Q.While the pressure gradient force is from north to south, i.e. from the subtropical high pressure to the equator in the northern hemisphere, why are the winds north easterlies in the tropics.
The pressure gradient pulls air straight from the subtropical high to the equator, but the Earth’s rotation bends that moving air to the right in the northern hemisphere, so what starts as a north-to-south flow arrives as a northeast-to-southwest wind — the northeast trade winds.
The question is a classic trap, and it deserves a patient answer. You have correctly identified the driving force: in the northern hemisphere, the subtropical high-pressure belt sits around 30°N, and the equatorial low-pressure trough sits near the equator. Air, being lazy, wants to flow from high pressure to low pressure, which means it should move due south — from 30°N straight down to the equator. That is the pressure gradient force, and it points from north to south. If the Earth did not rotate, the winds would blow due north or due south, simple as that.
But the Earth does rotate, and that rotation changes everything. As air begins to move southward, the Earth beneath it is also moving — and moving faster at the equator than at higher latitudes. Think of a point on the Earth’s surface at 30°N: it traces a smaller circle each day than a point on the equator, so it moves eastward more slowly. When air leaves the subtropics heading south, it carries that slower eastward speed with it. As it moves into lower latitudes, the ground beneath it is racing eastward faster than the air is. So relative to the ground, the air appears to lag behind, or drift westward, even as it continues southward.
That apparent deflection is the Coriolis effect. In the northern hemisphere, it bends any moving object to the right of its path. For air moving due south, “right” means toward the west. So the wind does not continue due south; it is continuously nudged southwestward. By the time it has travelled a good distance, its direction is no longer north-to-south but northeast-to-southwest. That is exactly what a northeast trade wind is — a wind that comes from the northeast and blows toward the southwest.
The name “northeast trade wind” tells you the direction it comes from, not the direction it goes. A northeast wind blows from the northeast toward the southwest. This naming convention is the same for all winds — a westerly comes from the west, an easterly comes from the east.
So the apparent contradiction dissolves once you separate the two forces. The pressure gradient force sets the air in motion southward. The Coriolis effect then deflects that motion, and the deflection is not a one-time kick but a continuous bending. The air never actually travels due south for long; it is constantly being turned. The result is a steady, reliable wind blowing from the northeast toward the southwest across the tropics.
The pressure gradient force and the Coriolis effect are not competing explanations — they work together. The pressure gradient starts the wind; the Coriolis effect bends it. In the northern hemisphere, the bend is always to the right, which turns a southward flow into a southwestward flow, giving the northeast trades.
There is one more subtlety worth noting. The Coriolis effect is strongest at the poles and zero at the equator. Near the equator itself, the deflection is weak, so the winds blow more directly southward. But over the broad tropical belt, the deflection is strong enough to give the trades their characteristic northeast direction. The same logic, mirrored, explains the southeast trades in the southern hemisphere, where the deflection is to the left.
In short, the winds are northeast trades because the pressure gradient pulls air south, but the Coriolis effect bends that southward flow to the right — toward the west — turning it into a northeast-to-southwest wind. The pressure gradient gives the direction of the push; the Earth’s rotation gives the direction of the wind.
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