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. It plays a critical role in the Earth's energy budget and weather phenomena.
Option Analysis:
- It is a constant gas. is Incorrect: Water vapour is not a constant gas. Unlike Nitrogen or Oxygen, its concentration varies significantly over time and space.
- It increases from the equator to the poles. is Incorrect: The quantity of water vapour decreases from the equator towards the poles. This is because warmer air (found near the equator) has a higher capacity to hold moisture compared to the colder air at the poles.
- It decreases with altitude. is Correct: Water vapour decreases rapidly with altitude. Approximately 90% of the total water vapour in the atmosphere is concentrated within the first 5 km from the Earth's surface.
- It is opaque to incoming solar radiation. is Incorrect: Water vapour is largely transparent to incoming shortwave solar radiation. However, it is opaque to outgoing longwave terrestrial radiation, meaning it absorbs heat radiated by the Earth, contributing to the greenhouse effect.
Key Takeaway: Water vapour is a variable greenhouse gas that decreases in concentration with increasing altitude and latitude (from the equator to the poles).