Insolation: Sun is the main source of energy for the earth’s surface and the atmosphere. Solar energy is called insolation. It reaches the surface travelling at a speed of 186,000 miles per second. According to Trewartha, “the energy received on the earth’s surface, in the form of in small waves and travelling at a speed of one lakh eighty – six thousand miles per second, is called insolation.
Terrestrial radiation: The earth receives heat through insolation only. When the surface gets heated, than radiation takes place from it because every hot object radiates heat. The radiation taking place from the Earth is in the form of long waves. The atmosphere is not directly warmed by the heat from the sun but it is heated with radiated heat obtained from the Earth. This is called terrestrial radiation.