Q. Normally, the temperature decreases with the increase in height from the Earth's surface, because
- the atmosphere can be heated upwards only from the Earth's surface
- there is more moisture in the upper atmosphere
- the air is less dense in the upper atmosphere
Select the correct answer using the codes given below; (UPSC Prelims 2012)
Answer:
1 and 3 only
Notes: The correct answer is
[C] 1 and 3 only. In the troposphere (the lowest layer of the atmosphere), temperature typically decreases with altitude at an average rate of
6.5°C per kilometer, a phenomenon known as the Normal Lapse Rate.
- Heating from the Earth's surface (Statement 1 is Correct): The atmosphere is largely transparent to incoming short-wave solar radiation. However, it is heated primarily by long-wave terrestrial radiation emitted by the Earth's surface. As a result, the layers of air closest to the ground are heated first and most intensely through conduction and convection. As you move further away from this "heat source" (the ground), the temperature drops.
- Air Density (Statement 3 is Correct): In the upper atmosphere, the air is much thinner or less dense due to the decrease in gravitational pull. Lower density means there are fewer molecules (like CO2 and water vapor) to absorb and retain the outgoing heat from the Earth. Denser air near the surface acts like a blanket, trapping heat more effectively than the rarified air at higher altitudes.
- Moisture Content (Statement 2 is Incorrect): In reality, there is less moisture (water vapor) in the upper atmosphere. Water vapor is a potent greenhouse gas that absorbs heat; its concentration is highest near the Earth's surface and decreases rapidly with height. Therefore, the lack of moisture in the upper atmosphere actually contributes to lower temperatures, making the statement that there is "more moisture" factually wrong.
Key Concepts of Atmospheric Heating:| Mechanism | Role in Heating |
| Terrestrial Radiation | The primary source of heat for the lower atmosphere. |
| Conduction | Transfer of heat from the warm Earth to the air layer touching it. |
| Convection | Vertical transfer of heat through rising air currents. |
| Greenhouse Effect | Gases like CO2 and Water Vapor trap heat; they are most concentrated at low altitudes. |