Winds and ocean currents

Wind is air moving from high pressure to low pressure. These pressure differences come mainly from uneven heating, as in the land and sea breezes you have studied. A handy rule: a wind is named after where it comes from.

Breezes in the hills (Shimla, Dehradun, Himalayan valleys):

• By day: the sunny mountain slopes heat up faster than the valley floor. Warm air rises over the slopes, and cooler air flows up from the valley: a valley breeze.
• After sunset: the slopes cool faster. Cold, heavy air slides down into the valley: a mountain breeze.

Reason backwards: a mountain breeze comes from the mountain, so the mountain has higher pressure, so it is colder, so it must be night.

Planetary winds: the big pattern. Follow a parcel of air:

• At the equator, strong heating makes warm air rise: a belt of low pressure.
• High up, it heads towards the poles, cools, and sinks around 30° N and S: belts of high pressure. From there, air flows back along the surface towards the equator.
• Some flows towards the poles instead and rises again around 60°, where it meets cold polar air: low pressure.
• At the poles (90°), cold, heavy air sinks: high pressure.

So the belts go: low (0°), high (30°), low (60°), high (90°).

Bent by the spinning Earth. Because the Earth rotates, winds don't flow straight. They bend to the right in the Northern Hemisphere and to the left in the Southern Hemisphere, so they follow curved paths.

Ocean currents are rivers of water moving through the sea. They are driven by:

• planetary winds, which drag the surface water along;
• differences in temperature and saltiness: colder, saltier water is heavier and sinks; warm water flows towards the poles on the surface and cold water flows back deep down;
• the Earth's rotation, and the continents blocking the way.

The rotation turns currents into huge circles called gyres: clockwise in the Northern Hemisphere, anticlockwise in the Southern.

Currents carry heat. The warm North Atlantic Drift (part of the Gulf Stream) carries warm water across the Atlantic and keeps ports in north-western Europe ice-free in winter, even so far north. Currents also carry nutrients that feed huge sea ecosystems.

In India, scientists at the Indian Institute of Tropical Meteorology (IITM), Pune run computer models that link the air, oceans, land and ice to forecast the monsoon, using data from satellites, buoys in the Indian Ocean, and even stations in Antarctica.