warm air mass. The same air mass may be called warm or cold with reference to the temperature of the surface over which it moves. For examples, if a warm air mass moves from a cold land surface to the warm oceanic surface, it will be known as a cold air mass. The most important features of the air masses are that they are independent in the respect of their temperature and humidity characteristics. Air masses with the different densities even when they come in the contact with the each other do not merge together but they retain their identity.

Classification of the Air masses

Generally, the four air mass classifications categorized according to the source region.

1: Polar latitudes P : located poleward of the 60 degrees north and the south.

2: Tropical latitudes T : located within about the 25 degrees of the equator.

3: Continental c : located over the large land masses- dry.

4: Marine m : located over the oceans--- moist.

We can then make the combinations of the above to describe various types of the air masses.

A: cP : continental polar cold, dry, stable

B: cP : continental tropical hot, dry, stable air aloft—unstable surface air.

C: mP : maritime polar cool, moist and unstable.

D: mT : maritime tropical warm, moist usually unstable.

Air masses can control the weather for the relatively long time period from a period of the days of the months. Most weather occurs along the periphery of these air masses at the boundaries called the fronts.

Fronts

Fronts are the transition zone between the air masses with distinctly different properties. The narrow frontal zones are marked by the abrupt changes in the various weather elements such as temperature air pressure humidity etc. in air mass the changes in the air masses in the different climatic elements are only gradual. But the boundary lines of the air masses are distinguished by the rapid changes in the weather elements.

General frontal characteristics

Fronts may differ in their types location or areal extent but the following characteristics are more or less common to all them.

1: Temperature

Larger differences in the air temperature are recorded across a front. But the changes in the temperature may be abrupt or gradual depending on the nature of the opposing air masses. Larger the temperature difference, the thinner the frontal zone and the vice versa. Besides, the fronts are always characterized by the temperature inversion layers.

2: Air pressure

The wedge formed by the bending of the isobars across the front always points toward the higher pressure. It is noteworthy that mostly the fronts lie in the trough of the low pressure.

3: Winds

Southwesterly wind in a tropical air mass gives way to the northwesterly wind in the polar air mass across the front. The lines of the abruptly shifting winds were formerly known as wind-shift lines

4: Cloud and precipitation

The type of the clouds and the precipitation falling from them depend on the slope of the front and the amount of the moisture in ascending mass of the air.

References:

.S.C., Santee. Environmental Science. India, New Center: New Center Book Agency (P) Ltd, 2004.

Lal, D.S. Climatology, Sharda Pustak Bhawan, Allahabad.(2010)