Which one of the following is not a factor that causes variations in

Which one of the following is not a factor that causes variations in ‘insolation’?

Rotation of the Earth on its axis
Angle of inclination of the Sun's rays
Length of the day
Vegetation cover on the Earth's surface
This question was previously asked in
UPSC CBI DSP LDCE – 2023
The correct option is D.
– Rotation of the Earth on its axis causes the daily cycle of day and night, leading to daily variations in insolation received at any point.
– Angle of inclination of the Sun’s rays varies with latitude and season (due to Earth’s tilt), affecting the intensity of insolation received per unit area. Lower angles (higher latitudes/winter) result in less intense insolation.
– Length of the day varies with latitude and season, determining the duration for which insolation is received, thus affecting the total amount of insolation received daily.
– Vegetation cover on the Earth’s surface affects how much of the received insolation is absorbed, reflected (albedo), or used in processes like transpiration. It does **not** affect the amount of insolation arriving from the sun at the top of the atmosphere or at the surface before interaction with the cover.
Factors affecting insolation received from the sun include solar constant, distance from the sun (Earth’s elliptical orbit), angle of incidence of sun’s rays (latitude, season, time of day), length of the day, and atmospheric transparency (clouds, aerosols, gases). Vegetation affects the Earth’s energy balance *after* insolation arrives.