#scifi #climate Water is made up of two of the commonest atoms in the Universe—hydrogen and oxygen. Specifically, it is composed of two hydrogen atoms attached to an oxygen atom. For such a simple compound, it has some rather incredible properties.

It is as close to a universal solvent as any common molecule—something we do not always appreciate simply because the things dissolved by water mostly have been dissolved or (in the case of living things) have evolved membranes that keep water out. This probably played an important role in the evolution of life, and the search for extraterrestrial life is still focused on the search for liquid water.

It is both the brightest and the darkest substance on our planet—brightest in the form of snow or cloud tops (albedo 50% to 80%) and darkest in the form of deep ocean water (albedo 3% to 5%, though dark coniferous forests may be as dark.) This makes it very important in the overall albedo of Earth.

Water has a very high specific heat—which means it takes a lot of energy to change its temperature. It takes about five times as much energy to raise the temperature of a mass of water by a degree as it does to raise the temperature of the same mass of dry soil, rock or most building materials by the same amount. The result of this is that an enormous amount of energy can be stored in the oceans.

High as this specific heat is, the latent heat of water is even higher. Latent heat? This is the energy required to change phase—to evaporate water or melt ice. The melting of ice takes about 80 times the energy it takes to raise the temperature of the resulting water by 1 degree C. But this is dwarfed by the energy needed to evaporate water: 600 times the energy needed to raise the temperature by 1 degree C.

Why is this important? Because when the water condenses to form clouds, it gives that heat back to the air. But why does it condense?

Air can hold only so much water in vapor form. Further, the limiting amount is primarily controlled by temperature. Very roughly, when the temperature rises by 20 degrees F, the amount of water the air can hold doubles. When the air is at forty below (not a totally unreasonable temperature for cloud tops) it can hold about 3% as much water vapor as it can hold at freezing. At 68 degrees, it can hold about 3.8 times as much as it can at freezing. And at 80 degrees, it can hold almost 6 times as much as it can at freezing.

We’ve all seen condensation on iced drinks or cold windowpanes, when the air is holding more water vapor than would be possible at the temperature of the glass. If the air itself cools, this condensation will occur on dust particles in the air, and the result will be fog.

But when air goes up, whether it is rising because of buoyancy or because it is rising over terrain or a colder air mass, it expands and cools. The result is fog above the ground—a cloud. And as the water condenses to form a cloud, it releases the same latent energy to the air as it took to evaporate the water in the first place.

The result is a transfer of energy from the solar-heated surface or ocean to the higher parts of the atmosphere. We think of thunderstorms in terms of rain, lightning and tornadoes but they are in fact one of the most effective means of transferring energy from the Earth’s surface to the upper troposphere.

The same is true of frontal storms, but here the transfer is not only from the surface to higher in the atmosphere, but horizontal as well as vertical—from the equatorial regions to the poles.

Hurricanes deserve a post to themselves, but they also transfer energy from the ocean to high in the atmosphere. In fact, any time it rains, we are seeing a side effect of transfer of energy.

What would happen without this transfer of energy by latent heat? The winds would have to blow much harder to transfer the same amount of energy, and there would be much more contrast in temperature between poles and equator, and between the surface and the same height in the atmosphere. But the albedo and the greenhouse effect of water vapor would also change, so the overall effect on temperature would be hard to calculate and would depend very much on the albedo of the rock making up the surface of the planet.