Question

Why the wind is never seen

Answer 1

it isn't a physical thing (unlike clouds) which is why you can't see it, but you can see it's effects and measure the rate at which air is moving from one place to another.

Extra Credit

The Earth is spherical and it's axis is inclined relative to the plane of its orbit.This means that different parts of the planet receive different amounts of solar radiation - so they tend to be different temperatures.