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.