The correct option is Water Vapour.
Explanation
The gases in the atmosphere are broadly classified into two categories based on the stability of their concentration:
- Permanent Gases: These gases maintain a constant proportion in the lower atmosphere. Examples include Nitrogen, Oxygen, and Argon.
- Variable Gases: The concentration of these gases changes with time and place. Examples include Water Vapour, Carbon Dioxide, and Ozone.
Option Analysis
- Nitrogen Nitrogen: Incorrect. Nitrogen is the most abundant gas in the atmosphere, constituting approximately 78.08% by volume. It is a permanent gas with a fixed proportion.
- Oxygen Oxygen: Incorrect. Oxygen constitutes approximately 20.95% of the atmosphere by volume. It is a permanent gas essential for combustion and respiration.
- Water Vapour Water Vapour: Correct. Water vapour is a highly variable gas. Its volume in the atmosphere varies from zero to four %. It decreases from the equator towards the poles and absorbs parts of the insolation from the sun, acting like a blanket for the earth.
- Argon Argon: Incorrect. Argon is an inert, permanent gas constituting approximately 0.93% of the atmosphere.
Key Takeaway
Water Vapour is the most significant variable gas in the atmosphere, with concentrations ranging from nearly 0% in dry, cold regions to about 4% in warm, humid tropics.