Atmospheric Circulation and Weather Systems — Class 11 Geography

Atmospheric pressure is the weight of a column of air; at sea level it averages 1,013.2 millibars and decreases with height (about 1 mb per 10 m near the surface). Horizontal pressure is shown by isobars, lines joining places of equal pressure. The world has four major pressure belts: the equatorial low, the subtropical highs (around 30°), the subpolar lows (around 60°) and the polar highs. Winds blow from high to low pressure and are affected by the pressure gradient force, the frictional force and the Coriolis force, which deflects winds to the right in the northern hemisphere and to the left in the southern. When pressure gradient and Coriolis force balance, the geostrophic wind blows parallel to isobars. The general circulation of the atmosphere works through three cells — the Hadley cell, the Ferrel cell and the polar cell — and is linked to ocean circulation and to events like El Nino and ENSO. Local winds include land and sea breezes and mountain and valley winds. Air masses form over homogeneous source regions, and fronts (cold, warm, stationary and occluded) form where they meet. The chapter also describes extra-tropical cyclones (along the polar front) and tropical cyclones (violent storms over warm oceans with an eye and eye wall), thunderstorms and tornadoes.