NCERT Class 11 - Fundamentals of Physical Geography, Chapter 10

Pressure Belts, Winds and Jet Streams

Air moves from high pressure to low pressure, but the earth's rotation deflects it, so winds blow around pressure systems rather than straight into them. Four pressure belts in each hemisphere organise the global wind pattern.

The pressure belts

• The belts shift north and south with the apparent movement of the sun, which is central to understanding seasonal rainfall, including the Mediterranean climate.
• Pressure falls with altitude, so maps always show pressure reduced to sea level to make places comparable; the average sea level pressure is about 1013 millibars.
• Lines joining places of equal pressure are called isobars; closely spaced isobars mean a steep gradient and strong winds.

Forces acting on wind

• Pressure gradient force drives air from high to low pressure and is stronger where isobars are closer together.
• Coriolis force arises from the earth's rotation and deflects moving air to the right in the northern hemisphere and to the left in the southern. It is zero at the equator and strongest at the poles.
• Frictional force slows wind near the surface and reduces the deflection, which is why surface winds cross isobars at an angle while upper air winds blow parallel to them.
• Where the pressure gradient and Coriolis forces balance in the upper air, the resulting wind is called a geostrophic wind.

Types of winds

• Planetary or permanent winds - trade winds blow from the subtropical high to the equatorial low, from the north-east in the northern hemisphere and the south-east in the southern; westerlies blow from the subtropical high towards the subpolar low; polar easterlies blow from the polar high.
• Seasonal winds - the monsoon, which reverses direction between summer and winter.
• Local winds - hot and dry winds such as the loo of northern India, the sirocco of the Mediterranean and the harmattan of West Africa; warm descending winds such as the chinook of North America and the foehn of the Alps; and cold winds such as the mistral of France and the bora of the Adriatic.
• Land and sea breezes - by day the land heats faster and air blows from sea to land; by night the land cools faster and the breeze reverses.
• Mountain and valley breezes - by day warm air rises up the slope as a valley breeze; by night cold dense air drains down as a mountain breeze.

Jet streams

• Jet streams are narrow, fast bands of wind in the upper troposphere, blowing from west to east at very high speed.
• The subtropical westerly jet lies over the subtropics; the polar front jet lies where polar and mid-latitude air meet.
• The tropical easterly jet blows from east to west over southern Asia in summer.
• For India, the shift of the subtropical westerly jet north of the Himalayas in early summer is closely linked with the onset of the south-west monsoon, and its return south is linked with the withdrawal.
• Jet streams also steer western disturbances, the weather systems that bring winter rain to north-western India and snow to the Himalayas.