Winds and Currents
The gentlest summer breezes and the fiercest blasts of winter are produced by the unequal heating of air. We have seen that the air nearest to a stove or hot object becomes hotter than the adjacent air, that it tends to expand and is replaced and pushed upward and outward by colder, heavier air falling downward. We have learned also that the moving liquid or gas carries with it heat which it gradually gives out to surrounding bodies.
When a liquid or a gas moves away from a hot object, carrying heat with it, the process is called convection
Convection is responsible for winds and ocean currents, for land and sea breezes, and other daily phenomena.
The Gulf Stream illustrates the transference of heat by convection. A large body of water is strongly heated at the equator, and then moves away, carrying heat with it to distant regions, such as England and Norway.
Owing to the shape of the earth and its position with respect to the sun, different portions of the earth are unequally heated. In those portions where the earth is greatly heated, the air likewise will be heated; there will be a tendency for the air to rise, and for the cold air from surrounding regions to rush in to fill its place. In this way winds are produced. There are many circumstances which modify winds and currents, and it is not always easy to explain their direction and velocity, but one very definite cause is the unequal heating of the surface of the earth.