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Philippines: Typhoon Nesat

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The mainland Philippines was battered by Typhoon Nesat that nearly devastated the most populous island of Luzon, forcing evacuation of more than 100000 people and leaving almost 300 dead. Nesat made landfall in Aurora province of Philippines hitting there with a   maximum winds of 140 kilometers per hour and gusts of up to 170 kph. It is here mentioned that Philippines gets hit by an average of 20 cyclones each year that form over the Pacific Ocean to the east. In September 2009, Tropical Storm Ketsana submerged most of the nation’s capital.

Understanding Typhoon: A typhoon is a violent tropical cyclone, in meteorological term, which is a low pressure system occurring in tropical oceans. The winds above the ground circulate around the centre counter-clockwise for a typhoon occurring in the northern hemisphere and clockwise for that occurring in the southern hemisphere.

Atmospheric Pre-conditions for typhoons: Several atmospheric ingredients must come together for a typhoon to form. Since a typhoon is just another term for hurricane, the same conditions apply for both. There are perhaps seven atmospheric conditions which, if met, could cause a typhoon to form. A pre-existing disturbance, warm ocean water, low atmospheric stability, sufficient Coriolis force, moist mid-atmosphere, and upper atmosphere divergence are all important factors for typhoon formation. These factors are important in that a tremendous amount of heat energy is transported from the tropics northward to the higher latitudes. The typhoon is a large heat engine, where great amounts of heat are being produced from the process of latent heat of condensation. This occurs as water vapour is being evaporated from the ocean surface and condensed into cloud droplets.

Mechanisms to cause typhoon Formation: If all of the pre-conditions are met, typhoon formation then becomes possible. There are several types of atmospheric disturbances that can cause a typhoon to develop. The most common mechanism to cause a typhoon to develop is the monsoon trough. This is an extension of the Inter-Tropical Convergence Zone where cyclonic spin has developed. The ITCZ is a trough of low pressure created by the convergence of the northeast and southeast trade winds. This “trade wind” trough does not contain the spin to initiate typhoon development. Typhoon is caused mostly by the monsoon trough in six of the seven hurricanes/typhoon formation basins of the world.

Another cause of typhoon formation is from the Tropical Upper Tropospheric Trough (TUTT). The upper air low is cold in nature as compared to its surrounding environment. A typical low pressure centre that forms a typhoon starts at the lower levels and is warm compared to its environment. If a TUTT low remains over the warm ocean waters for several days, sometimes it gradually warms and takes on tropical characteristics. At this point the low “reaches” down to the surface from the upper atmosphere where the development process transforms.

Another cause of typhoon development is from a front which stalls over tropical waters. Inherently, a front has a cyclonic spin associated with it. If winds aloft become favorable with little wind shear, showers and storms could become more numerous and cause a typhoon to form.