i) The solar constant which depends on
(a) energy output of the sun and
(b) distance from the earth to sun.
ii) Transparency of the atmosphere.
iii) Duration of the daily sunlight period.
iv) Angle at which the sun's rays strike the earth.
A transparency of the atmosphere has a more important bearing upon the amount of insolation which reaches the earth's surface. The areas having heavy dust, clouds, water vapour and cloudiness or polluted air will receive less direct insolation.
The transparency of atmosphere depends on the latitude of a place. At middle and high latitudes the sun's rays must pass through thicker layers of reflecting / scattering material and it is not so at tropical latitudes.
francis1897 answered the question on January 11, 2023 at 07:13