Pressure, Winds and Cyclones

Forces that shape the wind:

• Isobars are lines joining places of equal pressure. Closely spaced isobars show a steep pressure gradient and strong winds; widely spaced isobars show a gentle gradient and light winds.
• The Coriolis force comes from the Earth's rotation. It turns the wind to the right in the northern hemisphere and to the left in the southern hemisphere.
• It is zero at the equator and greatest at the poles (it grows with latitude).
• Cold land builds high pressure. In winter a high forms over north-west India, so a place like Jaisalmer has higher winter pressure than Chennai or Guwahati. High places like Leh have low pressure because there is less air above them.

The global pattern:

• Equatorial low (0 to 5°): hot air rises; calm air called the doldrums; the trade winds meet here at the ITCZ (Inter-Tropical Convergence Zone).
• Subtropical highs (about 30°): air that rose at the equator sinks here. Most hot deserts lie under them.
• Subpolar lows (about 60°) and polar highs.
• Trade winds blow from the subtropical highs to the equator; westerlies from the subtropical highs towards the subpolar lows; polar easterlies from the poles.
• Hadley cell (0 to 30°): driven by heating at the equator. Ferrel cell (30 to 60°). Polar cell (60 to 90°).
• NCERT: the pattern of planetary winds depends on latitudinal variation of heating, the pressure belts and their seasonal shift with the Sun, the distribution of continents and oceans, and the rotation of the Earth.

What every cyclone shares, and how the two kinds differ:

• Every cyclone has low pressure at the centre, ringed by closed isobars. (High pressure at the centre is an anticyclone.)
• Winds turn anticlockwise round a cyclone in the northern hemisphere and clockwise in the southern.
• Tropical cyclones form over warm seas (above about 27 °C), move east to west with the trade winds, and have a calm, clear centre called the eye.
• Temperate cyclones form along fronts in the middle latitudes and move west to east with the westerlies.

Named winds and storms:

• Bora: a cold, dry wind blowing down from the mountains onto the eastern shore of the Adriatic Sea.
• Mistral: a cold wind blowing down the Rhone valley in France.
• Bise: a cold, dry wind of Switzerland.
• Loo: a hot, dry wind over the plains of north India in summer afternoons.
• Kalbaisakhi (Nor'westers): violent evening thunderstorms of West Bengal and Assam before the monsoon.
• Mango showers: NCERT's name for the pre-monsoon showers common in Kerala and coastal Karnataka at the end of summer, which help mangoes ripen early. NCERT also names blossom showers, which make coffee flowers blossom in Kerala and nearby areas.
• Blizzard: a violent snowstorm with strong wind, found in high snowy mountains and polar lands. In India, the Himalayan heights of states such as Arunachal Pradesh.