For the transfer of heat from the sun to the earth, the energy emitted from the sun has to travel through the vacuum. This is possible by the radiating electromagnetic wave by the sun which can easily travel in a vacuum. This mode of heat transfer is known as Radiation.
To know, why it takes place only by radiation, we should know all possible modes of heat transfer and their requirements.
Let’s discuss different types of heat transfer.
Types of heat transfer:
There are three different modes of heat transfer as discussed below.
1) Conduction: Conduction is the transfer of heat energy at the molecular level by means of collision between molecules and due to the movement of free electrons.
The heat transfer by conduction is effectively carried out in solid material because in solid material, the molecules are very close to each other, and hence the rate of collision between molecules is higher.
Necessary conditions for conduction heat transfer:–
1) The two bodies or molecules should be in close contact with each other.
2) The temperature difference should be present between these two bodies.
2] Convection: Convection is the transfer of heat by means of the bulk movement of the molecules of the fluid.
In convection heat transfer, the heat from the surface of the body is absorbed by the fluid particles which are in contact with the surface because of the conduction, and this absorbed heat is carried away because of the motion in fluid molecules which is known as advection.
Therefore the convection is also known as a combination of conduction and advection.
Necessary conditions for convection heat transfer:–
1) The convective medium or fluid should be in contact with the heat transferring body.
2) The convective medium should have a macroscopic or bulk motion due to the density gradient or forced movement.
3) The temperature difference should be present between medium and heat transferring body.
3] Radiation heat transfer: Radiation is the mode of heat transfer by means of electromagnetic waves which depend on the temperature of radiating body.
The heat transfer by radiation doesn’t require a medium for its propagation.
How is heat transferred from the sun to the earth?
As we have already discussed necessary conditions for conduction heat transfer, thus in this case, heat transfer by conduction is not possible because there is no solid contact between the sun and the earth.
No medium is present in the path from the sun to the earth, therefore the heat transfer by convection is also not possible.
The only mode by which heat from the sun touches the earth’s atmosphere is by means of radiation.
The radiation by the sun is created due to the nuclear fusion reaction in the core of the sun.
The surface of the sun emits radiation approximately equal to 6.4 million W/m². These electromagnetic waves of radiation are propagated with a lower wavelength in all directions.
These electromagnetic waves don’t require a medium for their propagation therefore they can travel through the vacuum present in space.
Some parts of the radiation reach near the earth’s atmosphere at a heat flux rate of 1370 W/m².
After reaching into the atmosphere, the part of the radiation is reflected by the layer of atmosphere and clouds, and some part of the radiation is absorbed by the atmosphere while the remaining part of radiation directly comes on the earth’s surface.