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Extratropical Cyclones

Mid-latitude depressions, temperate cyclones, frontal depressions, and wave cyclones are all names for extratropical cyclones....

Mid-latitude depressionstemperate cyclonesfrontal depressions, and wave cyclones are all names for extratropical cyclones. In both hemispheres, these are active above the mid-latitudinal zone between 35° and 65° latitude.

  • The movement is from west to east, and it is most noticeable during the winter months.
  • Polar and tropical air masses collide and generate fronts in these latitude zones.
Origin and Development of Temperate Cyclones
Polar Front Theory
  • According to this theory, warm-humid air masses from the tropics collide with dry-cold air masses from the poles, forming a polar front as a discontinuous surface.
  • Such circumstances can be seen along the Tropopause and in subtropical high and subpolar low-pressure belts.
  • The warm air from beneath is pushed upwards by the cold air. As a result of the reduced pressure, a void is generated. The surrounding air rushed in to fill the void, and when combined with the earth’s rotation, a cyclone forms, which moves westward (Jet Streams).
  • Mid-latitude depressionstemperate cyclonesfrontal depressions, and wave cyclones are all names for extratropical cyclones.
  • In both hemispheres, these are active above the mid-latitudinal zone between 35° and 65° latitude.
  • The movement is from west to east, and it is most noticeable during the winter months.
  • Polar and tropical air masses collide and generate fronts in these latitude zones.
Air Masses

It is defined as a large body of air having little horizontal variation in temperature and moisture. The homogenous surfaces, over which air masses form, are called the source regions.

The air masses are classified according to the source regions. There are five major source regions. These are:

  • Warm tropical and subtropical oceans;
  • The subtropical hot deserts;
  • The relatively cold high latitude oceans;
  • The very cold snow covered continents in high latitudes;
  • Permanently ice covered continents in the Arctic and Antarctica.

When two different air masses meet, the boundary zone between them is called a front. The process of formation of the fronts is known as frontogenesis. There are four types of fronts:

  • Stationary: When the front remains stationary, it is called a stationary front.
  • Warm: When the warm air mass moves towards the cold air mass, the contact zone is a warm front.
  • Cold: When the cold air moves towards the warm air mass, its contact zone is called the cold front
  • Occluded: When an air mass is fully lifted above the land surface, it is called the occluded front.
Stages of Temperate cyclone
  • The development of temperate cyclones can be studied in five stages namely
  1. Stage I: Stationary/ Beginning
  2. Stage II: Beginning of Young Adult
  3. Stage III: Mature
  4. Stage IV: Occlusion
  5. Stage V: Dissipation
  • Warm air blows from the south and cold air from the north of the front in the northern hemisphere.
  • When the pressure drops along the front, warm air goes north and cold air moves south, causing an anticlockwise cyclonic circulation to form (northern hemisphere). The Coriolis Force is to blame for this.
  • A well-developed extratropical cyclone with a warm front and a cold front results from the cyclonic circulation.
  • Between the forward and rear cold air or cold sectors, there are pockets of warm air or warm sectors.
  • Warm air glides over cold air, causing precipitation as a series of clouds form across the sky ahead of the warm front.
  • The warm air is pushed up by the cold front as it approaches from behind. Cumulus clouds form along the cold front as a result.
  • The cold front advances quicker than the warm front and eventually overtakes it.
  • The cyclone evaporates once the warm air is completely pushed up and the front is occluded (occluded front).
  • Wind circulation activities at the surface and aloft are intricately interrelated.
  • As a result, the temperate cyclone has a lot of frontogenesis going on, mostly with occlusion type fronts.
  • Individual frontal cyclones usually last 3 to 10 days and move in a west to east path.
  • The orientation of the polar jet stream in the upper troposphere controls the precise movement of this weather phenomenon.
Seasonal Occurrence of Temperate Cyclones
  • Temperate cyclones are most common in the winter, late autumn, and early spring. Rainstorms and overcast weather are commonly connected with them.
  • During the summer, all temperate cyclone routes travel northwards.
  • There are few temperate cyclones over the subtropics and warm temperate zone, despite a large concentration of storms over the Bering Strait, the United States, and the Russian Arctic and sub-Arctic zones.
Distribution of Temperate Cyclones
  • Temperate cyclones form in the world’s mid-latitude regions, as previously stated.
  • This region is also referred to as extra-tropical (beyond tropical).
  • Temperate cyclones are mostly found between 350 and 650 degrees north and south latitude.
  • Their traces have been discovered beyond 650 degrees latitude in both hemispheres.
  • The prevalence and distribution of temperate cyclones reveal a well defined zone over the globe’s map.
  • Temperate cyclones can be found practically all year round, with some seasonal changes.
  • However, they are more common in the winter since there is a higher temperature disparity.
  • As a result, winter is the best time for temperate cyclones to form, and their frequency is also higher.
Characteristics of Temperate Cyclones
  1. Size and Shape:
  • The temperate cyclones are asymmetrical and inverted ‘V-shaped.
  • They cover a distance of 500 to 600 kilometres.
  • They may cover a distance of 2500 kilometres throughout North America (Polar Vortex).They range in height from 8 to 11 kilometres.
  1. Wind Velocity And Strength:
  • Winds are stronger in the eastern and southern parts of the country, and in North America than in Europe.
  • The wind speed increases as the storm approaches, but drops as it passes.
  1. Orientation And Movement:
  • In temperate cyclogenesis, the jet stream is quite important.
  • Temperate cyclones are similarly influenced by jet streams.
  • These cyclones are orientated east-west because they move with the westerlies (Jet Streams).
  • If the storm front is moving east-west, the centre will travel east quickly.
  • The centre moves northwards if the storm front is oriented northwards, but the pressure difference decreases after two or three days, and the cyclone evaporates.
  • If the storm front is directed south, the core of the storm moves fairly far south, even up to the Mediterranean region .
Associated Weather
  • A temperate cyclone’s arrival is signalled by a drop in temperature, a drop in mercury level, wind shifts, a halo around the sun and moon, and a thin layer of cirrus clouds.
  • A little drizzle follows, eventually turning into a torrential downpour.
  • The approach of the warm front, which halts the fall in mercury levels and raises the temperature, changes these conditions.
  • Rainfall ends and clear weather reigns until an anticyclonic cold front comes, bringing with it a drop in temperature, cloudiness, and rain with thunder.
  • After that, the weather becomes clear once more.
  • When temperate cyclones move slowly and there is a significant difference in rainfall and temperature between the front and back of the cyclone, more rain falls.
  • Anticyclones are usually accompanied by these cyclones.
Significance of Temperate Cyclones
  • Temperate cyclones bring rain to most of North and North-western India, filling the void left by the South West monsoon.
  • Temperate cyclones are extremely important in agriculture.
  • Because of the Temperate cyclones, most Rabi crops, especially wheat, in North and North-western India produce superior results.
  • They also play an important role in meeting water demands for human consumption .

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