Small pressure gradients at certain altitudes produce strong horizontal winds. Air cannot flow directly from high to low pressure areas due to the Earth's rotation which is faster at the equator than at the poles. Air moving from the equator to the poles moves eastward faster than the underlying surface and thus curves to the right in the northern hemisphere. Air moving from the poles to the equator moves slower than the underlying surface and thus also curves to the right. This is known as the Coriolis effect and produces the global wind patterns.
Air flowing out of the subtropical highs towards the equator forms the trade winds.
On the polar side of the subtropical highs, the winds are deflected to give rise to the prevailing weather. Friction slows the surface wind and weakens the Coriolis effect, causing the air to curve towards low pressure. The amount of change depends upon the surface.