TEMPERATE CYCLONES (EXTRA-TROPICAL / MID-LATITUDE CYCLONES)
Geography
Introduction
- Temperate cyclones are storm systems that develop in the mid and high latitudes, away from the tropics.
- They are also known as Extra-Tropical Cyclones, Mid-Latitude Cyclones, Frontal Cyclones, and Wave Cyclones.
- They are low-pressure systems associated with cold fronts, warm fronts and occluded fronts.
- They occur between 35° to 65° latitudes in both hemispheres and generally move from west to east under the influence of westerlies and jet streams.
Origin and Development – Polar Front Theory
- According to the Polar Front Theory, warm and moist tropical air masses meet cold and dry polar air masses.
- This meeting zone forms a polar front which acts as a surface of discontinuity.
- Such conditions exist near subtropical high pressure belts, subpolar low pressure belts and near the tropopause.
Development Process
- Cold air pushes warm air upward from below creating a low pressure area.
- Surrounding air rushes in and due to Earth’s rotation (Coriolis force) cyclonic circulation develops.
- The system starts moving with the westerlies and jet streams.
- Warm front forms ahead and cold front forms behind.
- A warm sector is trapped between two cold air masses.
Occlusion Stage
- The cold front moves faster and overtakes the warm front.
- Warm air is completely lifted up forming an occluded front.
- The cyclone then starts losing energy and finally dissipates.
- Thus, a temperate cyclone is a result of intense frontogenesis and occlusion.
Life Span
- Usually lasts from 3 to 10 days.
- Moves from west to east.
- Path is controlled by the polar jet stream.
Seasonal Occurrence
- Mostly occur in winter, late autumn and spring.
- During summer, cyclone tracks shift northwards.
- High concentration occurs near Bering Strait and Arctic and sub-Arctic regions.
Distribution
- Major regions include:
- USA and Canada (Great Lakes, Rockies, Sierra Nevada),
- Iceland to Barents Sea to Siberia belt,
- Mediterranean basin up to Russia and India (Western Disturbances),
- Antarctic frontal zone.
Indian Context
- Mediterranean cyclones enter India in winter as Western Disturbances.
- They cause rainfall in Punjab, Haryana, Rajasthan and Western UP and are important for rabi crops.
Characteristics
- Size and Shape
- Asymmetrical and inverted V shaped.
- Usually 500–600 km in size but may extend up to 2500 km.
- Height is about 8 to 11 km.
Wind Velocity and Strength
- Stronger in eastern and southern parts.
- Stronger in North America than Europe.
- Wind speed increases before arrival and decreases after passage.
Orientation and Movement
- Controlled by jet streams.
- Move with westerlies.
- If front is east–west, cyclone moves eastward.
- If front is northward, cyclone moves northward and weakens.
- If front is southward, cyclone moves deep south (Mediterranean / Western Disturbances).
Structure
- North-west sector is cold sector.
- North-east sector is warm sector.
Associated Weather
- Before Arrival
- Fall in temperature and pressure.
- Wind shift.
- Halo around sun and moon.
- Cirrus clouds appear.
During Warm Front
- Light drizzle and continuous rain.
- Temperature rises and pressure stabilizes.
Warm Sector
- Rain stops and temporary clear weather.
During Cold Front
- Sudden fall in temperature.
- Thunderstorms and heavy rainfall.
After Cyclone Passes
- Clear weather and cold dry air.
Importance
- Major source of winter rainfall in mid-latitudes.
- Western Disturbances are crucial for Indian agriculture.
- Help in heat exchange between tropics and poles.
- Strongly influence weather of Europe, North America and North Asia.
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Subject: Geography
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