Water Vapour

Water vapour is the most variable of the atmosphere’s gases, and its amount changes sharply with place, altitude, and temperature. Unlike the permanent gases that stay in roughly fixed proportions, water vapour is present in very different quantities depending on whether you are over a hot ocean, a desert, or a polar ice cap. This variability matters because water vapour is not just a passive component — it actively shapes weather, climate, and the heat balance of the planet.

The first thing to know is that water vapour decreases with altitude. The lower layers of the atmosphere hold far more moisture than the upper layers, simply because evaporation happens at the surface and the air thins out as you go up.

Its concentration also varies dramatically across the globe. In the warm and wet tropics, water vapour may account for four per cent of the air by volume — the highest share anywhere on Earth. In contrast, in the dry and cold areas of deserts and polar regions, it may fall to less than one per cent of the air. So the same gas that makes up a noticeable slice of tropical air is almost negligible in the coldest, driest places.

There is also a clear latitudinal pattern: water vapour decreases from the equator towards the poles. This follows directly from temperature — warm equatorial air can hold far more moisture than cold polar air, and the abundant sunshine and evaporation in the tropics keep the air humid throughout the year.

Beyond its quantity, water vapour plays a central role in the Earth’s heat budget. It absorbs parts of the insolation coming from the sun, and it also preserves the earth’s radiated heat — the longwave energy the planet sends back out towards space. By trapping this outgoing heat, water vapour acts like a blanket around the Earth. This blanket effect is what keeps the planet from becoming either too cold or too hot; without it, the surface would swing between extremes far beyond what life could tolerate. …