Revision summary
A cyclone is a closed low with inward spiralling winds. Temperate cyclones are mid-latitude frontal storms, not tropical ocean cores. They grow from a wave on the polar front between warm and cold air masses. Coriolis force and jet-stream divergence aloft deepen the low. Western disturbances that wet north-west India in winter belong to this family.
Model answer
Introduction
A cyclone is a closed low-pressure system in which winds spiral inward—anticlockwise in the Northern Hemisphere—around a centre. Temperate cyclones are the mid-latitude family of that system, born on air-mass fronts, not on a warm tropical ocean core.
Body
What a cyclone is
- Dynamically, a cyclone is an extra-tropical or tropical depression with cyclonic vorticity, cloud, and often precipitation.
- In school geography the word covers both tropical cyclones (hurricanes, typhoons, Bay of Bengal storms) and temperate or extra-tropical cyclones.
- This question asks for the second: travelling depressions of the westerlies, typically between about 35° and 65° latitude.
Causes of origin of temperate cyclones
- Polar front theory (Bjerknes and the Bergen school): warm subtropical westerly air meets cold polar air along a front; a wave on that front deepens into a low.
- Strong temperature and density contrast between air masses supplies potential energy that converts to kinetic energy of the storm.
- The Coriolis force, sufficient away from the equator, turns inflow into rotation; without it a front does not become a cyclone.
- Upper-air support from the polar front jet stream—divergence aloft, troughs in the Rossby waves—lets surface pressure fall and the wave grow.
- Moisture and a trigger (orographic, or a pre-existing disturbance) feed cloud and rain along warm and cold fronts as the mature cyclone occludes.
- These systems travel west-to-east with the westerlies, bringing winter rain and snow to north-west India as western disturbances, which are temperate cyclones of Mediterranean origin reborn over Iran–Pakistan.
- They are not caused by a warm ocean ‘eye’ or by the monsoon trough; those are tropical mechanisms.
Flow diagram
flowchart TD F[Polar front contrast] --> W[Frontal wave] J[Jet divergence Coriolis] --> W W --> T[Temperate cyclone] T --> D[Western disturbance India]
Conclusion
A cyclone is an inward-spiralling low. Temperate cyclones originate on the polar front where contrasting air masses, Coriolis rotation, and jet-stream divergence turn a frontal wave into a travelling mid-latitude storm.
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Describe the areas of Uttar Pradesh affected by soil erosion and also identify the factors responsible for it.
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Is a Bay of Bengal cyclone a temperate cyclone?
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Why do temperate cyclones matter for Uttar Pradesh?
Winter western disturbances can give rain or cold waves into the western plains; they are the temperate-cyclone link in the state syllabus.
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