Competitive Exams: Fronts

Usually, air mass from one region gradually moves to the other region occupied by some other air mass. When a warmer and a lighter air mass moves against a cold and more dense air mass, the former rides over the latter. Such a front is called a warm front. On the contrary, if the cold air mass forces its way under a mass of warmer air and pushes the latter upwards, the front will be called a cold front.

  1. General Frontal Characteristics

    1. Temperature: Great difference in temperature are recorded across a front. But the change in the temperature may be abrupt or gradual depending upon the nature of the opposing air masses. The width of frontal transition zone is dependent on the temperature contrast. Besides the fronts are always characterized by the temperature inversion layers because of ascent of warm air over a wedge of cold and dense air mass.

    2. Air Pressure: There is an abrupt change in the pressure as well as the pressure gradient across a front.

    3. Winds: Abrupt wind shift at the fronts.

    4. Clouds and Precipitation: Frontogenesis:

    The term was for the first was used by Tor Bergeron. It is a process of regeneration of the old and the decaying fronts or it means the creation of altogether new fronts.

Frontolysis

The process means the dying of a front. Fronts do not come into existence out of a sudden, rather they appear only after the process of frontogenesis have been in operation for quite some time. In the same way the act of vanishing of the existing fronts is not accomplished suddenly. The process of frontolysis must continue for some time in order to destroy an existing front. Convergence of the wind toward a point or contraction toward a line augments the process of frontogenesis. On the contrary, divergence of the wind from a point, or is helpful to the process of frontogenesis.

Frontogenesis there for is likely to occur when fronts move into regions of divergent air flow. That is why in crossing the subtropical high pressure regions, the front generally disappears. Cyclones facilitate development of fronts whereas anticyclones do not allow the formation of the fronts.

Classification of fronts

  1. Cold front

  2. Warm front

  3. 0ccluded front

  4. Stationary front

Cold Front

A cold front is a front along Which cold air is invading the warm air zone. Since the colder air masses denser, it remains at the ground and forcibly uplifts the warmer and lighter air mass. In fact, when pressure distribution is such as to force the cold air to advance and the warm air to retreat, the zone of transition is called a cold front. The steepness of the front is closely related with its velocity. Higher velocity results in the steeper slope, while the lower velocity makes the slope of the front rather gentle. The slope of the cold front varies from 1: 50 to 1: 100. Depending upon the instability of the overrunning warm air, convective clouds or even thunderstorms may occur along the leading edge of the cold front. The type of front slopes backward instead of forward, so there is no warning far in advance often approaching cold front and no preceding cloudiness until the front is near. The cold front in general is associated with narrow band of cloudiness and precipitation. The cold front passes more rapidly. The sky becomes clear soon after the passage of the front. However, the weather produced along the cold front is valid. At the actual front, the clouds are of Nimbostratus and Cumulonimbus type which produce heavy rainfall. In certain cases precipitation falls ahead of the front, while on occasions it is behind the same. If cold front moves rapidly, the secondary cold fronts may develop at some distance behind. With the passage of cold front, the sky becomes rapidly clear and the weather improves. There is a sudden drop in the temperature. The wind shift from south to west or northwest generally accompanying the frontal passage. There is marked decrease in the specific and relative humidity. The weather after a cold front has passed, is dominated by subsiding and relatively cold air mass. In winter, the passage of a cold front is followed by a cold wave which further reduces the surface temperature.

Warm Front

The slope of warm front is 1: 100 to 1: 200. Cirrostratus clouds halos around the sun and the moon. Mackerel sky is produced by Cirrocumulus clouds. As the front approaches the viewer, the clouds become lower and thicker. The thick cloud sheet overlying the surface position of the front gives steady precipitation extending over a long distance ahead of the front. Warm fronts usually yield moderate to gentle precipitation over a relatively larger area for several hours. This is in conformity with the gentle slope of the front. Convective activity is generally absent along a warm front. The passage of a warm front is, marked by a rise in the temperature and pressure. The specific humidity arises rapidly.

Occluded Front

An occluded front is a front formed when a cold front overtakes a warm front, the cofd front moves rapidly than the warm front. Ultimately, the cold front overtakes the warm front and completely displaces the warm air at the ground. A long backward swinging occluded front comes into the existence.

There are two types of occlusion:

  1. Cold front occlusion which is most common

  2. Warm front occlusion. The weather that is produced along an occluded front is usually a combination of the cold front and the warm front weather conditions. Stationary Front: It is a front in which the surface position of the fronts do not move.

Zones of Frontogenesis

  1. The fronts do not form everywhere. There development is confined to certain defined zones. Fronts usually develop in those areas of the world where air masses have strong temperature contrasts.

    1. Atlantic Polar Front: This is the most important zone, which is developed maximum in winter. It is an area between Great lakes, Iceland, Portugal and West Indies.

    2. Atlantic Arctic Front:

    3. Mediterranean Front: This front lies over Mediterra. Nean and Caspian Sea region which develops in winter.

    4. Pacific Arctic Front: This front lies between Rocky mountains and Great Lake regions. In winter, it shifts equatorward.

Polar Pacific Front

  1. In winter, two pacific polar fronts develops:

    1. near the coast of North America

    2. near the Asian coast.

  2. The winter rainfall along the Pacific coast of North America is produced by the storms developed on these fronts. The Polar fronts over western Atlantic and Pacific deyelop 10 degrees further north in summer than in winter. The summer Polar Fronts develop over Eurasia in Middle North America. In the southern hemisphere the average position of the polar front is about 45 degrees south in January. In July, there are two polar fronts

    1. originating over South America

    2. the other at 170 degrees west.