| Hurricane is a tropical cyclone, formed in tropical latitudes — an atmospheric whirlwind with reduced atmospheric pressure in the centre. Unlike extra-tropical cyclones, it is often coupled with stormy wind speeds. About 80 tropical cyclones are encountered annually worldwide.
In the Far East and in South East Asia, tropical cyclones are called typhoons and in North and South America — hurricanes (Latin: huracán, which are named after the American Indian god of wind “Huracan”. It is accepted that the storm transforms into hurricane when the wind speed is more than 120 km/hr and at wind speed of 180 km/hr, hurricane is called powerful hurricane.
Structure, occurrence and development
Tropical cyclones are allocated in separate group as they differ from other extra-tropical cyclones by the occurrence, development and some structural peculiarities.
Usually tropical cyclones are small (in comparison with other cyclones) in size, constituting about 200-300 km in diameter and at the same time, pressure in the center of cyclone drops to 0.95 (and sometimes even upto 0.9) atmospheres and both these factors ensure very big barometric gradients. Winds attain force of storm and hurricane. Coriolis force (deflecting force of Earth’s rotation) is the reason for occurrence of rotation of cyclone and hence winds in tropical cyclones of northern hemisphere move anticlockwise and southern hemisphere — clockwise.
In spiral whirlpools of air, the wind speed reaches upto 240-320 km/hr. Warm air, which lowers towards the surface of earth (or water), is located in the calm center “Eye” of cyclone. Sizes of such eye across can vary from 6.5 to 48 km. Presence of warm air in the center contributes towards drop of atmospheric pressure near the surface. Warm moist air twists in a spiral round the “Eye”. Condensation causes the formation of cumulonimbus clouds, accompanied by emission of heat, which in turn strengthens spiral ascent of air around the center of cyclone.
In lower layers, air mass flows inside the cyclone and in upper layers, this convergence (convergence) of wind field is blocked by stronger divergence. It leads to strong ascending movement of air in entire area of cyclone and to development of powerful cloudy system with heavy showers and thunderstorms.
Only small (with diameter from 8 km) inner portion of cyclone, which is called “Eye of hurricane” or “Eye of typhoon” is free from powerful clouds.
Tropical cyclones arise mainly in intratropical convergence zone above over-heated oceanic areas. Thus, such convergence zone should be located at least 5 ° from equator (in overwhelming majority of cases, not less than 10° from equator) so that the deflecting force of Earth’s rotation (coriolis force) was sufficiently more.
Formed tropical cyclones move together with air masses from East to West, thus gradually deviating towards high latitudes.
Main source of energy of tropical cyclones — heat emission during condensation of water vapor in high altitudes and this can be explained by the fact that they quickly fade after falling on land. Also it is known, that the temperature near water surface should rise upto minimum of 27°С so as to give rise to cyclone.
Part of tropical cyclones fall beyond the limits of tropics, turning thus towards East and their properties, in future, appear to be the properties extra-tropical cyclones.
Hurricane activity in Atlantic is usually observed from the beginning of June till the end of November, time of existence of tropical cyclones can reach up to three weeks. By calculations of meteorologists, 10 tropical storms are formed on an average around Atlantic in a season; 6 of them turn into hurricanes and two — into strong hurricanes.
Areas of occurrence:
Almost all typhoons are formed in area upto 30° from equator and 87 % of all typhoons are formed in area upto 20° from equator. Since rotation of tropical cyclones is initiated and supported at the expense of Coriolis force, then cyclones never ever form and do not move towards 10° zone from equator where Coriolis force is weak. Occurrence of tropical cyclones in this area is possible only if there are other factors causing rotation, however, such conditions are very rare and also probability of occurrence of tropical cyclone in these latitudes is estimated at less than one cyclone in 100 years.
Tropical cyclones arise usually in following areas:
* Northern hemisphere: Pacific ocean to East from Philippines and South China sea, Pacific ocean to West from California and Mexico, Atlantic ocean to East from Greater Antilles, Bay of Bengal and Arabian sea;
* Southern hemisphere: Pacific ocean to the East from New Guinea, Indian ocean to the East from Madagascar and to northwest of Australia;
Cyclones, similar to tropical cyclones, were observed in Mediterranean Sea in September 1947, September 1969, January 1982, September 1983 and in January 1995, however, common opinion amongst the scientists is missing with respect to nature of these cyclones.
Consequences:
Powerful winds (up to 70m/sec with gusts up to 100m/sec) and large quantity of residues (up to 1000mm/day), which are characteristic for tropical cyclones, lead to catastrophic devastations on land and stormy heaving in the sea.
Floods during tropical cyclones are caused not only by residues but also by run-up of sea water onto low-lying coasts. |
Hurricane Gustav largely spared New Orleans, days after an evacuation order by Mayor Ray Nagin. About 2 million people left coastal Louisiana and New Orleans, anticipating that Gustav's visit may be another Katrina-size catastrophe. Coincidentally, Gustav was the name of several Swedish kings. Also, the National Hurricane Center has has four tropical storms that are three are forecast to be become hurricanes.
