(i) The amount of insolation a place receives depends upon the inclination of the rays. This depends on the latitude of the place. The higher the latitude, the less is the angle they make with surface of the earth resulting in slant sun rays. The area covered by vertical rays is always less than the slant rays. If more area is covered, the energy gets distributed and the net energy received per unit area decreases. Moreover, the slant areas are required to pass through greater depth of the atmosphere resulting in more absorption, scattering and diffusion.

The axis of the earth is tilted at an angle of 23.5°. So the sun’s rays are different in both the hemispheres at the same time. The sun’s rays fall vertically on the tropic of cancer from 1^st March to 21^st July and on Tropic of Capricorn from 23^rd September to 22^nd December. As we go towards the poles, the temperature keep decreasing as the earth is tilted. Therefore the latitude and tilt in the axis of rotation of the Earth affect the amount of radiation received at the earth’s surface.

(ii) The sun’s energy reaches the earth’s surface by the process of radiation. The earth maintains the temperature by absorbing the rays and reflecting the terrestrial radiation. Of the total energy radiated by the sun, some amount of energy is reflected, radiated and absorbed. Out of the remaining part, 35 units are reflected back to the space before reaching the earth’s surface. Of these 27 units are reflected back from the top of the clouds and 2 units from the snow and ice –covered areas of the earth. The remaining 65 units are absorbed, 14 units within the atmosphere and 51 units by the earth’s surface. The earth radiates back 51 units in the form of terrestrial radiation. Of these 17 units are radiated to space directly and the remaining 34 units are absorbed by the atmosphere. 48 units absorbed by the atmosphere are also radiated back into space. Thus, the total radiation returning from the earth and the atmosphere respectively is 65 units which balance the total of 65 units received from the sun. This is known as the heat balance of the earth’s surface.

(iii) In general the effect of latitude on latitude is well pronounced on the map as the isotherms are generally parallel to the latitude. The deviation from this general trend is more pronounced in January than in July especially in the northern hemisphere. In the northern hemisphere the land surface area is much larger than in southern hemisphere. Hence the effects of land mass and ocean currents are well pronounced. In the January the isotherms deviate to the north over the ocean and to the south over the continent. This can be seen in the Atlantic Ocean. The presence of warm ocean currents, gulf streams, and North Atlantic drift make the Northern Atlantic Ocean warmer and the isotherms bend towards the north. Over the land the temperature decreases sharply and isotherms bend south in Europe. The effect of ocean is well pronounced in the southern hemisphere. Hence the isotherms are more or less parallel to the latitudes and the variation in temperature is more gradual than in the northern hemisphere. The isotherm of 20°C, 10°C and 0°C runs parallel to 35°S, 45°S and 60°S latitudes respectively.