6. Fronts

When two different air masses with distinct properties (temperature, moisture, density, pressure etc.) meet, the boundary zone between them is called a front. These air masses are brought together by converging movements in the general atmospheric circulation. The process of formation of the fronts is known as frontogenesis while Frontolysis is the end stage of a front (table 3). The fronts do not mix readily. In fact, they come in contact with one another along sloping boundaries. These sloping boundaries are actually a transition zone across which a sharp contrast in weather condition occurs. The air masses are of vast size covering tens of thousands of square kilometers. Therefore, frontal zones of discontinuity about 15 to 200 kms wide are relatively narrow. So on the weather map they are represented by only a thick line. A front can be recognized with following observations:

♤ Sharp temperature changes over a relatively short distance. Sometimes change of 100 to 200 C may be observed.

♤ Change in moisture content

♤ Rapid shifts in wind direction

♤ Pressure changes

♤ Clouds and precipitation patterns

Frontogenesis	Frontolysis
creation of altogether new fronts	destruction or dying of a front
Only after the process of frontogenesis have been in operation for quite some time, front do come into existence	Process of frontolysis must continue for some time in order to destroy an existing front.
is likely to occur when the wind blow in such a way that the isotherms become packed along the leading edge of the intruding air mass	likely to occur when fronts move into regions of divergent air flow on crossing the sub-tropical high pressure regions, the fronts generally disappear
Convergence of the wind toward a point or contraction towards a line augments the process of Frontogenesis.	divergence of the wind from a point is helpful to the process of frontolysis
Cyclonic wind shear witnesses the creation of fronts. Contrarily, the areas of anti-cyclonic wind shear do not allow the formation of fronts. Even the pre- existing fronts degenerate in such areas.

Table 3 – difference between frontogenesis and frontolysis Figure 26 – Fronts: (a) Warm front; (b) Cold front 6.3. Stationary Front 6.4. Occluded Front Figure 27- cold front occlusion and warm front occlusion Figure 28 – symbols used for Fronts 7.1. Extra-Tropical Cyclones Figure 29 – life cycle of a extra-tropical cyclone Figure 30 – world: pathways of cyclones (Numbers indicate average frequency of cyclones)7.2. Tropical Cyclones Figure 31 – tropical cyclone Figure 32 – different names of tropical cyclone Impact on humans Table 7 – comparison between tropical and extra-tropical cyclone Figure 33 – three stages in the development of a thunderstorm: (a) cumulus stage; (b) Mature stage; (c) Dissipating stage 7.3.2. Why Thunders are Cause of Concern