Correct Option
The correct option is 2 only.
Explanation
Water vapour is a highly variable gas in the atmosphere, playing a crucial role in weather phenomena and the Earth's heat budget. Unlike permanent gases like nitrogen and oxygen, its concentration fluctuates significantly based on location and temperature.
- Statement 1 is Incorrect: Water vapour does not constitute a constant percentage of the atmosphere. It is a variable gas, ranging from zero to four % by volume of the atmosphere. Its concentration is highest in warm and wet tropical regions (up to 4%) and lowest in dry and cold areas, such as deserts and polar regions (less than 1%).
- Statement 2 is Correct: Water vapour is defined as water in its gaseous state. It enters the atmosphere primarily through evaporation from water bodies and transpiration from plants.
- Statement 3 is Incorrect: The amount of water vapour generally 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 found at higher latitudes.
Key Takeaway: Water vapour is a variable gas that decreases in concentration with both increasing altitude and increasing latitude (from the equator towards the poles).