Short Notes On Cyclones

Cyclones

cyclone, atmospheric pressure distribution in which there is a low central pressure relative to the surrounding pressure. The resulting pressure gradient, combined with the Coriolis effect, causes air to circulate about the core of lowest pressure in a counter clockwise direction in the Northern Hemisphere and in a clockwise direction in the Southern Hemisphere. Near the surface of the earth, the frictional drag on the air moving over land or water causes it to spiral gradually inward toward lower pressures. This inward movement of air is compensated for by rising currents near the center, which are cooled by expansion when they reach the lower pressures of higher altitudes. The cooling, in turn, greatly increases the relative humidity of the air, so that "lows" are generally characterized by cloudiness and high humidity; they are thus often referred to simply as storms.

Temperate Cyclones

Temperate cyclones are atmospheric disturbances having low pressure in the centre and increasing pressure outward. The origin and influence of these cyclones is in the temperate zone due to which they are known as temperate cyclones. They are more active in winter in the Northern hemisphere but due to prominence of water areas in the Southern hemisphere, they are active throughout the year, since, they travel from West to East they are also termed as Western disturbances.

Life Cycle of a Temperature Cyclone

There are six stages of life cycle of a temperate cyclone.

First Stage involves the convergence of two ari masses of contrasting physical properties and directions. Initially, the air masses (warm and cold) move parallel to each other and a stationary front is formed. This is called initial stage.

Second Stage is also called as incipient stage, during which the warm and cold air masses penetrate into the territories of each other and thus a wave-like front is formed.

Third Stage is the mature stage when the cyclone is fully developed  and isobars become almost circular.

Fourth Stage warm sector is narrowed in extent due to the advancement of cold front than warm, as cold front comes nearer to warm front.

Fifth Stage starts with the occlusion of cyclone when the advancing cold front finally overtakes the warm front and an occluded front is formed.

Sixth Stage warm sector completely disappears, occluded front is eliminated and ultimately cyclone dies out.

Tropical Cyclones

Cyclones developed in the regions lying between the tropics of Capricorn and Cancer are called tropical cyclones. All types of cyclones originate because of frontogenesis. Inspite of the absence of two contrasting air masses in the equatorial region fronts are formed due to meeting of land and sea winds. This frontal concept of the origin of tropical cyclone is no longer acceptable because tropical cyclones in no case are related to fronts.

They are characterized by the following salient features-:

-Tropical cyclones move at the speed of about 32 km per hour while hurricanes attain the velocity of 180 km per hour or more.

-On an average, their diameters range between 80 km and 300 km but some time they become so small their diameter I restricted to 50 km or even less.

-Like temperate cyclones, tropical are not characterized temperature variation in their different parts because they do not have different front.

-Generally, direction is Westerly up to 15° latitude from the equator, pole–ward between 15° to 60° latitudes, and again Westerly. These cyclones are weaker when they enter sub-tropical regions.

-Tropical cyclones become disastrous natural hazards because of their high wind speed of 180 km to 400 km per hour, high tidal surges-high rainfall intensity, very low atmospheric pressure.