Previous Year Question
Consider the following statements.
High clouds primarily reflect solar radiation and cool the surface of the Earth.
Low clouds have a high absorption of infrared radiation emanating from the Earth’s surface and thus cause warming effect.
Which of the statements given above is/are correct?
Correct Answer: D
Detailed Solution
Clouds influence Earth’s energy balance through two opposing processes:
They produce a cooling effect by reflecting incoming short-wave solar radiation back to space.
They produce a warming effect by absorbing and re-emitting outgoing long-wave infrared radiation from Earth.
The net effect of a cloud depends upon its altitude, thickness, temperature and the amount of water droplets or ice crystals present.
Statement 1 is incorrect:
High clouds such as cirrus are generally thin and composed largely of ice crystals.
Because they are thin, most incoming solar radiation passes through them and reaches Earth’s surface; they are therefore not highly effective reflectors of solar radiation.
High clouds are extremely cold. They absorb infrared radiation emitted by Earth and re-emit it to space at a much lower temperature.
This reduces outgoing energy, and their usual net effect is warming, not cooling.
Statement 2 is incorrect:
Low, thick clouds reflect a substantial proportion of incoming solar radiation back to space.
Low clouds also absorb infrared radiation, but their temperature is relatively close to the surface temperature. Their greenhouse warming effect is therefore weaker than that of high clouds.
Their strong solar-reflection cooling effect generally exceeds their infrared warming effect. Consequently, the usual net effect of low clouds is cooling.
Thus, the question reverses the principal effects of high and low clouds, making both statements incorrect.