Paradoxically, the "gas" which is most important to greenhouse warming is one not normally thought of as a gas at all: water (H2O). We are used to thinking of water in its pure liquid form and seldom as gas or steam. The water vapour in the atmosphere is essentially molecules of steam - evaporated water - bounding around very thinly diluted among the gaseous nitrogen and oxygen molecules that make up the atmosphere. This water vapor can be experienced as humidity.

Water vapour plays a number of critical roles in affecting both climate and weather. The amount of water vapor in the atmosphere is not at all uniform - far from it - but changes drastically and abruptly, often in a matter of a few hours, to cause, for example, thunderstorms.

It takes a lot of energy to evaporate water. A molecule of water vapor "contains" much more energy than a molecule of liquid water. And quite a bit of water is evaporated every day as the Sun shines down on Earth's vast oceans. In short, water vapor is one of the most important "storehouses" of energy in the atmosphere and in the climate system.

Water loses energy when it condenses into the tiny suspended droplets which make clouds, or into the larger drops which constitute rain. The energy does not disappear, but instead heats the atmosphere. Thus, energy is redistributed through the process of evaporation and condensation.

When water vapor condenses to form clouds, it has another important effect: it "shades" Earth's surface and lower atmosphere. In the greenhouse analogy, rolling down a shade over the greenhouse would cool off the interior, just as it would cool a sunny room (Cloud formation is an important process in the climate system, but one that is hard to quantify and model). When clouds shade the Earth, some of the incoming solar energy is reflected back into space. Some also is absorbed by the clouds and re-radiated upwards and downwards. Thus some of the energy is caught at altitudes higher than Earth's surface, but still in the atmosphere.

But water in its vaporous state has an important heat-trapping "greenhouse effect". This is explained by water vapor's being relatively transparent to the shorter wavelengths at the visible and ultraviolet end of the light spectrum (the form which much incoming solar radiation takes). But after this energy has warmed the Earth's surface and been re-radiated upward in the infrared bands of the spectrum, water vapor readily absorbs it - trapping heat in the lower atmosphere (troposphere). Thus the water vapor is like the heat-trapping "glass" in the greenhouse analogy.

Eventually, this trapped heat finds its way upward and outward and is re-radiated into space. But first it works its way through various parts of the atmosphere. Because incoming solar radiation (now outgoing heat radiation) is thus delayed in returning to space, the temperature of the lower atmosphere is greater than it would be without the water vapor.

Human activities add and subtract water vapor from the atmosphere. But these amounts are insignificant compared to the amounts added and subtracted by natural processes. Water vapour is nonetheless important because its atmospheric-warming effects are huge, and because they are hard to quantify, model and predict. In fact, the amount of water vapor in the atmosphere is determined by the climate - a classic "feedback loop", There is also substantial scientific uncertainty about water's role as vapor or clouds. Since it has both warming and cooling effects, water is a "wild card".

Related Articles:

- Gases Introduction
- Carbon Dioxide
- Halocarbons
- Methane
- Nitrous Oxide
- Ozone
- Water Vapour