Planet Building 8: Weather

We’ve talked about the need to transfer energy from the surface of a planet to high in the atmosphere, where greenhouse gasses can radiate it away. But solar energy is not absorbed uniformly over the face of a round planet, or uniformly in time if that planet is rotating. This means energy must also be transferred horizontally, and water plays an important role here, as well.

Since we’re close to the equinox, let’s look first at how incoming energy is distributed then. The sun at this time of year is almost directly over the equator, and on the horizon at the poles. This means that the incoming energy is at a maximum—in both time and space—at the equator, and zero at the poles.  Unless the poles are at absolute zero (- 460 degrees F) energy must be transferred from the equator to the poles.

Remember that liquid water can store an enormous amount of energy as heat, and about half of that transfer is actually carried out by the ocean currents. When you add the rotation of the planet, we have currents like the Gulf Stream and winds in temperate latitudes generally blowing from west to east. The result is that west coasts are generally warmer than east coasts—southern France, for instance, is at the same latitude as Maine. This will carry over to any planet with oceans, and because oceans are so good at storing energy, it is true at any time of year.

The atmosphere also carries energy from the equator to the poles. In fact, this transfer of energy is what drives the whole weather system. The eddies we call cyclones and anticyclones, the lows and highs on the weather map, carry energy from the regions of surplus near the equator to the regions that radiate energy away near the poles.

Some of this energy is carried by warm air moving poleward while cold air moves equatorward.  But a large part is carried by latent heat, just as in pure vertical transport. As the warm, moist air moves poleward, it tends to ride over colder air coming equatorward, and as it is lifted and cools, water condenses. Thus the latent heat added by evaporation over the tropical oceans is released to the air at much higher latitudes.

What about other times of year, such as the solstices?

The solstices are defined as the dates when one or the other of the planet’s poles are pointed most nearly directly at the sun. This affects day length as well as the angle at which the sun falls on the surface, with the surprising result that for the earth’s axial tilt (23 degrees 27 minutes) the maximum incoming energy at the top of the atmosphere is at the summer pole, and there is very little variation of incoming energy with latitude in the summer hemisphere.

Ice at the Earth’s poles keeps surface temperature low and the low angle of the sun means more energy is absorbed in the atmosphere, but high midsummer temperatures at the pole would be quite possible on a world without water. If the tilt were increased even farther, to around 70 degrees, the annual average incoming radiation at the poles would actually exceed that at the equator.

Another important way in which water affects temperature is its large capacity for storing heat, which means that in summer water is generally cooler than land. This leads to coastal climates being cooler in summer than those inland. It also leads to the summer monsoons, when hot air rising over the continents sucks in the cooler, moister air from the nearby oceans.

This is reversed in the winter hemisphere, where the water is usually warmer than land. Cold air flows out from the continents, often triggering storms when it moves over the warm, moist oceans.

Because the energy input near the equator is only a little less than during the equinoxes, while that of latitudes above the polar circles is zero, a great deal of energy must be transferred from the equator to the pole of the winter hemisphere. Thus the violence of winter storms, as energy is transferred both upward and toward the winter pole.

Finally, the daily cycle is hardly noticeable in water temperature, but it has a large effect on land temperature. The result is local winds from colder to warmer temperature near the surface—the sea breeze in the daytime and the land breeze at night. In some cases these moist, cool winds from the sea trigger thunderstorms when they move over the warm land.

All of these types of weather will be found on any rotating planet with oceans of liquid water. Don’t think they are “earth-only” effects in building a planet.

Planet Building 6: Water

#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.