Why Do Winds Blow the Same Way for Months? Bands That Circle the Planet

Solar heating is strongest near the equator, so air there warms, rises and flows away at height towards both poles. It cools and descends around thirty degrees of latitude, and the descending air spreads at the surface, with part of it returning towards the equator to replace what rose. That return flow would blow directly from north and south if the earth were still, and because the planet rotates, moving air is deflected, so the flow arrives from the northeast in the northern hemisphere and the southeast in the southern. The result is a broad band of steady wind on each side of the equator, blowing from the east with a component towards it, and the circulation cell producing it is named after the lawyer who described it in 1735.

The global pattern is a set of latitudinal belts, each with its own character:

• •The doldrums near the equator, where the two flows converge and the wind is light and variable
• •The trade wind belts on either side, steady and reliable
• •The horse latitudes around thirty degrees, where air descends and winds are weak
• •The westerlies in the middle latitudes, blowing from the west and far less steady
• •Stronger westerlies in the southern ocean, uninterrupted by land
• •Polar easterlies at the highest latitudes

The reliability of these winds determined how sailing routes worked, and the consequences ran for centuries. A ship crossing the Atlantic westward sailed south to pick up the trades and ran before them, and returning eastward sailed north into the westerlies, which produced a clockwise circuit that every vessel followed and which made the Atlantic a system rather than a set of point-to-point routes. The same logic organised the Pacific and shaped the routes linking Asia and the Americas. The name records the original meaning of the word trade, which was a regular course or track, and the association with commerce followed rather than preceded it. The belts of weak wind between were genuinely feared, since a becalmed ship consumed provisions and water while going nowhere.

The whole pattern shifts with the seasons, which matters enormously for the regions at its edges. The zone where the two flows converge migrates north and south through the year, following the sun with a lag, and the rains associated with it migrate too, which is what produces the wet and dry seasons across the tropics and what drives the seasonal reversal in the Indian Ocean. Regions sitting at the extreme of that migration receive rain for a short season and nothing for the rest, which is why the margins of the Sahara and comparable zones elsewhere are so vulnerable, since a small shift in how far the belt travels means a season with rain or without. The long-term position of these belts is also changing, with evidence that the descending branches have shifted poleward in recent decades.

The winds transport more than ships. Dust from the Sahara is lifted and carried across the Atlantic in quantities measured in hundreds of millions of tonnes a year, fertilising the Amazon basin with phosphorus and reaching the Caribbean, where it affects air quality and has been linked to coral health. Moisture carried westward supplies rainfall to the eastern sides of tropical landmasses. The winds drive ocean currents, pushing surface water westward and setting up the circulation of the tropical oceans, and the state of that wind field is what changes during the periodic warming of the Pacific that reorganises weather worldwide, since a weakening of the trades allows warm water to slosh eastward. Seeds, insects and birds cross oceans on them.

Air rising at the equator descends around thirty degrees and returns at the surface, deflected by the earth's rotation into a steady flow from the east. Sailing routes across the Atlantic and Pacific became clockwise circuits using the trades one way and the westerlies back. The same winds carry Saharan dust to the Amazon and drive the ocean currents whose changes reorganise global weather.