Pressure and Winds: Tropical and Temperate Cyclones, Air Mass, Cyclones

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Tropical and Temperate Cyclones

  • Air Mass: An air mass is an extensive portion of the atmosphere having uniform characteristics of temperature, pressure and moisture which are relatively homogeneous horizontally. An air mass develops when the air over a vast and relatively uniform land or ocean surface remains stationary for long time to acquire the temperature or moisture from the surface. The major source regions of the air masses are the high latitude polar or low latitude tropical regions having such homogeneous conditions. Air masses are of two kinds polar and tropical air masses. When cold air mass and warm air mass blow against each other, the boundary line of convergence separating the two air masses is termed as front. When the warm air mass, moves upward over the cold air mass the front formed in such a situation is termed warm front. On the contrary, when the cold air mass advances faster and undercuts the warm air mass and forces the warm air upwards, the front so formed is termed cold front. The frontal surface of cold front is steeper than that of a warm front. A prevailing air mass in any region polar, tropical, maritime or continental largely controls the regions general weather.

  • Cyclones: Cyclones are elliptical arrangement of isobars having low pressure at the centre with a convergence of winds within them. The wind direction in the cyclones is anti-clockwise in the northern hemisphere and clockwise in the southern hemisphere. Cyclones are of two types:

Cyclonic Circulation in Northern and Southern llemisphere

Cyclonic Circulation in Northern and Southern Llemisphere

Cyclonic Circulation in Northern and Southern llemisphere

  • Temperate Cyclones: Temperate cyclones are formed along a front in mid-latitudes between 35° and 65° N and S. They blow from west to east and are more pronounced in winter season. Atlantic Ocean and North West Europe are major regions of temperate cyclones. They are generally extensive having a thickness of 9-11 km and with 1040 and 1920 km short and long diameters respectively. The weather associated with the cyclone is drizzling rain and of cloudy nature for number of days.

  • Tropical Cyclones: Tropical cyclones are formed along the zone of confluence of northeast and southeast trade winds. This zone is known as the Inter Tropical Convergence Zone. Cyclones generally occur in Mexico, south-western and north Pacific Ocean, north Indian Ocean, and south Pacific Ocean. These cyclones differ from temperate cyclones in several ways. There are no clear warm and cold fronts as temperature seldom differs in Inter Tropical Convergence Zone. They do not have well-defined pattern of winds and are energised by convectional currents within them. Generally, these are shallow depressions and the velocity of winds is weak. The arrangement of isobars is almost circular. These are not extensive and have the diameters of 160-640 km. They are called hurricanes in the Caribbean Sea, typhoons in the China, Japan and Philippines, cyclones in the Indian Ocean and willy-willies in Northern Australia.

    • Tropical cyclones often cause destruction on the coasts. The steeper pressure gradient causing strong high velocity winds and torrential rainfall bursting upon a restricted area combine to create destructive storms. However about 8-48 km area around their centre called the eye of these stormy cyclones remains calm and rainless.

Track of Tropical Cyclone

Track of Tropical Cyclone

Track of Tropical Cyclone

In brief, it can be said that

  • Atmospheric pressure is the weight of the column of air at a given place and time. It is measured by an instrument called barometer. Unit of measurement of pressure is millibar. The distribution of atmospheric pressure varies both vertically and horizontally. It is shown on the maps through isobars which are the imaginary lines joining the places having equal air pressure. In high latitudes, atmospheric pressure is more than the pressure at low latitudes. The zonal character of horizontal pressure is commonly known as pressure belts.

  • There are four pressure belts spread over the earth. They are equatorial low-pressure belt, sub-tropical high-pressure belts, sub-polar low-pressure belts, and the polar highs. Pressure belts are not fixed; they shift northwards in summer and southwards in winter with the apparent movement of the sun. Pressure gradient is the difference in horizontal pressure between regions of high pressure and region of low pressure. The difference in air pressure causes movement of air called wind. There are wind systems that blow regularly on a daily pattern. Examples include the land and sea breezes, the mountain and valley breezes, and winds warmed as a result of compression. There is a close relationship between pressure gradient and wind speed. Due to Coriolis force, winds deflect from their original course. In northern hemisphere they deflect towards their right and in southern hemisphere towards their left. This is known as the Ferrell’s law. Winds are grouped under planetary, periodic and local winds. Planetary winds blow in the same direction throughout the year, while the other types of winds get modified due to certain reasons. Monsoon is seasonal winds while local winds below generally on diurnal basis.

  • Air masses are horizontal large bodies of air which have uniform temperatures and moisture contents. The boundary line between two different air masses is called a front. Air masses and front cause temperate cyclones in mid-latitudes. Another type of cyclones are tropical cyclones which originate on tropical oceans and influence the coastal areas. Sometimes they turn violent and cause heavy loss to life and property.

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