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Temperate Cyclones

Temperate Cyclones

Introduction

  • Temperate cyclones are also known as Extra-tropical cyclone where the term “Extra-tropical” signifies that this type of cyclone generally occurs outside the tropics with a latitude range between 30° and 60°.
  • They are generally, extensive having a vertical thickness ranging from 9 to 11 km and a diameter of about 1,000 km.
  • These systems and very often provide extensive precipitation over a very large region.
  • The general direction of movement of temperate cyclones is from west to east
  • with under the influence of westerly flow in mid-latitudes (westerlies and polar jet streams).
  • Jet stream plays a major role in temperate cyclone formation. It influences the path of temperate cyclones.
  • The temperate cyclones occur mostly in winter, late autumn and spring. During summer, all the paths of temperate cyclones shift northwards and there are only few temperate cyclone over sub-tropics and the warm temperate zone, although a high concentration of storms occurs over Bering Strait, USA and Russian Arctic and subArctic zone.

Characteristics of Temperate Cyclone

  • The Extra-Tropical Cyclones are storm systems emerging in the mid and high latitudes, away from the tropics.
  • They are low-pressure systems with associated cold fronts, warm fronts, and occluded fronts.
  • These cyclones are formed along the polar front.
  • In the beginning, the front is stationary.
  • Extra-tropical cyclones are also known as mid-latitude storms or baroclinic storms.
  • In the Northern hemisphere, cold air blows from the north of the front and warm air blows from the south.
  • When the pressure descents along the front, the cold air move towards the south and the warm air moves northwards setting in motion an anticlockwise cyclonic circulation.
  • The cyclonic circulation results in a well-built extratropical cyclone, with a cold front and a warm front.
  • There are pockets of warm air compressed between the forward and the rear cold air.
  • The warm air climbs over the cold air and a series of clouds appear over the sky ahead of the warm front and cause rainfall.
  • The cold front approaches the warm air from behind and pushes the warm air up.
  • As an outcome, cumulus clouds develop along the cold front.
  • The cold front moves faster than the warm front eventually surpassing the warm front.
  • The warm air is entirely lifted up and the front is occluded and the cyclone dissipates.
  • They can originate over the land and sea and cover a larger area.

Polar Front Theory

  • According to this theory, the warm-humid air masses from the tropics meet the dry-cold air masses from the poles and thus a polar front is formed as a surface of discontinuity.
  • Such conditions occur over subtropical high, sub-polar low pressure belts and along the Tropopause.
  • The cold air pushes the warm air upwards from underneath. Thus a void is created because of lessening of pressure. The surrounding air rushed in to occupy this void and coupled with the earth’s rotation, a cyclone is formed which advances with the westerlies (Jet Streams).

Stages Of Formation

  • FIRST STAGE: This involves the convergence of two air 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: It 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: It 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 front, 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.

Movements of Temperate Cyclones

  • Midlatitude cyclones or Temperate cyclones are essentially transient features, on the move throughout their existence. Four kinds of movement are involved:
  • The whole storm moves as a major disturbance in the westerlies, traversing the midlatitudes generally from west to east. The rate of movement averages 30 to 45 kilometers (about 20 to 30 miles) per hour, which means that the storm can cross North America in three to four days (often faster in winter than in summer).
  • The route of a cyclone is likely to be undulating and erratic, although it moves generally from west to east, often in association with the path of the jet stream.
  • The system has a cyclonic wind circulation, with wind generally converging counterclockwise (in the Northern Hemisphere) into the center of the storm from all sides.
  • The cold front usually advances faster than the center of the storm (the advancing dense, cold air easily displaces the lighter, warm air ahead of the front).
  • The warm front usually advances more slowly than the center of the storm, causing it to appear to lag behind. (This is only an apparent motion, however. The warm front is actually moving west to east, just like every other part of the system.)              Temperate Cyclones

Distribution Of Temperate Cyclones

  • USA and Canada
  • The belt extending from Iceland to Barents Sea and continuing over Russia and Siberia
  • Winter storms over Baltic Sea
  • Mediterranean basin extending up to Russia and even up to India in winters (called western disturbances) and the Antarctic frontal zone.
  • East China
  • (Temperate Cyclones)

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