The correct option is 2 and 3 only.
Explanation
Water vapour is a variable gas in the atmosphere, ranging from 0% to 4% by volume. Its distribution varies significantly across different latitudes and altitudes, playing a crucial role in the Earth's energy budget and weather systems.
Statement 1 is Incorrect:
The moisture-holding capacity of air is directly proportional to its temperature. Warm air can hold more moisture than cold air. Since temperatures are highest at the equator and decrease towards the poles, the concentration of water vapour decreases from the equator towards the poles. In the warm and wet tropics, it may account for up to 4% of the air by volume, whereas in the dry and cold polar regions, it may constitute less than 1%.
Statement 2 is Correct:
The primary sources of atmospheric moisture are evaporation from water bodies and transpiration from plants, both of which occur at the Earth's surface. Consequently, water vapour is most abundant near the surface and decreases rapidly with altitude. Approximately 90% of the total atmospheric water vapour is found within the lowest 5 km of the troposphere.
Statement 3 is Correct:
Water vapour acts as a greenhouse gas. It allows incoming shortwave solar radiation to pass through (transparency) but absorbs a significant portion of the outgoing longwave terrestrial radiation emitted by the Earth. This absorption traps heat, acting like a blanket that regulates surface temperatures, ensuring the Earth does not become too cold at night or excessively hot during the day.
Key Takeaway:
Water vapour concentration decreases with increasing latitude (equator to poles) and altitude. It functions as a vital greenhouse gas, moderating Earth's temperature extremes by absorbing terrestrial radiation.