The general circulation of winds arises from the global redistribution of heat from warm low latitudes to cold high latitudes, driven by the development of surface pressure gradients. Wind blows from high to low pressure regions, although airflow is deflected by the Coriolis force as a result of the Earth's rotation, and tends to follow more east-west trends rather than north-south trends.
Air movement at or near the equator is light. At sea the region became known to sailors as the Doldrums. Air from the subtropical zones in the Northern and Southern Hemispheres converges here in a zone called the Inter-Tropical Convergence Zone (ITCZ). These low latitude wind belts became known as the Northeast and Southeast Trades, which merchant ships used to cross the Atlantic Ocean from Europe to the New World. In the Indian Ocean, Northeast Trade winds blow throughout the winter months. During the Northern Hemisphere summer however, the ITCZ is shifted well to the north of the equator, when the midday Sun is overhead at the Tropic of Cancer at latitude 23.5° north. The Southeast Trade winds now cross the equator, and are deflected to the right by the Coriolis force, forming the Southwest Monsoons. This summertime airflow picks up considerable moisture crossing the Indian Ocean, and brings a heavy and prolonged wet season to India and Southeast Asia through April to September, known as the Monsoon.
The temperate mid-latitudes are influenced by a stream of westerly airflow. In the Northern Hemisphere the winds became known as the Southwest Antitrades, which prevail for much of the year. In the Atlantic, the Gulf Stream enhances the warmth of the southwesterly air masses, which influence the mild weather of the UK and Western Europe. This warm flow of air collides with the Polar Easterlies from the Arctic region, generating a zone of cyclonic low pressure, where frontal depressions frequently form. In the Southern Hemisphere, the westerlies are known as the Roaring Forties, which blow more or less continuously around the Earth due to the absence of significant landmasses.