Periodic Variations in Light-Diurnal and Seasonal
We know that rotation of the earth on its axis accounts for day-to-night variations in the amount of radiations falling at a given place and seasonal variations occur due to the orbiting of the earth around the sun. Since the earth's equatorial plane is inclined to its orbit at an angle of 23o.27'. the rays of the sun do not fall vertically on all parts of the earth. From March 22nd to Sept. 22nd (AutumnaTequinox), the northpole is inclined towards the sun. So the most intense solar beam is focused on the northern hemisphere. We in the northern hemisphere, have summer season and on the northpole the sun shines for 24 hours of the day. While on South pole it is dark for six months and the southern hemisphere has winter season. The opposite situation exists on the poles from September 24 to March 20 (spring equinox) when the northern hemisphere has winter season and southern hemisphere hassummer season (Figure shown below).
There is also horizontal variation in the distribution of radiation on the earth. Because the earth is nearly spherical in shape, parallel beam of incoming sunlight does not fall vertically on all parts of the earth. It strikes lower latitudes more directly than higher latitudes. Therefore, at higher latitudes the incident radiation falls obliquely on the surface, travels more through the atmosphere, and spreads over a greater area and thus is less intense than the vertical beam falling on or near the equator.