Question

What causes the intensity and duration of solar radiation received at any place to vary throughout the year?

Answer 1

due to axial tilt the light does not strike at 90 degree on the surface.in all places.
because of seasons the intensity changes.

Explanation:

at equator sun light strikes at 90 degree.But as you move away to poles the angle at which sun light strikes the surface changes and so intensity is reduced.Other things ;like clouds, fog effect the intensity.