Correct Option
The correct option is It decreases with altitude..
Explanation
Water vapour is a variable gas in the atmosphere, constituting between 0% and 4% of the air by volume. Its distribution is primarily determined by temperature and the proximity to moisture sources (evaporation from water bodies and transpiration from vegetation).
Option Analysis
- It increases from the surface upwards. is Incorrect: Water vapour is most abundant near the Earth's surface where evaporation occurs. As altitude increases, the temperature generally decreases in the troposphere, reducing the air's capacity to hold moisture. Consequently, the concentration of water vapour decreases rapidly with height. nearly 90% of the total atmospheric water vapour is found within 5 km of the surface.
- It is uniform across all latitudes. is Incorrect: The distribution of water vapour is highly variable and is not uniform across latitudes. It is influenced by the temperature of the air and the availability of water for evaporation.
- It decreases with altitude. is Correct: Water vapour decreases with altitude. This is due to the gravitational concentration of gases near the surface and the lapse rate (decrease in temperature with height), which limits the moisture-holding capacity of the upper atmosphere.
- It is highest at the poles. is Incorrect: Water vapour decreases from the equator towards the poles. Warm air at the equator can hold up to 4% water vapour by volume, whereas cold air at the poles holds very little moisture.
Key Takeaway: The concentration of atmospheric water vapour decreases vertically with altitude and horizontally from the equator towards the poles.