Temperate Cyclones
Notes
Temperate (extra-tropical or mid-latitude) cyclones form between about 35° and 65° latitude along the polar front, where warm, moist subtropical air meets cold polar air. They are explained by the Polar Front Theory of the Norwegian meteorologists (Bjerknes, 1918-20), with the warm front, cold front and warm sector.
Life cycle (cyclogenesis): the initial (stationary front), incipient (wave forms), mature (a well-developed warm sector), occlusion (the cold front overtakes the warm front) and dissipation.
Weather
- Warm front: a gentle slope; warm air rises over cold air; clouds (cirrus, altostratus, nimbostratus); steady, light rain or drizzle ahead of the front; temperature rises after passage.
- Cold front: a steeper slope; cold air pushes under warm air; cumulonimbus, heavy showers, thunderstorms; temperature falls sharply; clearing skies behind.
- They move west to east with the westerlies at 30-50 km/h.
Western Disturbances (India)
- Extra-tropical storms that originate over the Mediterranean, Caspian and Black Sea region and move east, steered by the subtropical westerly jet.
- They bring winter rain and snow to north-west India (Punjab, Haryana, Himachal Pradesh, J&K, Uttarakhand), locally called Mawath in some areas. They are beneficial for rabi crops (wheat) and replenish Himalayan snow, but also cause cold waves, fog, and hailstorms; with the monsoon they can trigger floods (Uttarakhand, 2013).
Basis | Tropical cyclone | Temperate cyclone |
|---|---|---|
Origin | Warm tropical oceans | Mid-latitudes along the polar front |
Energy source | Latent heat of condensation | Temperature contrast of air masses |
Fronts | No fronts | Warm and cold fronts |
Size | 100-1,000 km | 1,000-2,000 km |
Eye | Present | Absent |
Direction of movement | East to west | West to east |
After landfall | Dies out quickly | Can persist over land |
Season | Late summer and autumn | Mainly winter |
Key Points
- Formed at the polar front, 35-65°; Polar Front Theory (Bjerknes).
- Warm and cold fronts; occlusion; move west to east.
- Western Disturbances bring winter rain to north-west India; good for wheat.
- No eye in temperate cyclones.
- Tropical cyclones draw energy from latent heat; temperate from air mass contrast.
MCQs
Q1. Western Disturbances originate in the:
(a) Bay of Bengal
(b) Mediterranean region
(c) Arabian Sea
(d) Pacific Ocean
Ans: (b).
Q2. Which of the following is a feature of temperate cyclones?
(a) An eye at the centre
(b) Fronts
(c) Formation near the equator
(d) Movement east to west
Ans: (b).
Q3. Western Disturbances are especially beneficial for:
(a) Kharif rice
(b) Rabi wheat
(c) Cotton
(d) Jute
Ans: (b).