Correct Option
The correct option isIt forms when water evaporates from land and water bodies.
Explanation
Water vapour is the gaseous state of water and constitutes a variable component of the Earth's atmosphere. Unlike permanent gases (such as Nitrogen and Oxygen) which maintain a constant ratio, the concentration of water vapour fluctuates significantly based on temperature and location.
Option Analysis
- It is a permanent gas that does not change in quantity. is Incorrect: Water vapour is not a permanent gas; it is a variable gas. Its concentration varies from nearly 4% by volume in warm and humid tropical regions to less than 1% in cold and dry polar regions or deserts.
- It forms when water evaporates from land and water bodies. is Correct: Water vapour is added to the atmosphere primarily through evaporation from oceans, lakes, and soil, and through transpiration from plants. It represents the transition of water from a liquid state to a gaseous state.
- It is only found in the clouds, not in the air we breathe. is Incorrect: Water vapour is an invisible gas present in the ambient air, including the air humans breathe. Clouds are visible aggregates of tiny liquid water droplets or ice crystals formed by the condensation of water vapour, not the vapour itself.
- It is the main component of the Exosphere. is Incorrect: The overwhelming majority of water vapour (approximately 99%) is contained within the Troposphere (the lowest atmospheric layer). The Exosphere is the outermost layer, composed primarily of extremely low densities of hydrogen and helium.
Key Takeaway
Water vapour is a variable gas concentrated in the lower atmosphere (Troposphere), decreasing with altitude and latitude. It acts as a significant greenhouse gas and is the source of all clouds and precipitation.