Which of the following statements with reference to Water Vapour in the atmosphere is/are correct? 1. It has a role to play in the stability and instability in the air. 2. It acts like a blanket allowing the earth to neither become too cold nor too hot. Select the answer using the code given below:

Correct — C, Both 1 and 2. The examiner has lifted both statements from a single paragraph of the textbook, and both are physically sound. Water vapour governs stability because of latent heat. Evaporating a kilogram of water takes roughly two and a half million joules out of the surroundings, and every one of those joules comes back when the vapour condenses. A parcel of moist air that is forced to rise cools, reaches saturation, and then begins to condense — at which point the released latent heat slows its cooling, keeps it warmer than the air around it, and lets it go on rising by itself. That runaway ascent is instability, and it is what builds towering cloud and thunderstorms. Dry air has no such reservoir, cools faster as it rises, and settles back: stability. The blanket role is the greenhouse effect. Water vapour is largely transparent to incoming short-wave sunlight but absorbs strongly in the infrared that the warmed earth radiates back, holding that heat in the lower atmosphere and returning part of it to the surface. Take the vapour away and nights would be brutally cold; the same absorption also moderates the daytime heating. Deserts show the point in reverse — dry air, and a daily temperature range that can exceed twenty degrees.

• (a)1 only — Accepts the role in stability but denies the blanket, when water vapour is in fact the single largest contributor to the natural greenhouse effect — a bigger one than carbon dioxide.
• (b)2 only — Accepts the blanket but denies the link with stability. Latent heat released on condensation is exactly what makes moist air unstable and dry air stable, and it is the energy source of every thunderstorm and cyclone.
• (d)Neither 1 nor 2 — Rejects both, which would leave water vapour as an inert passenger in the atmosphere. It is the most important variable gas there is.

Water vapour is the atmosphere's great variable gas. Its share ranges from about four per cent of the air by volume in the warm wet tropics to under one per cent in deserts and polar regions, and it decreases both with altitude and from the equator towards the poles. It carries energy in latent form, releasing it wherever condensation happens, and it absorbs strongly in the infrared. Between those two properties it drives cloud, rain and storms, and it does more than any other gas to keep the earth's surface temperature within a liveable range.

A useful way to hold the two roles apart is to ask what water vapour is doing in each. In the stability role it is moving energy: heat taken up where evaporation happens is carried aloft with the vapour and released where condensation happens, which is why a humid afternoon builds cumulus and a dry one does not. In the blanket role it is trapping energy: it lets sunlight through and then intercepts the long-wave radiation the ground sends back up. Both statements are textbook wording, which is a signal in itself — when a two-statement item reads like consecutive sentences from a chapter, the answer is often that both are correct, and the examiner is testing whether you will second-guess a passage you have read.

• NCERT: water vapour 'acts like a blanket allowing the earth neither to become too cold nor too hot' and 'also contributes to the stability and instability in the air'.
• Water vapour is a variable gas: up to about four per cent by volume in the warm wet tropics, under one per cent in deserts and polar regions.
• It decreases with altitude and from the equator towards the poles, following the temperature of the air that has to hold it.
• Condensing a kilogram of water vapour releases roughly 2.5 million joules of latent heat — the energy that powers thunderstorms and tropical cyclones.
• Water vapour is the largest single contributor to the earth's natural greenhouse effect.

• Treating water vapour as a minor trace gas. It is variable, not minor, and at four per cent it outweighs argon several times over.
• Assuming warm air 'holds' more moisture because it has more room. The capacity rises with temperature because faster molecules escape the liquid more readily.
• Reading 'blanket' as reflection. Water vapour absorbs outgoing infrared and re-radiates it; it does not act as a mirror.

As a two-statement item on what water vapour does, or as a definition question on humidity, dew point or the form condensation takes at a given temperature.

• practice — not a real PYQ

  The release of latent heat when water vapour condenses in a rising parcel of air tends to make that air

  • (a)more stable, because it cools faster
  • (b)more unstable, because it cools more slowly than the surrounding air
  • (c)neutral, because latent heat is exactly balanced by expansion
  • (d)denser, because condensation adds mass

  Answer(b) more unstable, because it cools more slowly than the surrounding air — staying warmer than its surroundings, the parcel keeps rising on its own, which is what builds tall cloud.

• practice — not a real PYQ

  The proportion of water vapour in the atmosphere is highest over

  • (a)the polar regions
  • (b)the warm and humid tropics
  • (c)the mid-latitude deserts
  • (d)the upper stratosphere

  Answer(b) the warm and humid tropics — it can reach about four per cent of the air by volume there, against less than one per cent over deserts and the poles.