Air Masses- concept, classification and properties

Air Mass

An air mass is a vast body of air (a macro-scale phenomenon that can have global implications) in which the condition of temperature and moisture are much the same at all points in a horizontal direction. In other words, an air massis any large body of the lower atmosphere that has fairly uniform conditions of temperature and moisture. Air masses form an integral part of the global planetary wind system. An air mass takes on the temperature and moisture characteristics of the surface over which it forms. The processes of radiation, convection, condensation, and evaporation condition the air in an air mass as it travels. The region where the air mass develops is known as a source region of an air mass. The source region of an air mass is any large body of land or water from which the air derives the characteristics. Two conditions govern the occurrence of the principal source regions:

  1. They occur in the regions of relative calm in the general circulation, where air motion is sufficiently slow for homogenous air mass characteristics to develop.
  2. Source regions are usually the areas where the earth’s surface itself is fairly uniform, such as oceans, deserts or large ice and snow-covered areas.

Classification of Air Masses

Typically, air masses are classified according to the characteristics of their source region or area of formation. Air masses are also classified as being either continental or maritime in terms of moisture characteristics. They are also sub-categorised on the basis of thermodynamic and mechanical changes. The classification of air masses is shown in figure 2.3.

Where; C= Continental, P = Polar, K = heated from below, W= cooled from above,S = stable in upper side, U = unstable from upper side.

Fig. 2.3: Classification of Air masses

Continental polar air masses: These are characterised by dry, cold and stable conditions with very cold and dense air during winter at the source region. The weather during winter is frigid, clear and stable with very little cloudiness and precipitation. However, the weather is less stable during summer. The source regions for these air masses are the Arctic basin, northern parts of North America, Eurasia and Antarctica.

Maritime polar air masses: These are actually those continental polar air masses which have moved over the warmer oceans, got heated up and have collected moisture. The conditions over the source region are cool, moist and unstable. The weather during winter is characterised by mild temperatures, high humidity, overcast skies and occasional fog and precipitation. However, the weather is fair, clear and stable during summer. The source regions are the oceans between 400 and 600 latitudes.

Continental tropical air masses: These air masses are hot, dry, stable and localised and thus do not expand beyond the source. They remain dry throughout the year and very hot during summer with scanty rainfall. The source regions for these air masses are the tropical and sub-tropical deserts of Sahara, west Asia and Australia.

Maritime tropical air masses: These air masses are warm, humid and unstable at the source regions. The weather conditions during summer are characterised by high temperature, high humidity and conventional rainfall. However, the weather during winter has mild temperatures, overcast skies with fog and drizzle. The source regions for these air masses include the tropical and sub-tropical oceans.

Fronts

The air masses with different characteristics do not mix readily and try to retain their identity as far as temperature and moisture are concerned. The zone of convergence between two air masses is called a front. When two different air masses collide, a frontis formed. A front is not a sharp wall but rather a zone of transition that is often several kilometres wide. The ideal conditions for a front to form are temperature differences and converging air which should be strong enough to move one air mass towards another. The process of the formation of a front is called frontogenesis. Four types of fronts—warm, cold, occluded, and stationary—can be formed by air of different temperatures.

Warm Front: A warm front marks the advance of a warm air mass as it rises up over a cold one. Warm fronts occur in places where warm air advances into relatively cool air. This interaction causes the warmer air to slowly slide over the top of the underlying cooler air (Fig. 2.4). This process of gradual overriding of cooler air actually occurs in the upper atmosphere ahead of the surface warm front and causes the lifting air to cool adiabatically. A warm front is the transition zone in the atmosphere where an advancing warm subtropical, moist air mass replaces a retreating cold, dry polar air mass. As the air cools, clouds form, beginning with high level cirrus clouds at the top of the upwardly moving air mass. As the warm front approaches, these clouds change to progressively lower stratus clouds, culminating in rain-producing nimbostratus clouds at the surface front. When these conditions evolve, the sky may be overcast with a slow but steady rate of precipitation that may last a day or two. This precipitation can be in the form of drizzle, light rain, and even snow. The weather conditions of fronts are shown in table 2.1.

Fig. 2.4: Uplift of air at a warm front.

Cold Fronts: A cold frontmarks the line of advance of a cold air mass from below, as it displaces a warm air mass. Cold frontsoccur when cool air moves into a region that was previously dominated by warmer air. The cold air is denser and heavier than the warm air ahead of it, so the warm air is forced to rise. In this fashion, a cold front is significantly different from a warm front because the cold air hugs the surface along a cold front and vigorously drives the warm air ahead of it aloft. Notice in Figure 2.5 that the edge of a cold front is very steep when compared to the warm front in Figure 2.4. Once the warm air begins to lift along the cold front, adiabatic cooling starts and vapour condenses, forming clouds. Because the air rises rapidly, it quickly cools adiabatically, which means that large amounts of latent heat energy are rapidly released as the water condenses. As a result, rainfall is intense and of short duration. If sufficient moisture is present and enough latent heat is quickly released, an intense thunderstorm can form, with the latent heat being the fuel for its development.

Fig. 2.5: Uplift of air along a cold front

Table 2.1:Weather conditions associated with warm and cold fronts

Weather condition / Warm Front / Cold Front
Before the Passing of Front / At the time of Front / After the Passing of Front / Before the Passing of Front / At the time of Front / After the Passing of Front
Temperature / Cool / Warming suddenly / Warmer then levelling off / Warm / Cooling suddenly / Cold and getting colder
Atmospheric Pressure / Decreasing steadily / Levelling off / Slight rise followed by a decrease / Decreasing steadily / Levelling off then increasing / Increasing steadily
Precipitation / Showers, snow, sleet or drizzle / Light drizzle / None / Showers / Heavy rain or snow, hail sometimes / Showers then clearing
Clouds / Cirrus, cirrostratus, altostratus, nimbostratus, and then stratus / Stratus, sometimes cumulonimbus / Clearing with scattered stratus, sometimes scattered cumulonimbus / Cirrus and cirrostratus / Cumulus and cumulonimbus / Cumulus

Occluded fronts: Occluded frontsareformed when cold and warm fronts merge. Occluded fronts are produced when a fast moving cold front catches and overtakes a slower moving warm front. Two types of occluded fronts are generally recognized. A cold type occluded front occurs when the air behind the front is colder than the air ahead of the front. When the air behind the front is warmer than the air ahead of the front a warm type occluded front is produced. Warm type occlusions are common on the west coast of continents and generally form when maritime polar air collides with continental polar or arctic air.

Stationary front: When the surface position of a front does not change, a stationary front is formed. As the name implies, the air masses around this front are not in motion. A stationary front can cause bad weather conditions that persist for several days.

Lecture prepared by: Mr Manzoor Ahmad Bhat