Man, it's been a month. A long hiatus. Now that I am somewhat stable over here in the desert, I have a little time to continue my series, do some other blogging, and in general just get back to writing, as much as I am able to.
In Part I, I talked about getting your planet set-up. In Part II, we discussed the placement of major land masses. So now let's talk turkey – or more precisely, water.
Water, that lifeblood of all things (or, at least the living things we know about), has a few magical weather properties. One is the ability to exist in all three states – solid, liquid, vapor – at the same temperature and pressure . More importantly, water has an enormous heat capacity compared to earth or air, which will presently become important. But we'll come back to that in Part IV. First, let's discuss how our water moves around. I am going to go on with the assumptions we made about our fantasy world being Earth-like in character.
I linked to the Hadley Cells and how they work. Because of near-constant heating at the Earth's equator, you almost always have heated air at the equator (scientifically known as the Inter-Tropical Convergence Zone). When air is heated, it rises. As air rises, it begins to cool. Air can only hold so much water vapor at a given temperature, and it is less at colder temperatures. So if air can hold X water at a temp, it might hold 1/2X at a lower temperature. The excess water precipitates out, first in the form of clouds, then rain and other phenomena. That constant rising air over the equator means a near-constant rain over the equator and tropical regions. This is all a fancy way of explaining why all the world's rainforests are in a thin equatorial band around the planet.
Further north and south, those same Hadley Cells produce a band of high pressure in the sub-tropics, about 20-30 degrees north and south of the equator. High pressure means the air is subsiding, or settling from above to the Earth. This is the antithesis of rain, meaning the sub-tropics tend to be very dry – hence the bands of deserts and desert-like environments around the 20-ish N/S of the equator.
Now this is where it gets complicated, because there are exceptions.
The American Southeast, by rights, should be a desert. It is about the same latitude as the Sahara. So what gives? Well, the band of high pressure sits directly over the Gulf of Mexico. High pressure (the big blue "H" you see on a weather map) rotates clockwise. So if you imagine a spiral pattern from that "H" over the Gulf, what do you see? You see air laden with Gulf moisture blowing up over the land, watering it ..,. whereas over the Sahara, a big "H" spinning draws up … dry air from the south. You can even see the difference between Georgia and Arizona. If the high pressure is some degrees south, winds from the southwest wrap moisture into Georgia. Winds from the southwest of Arizona bring dry air from the Mexican mountains.
These bands are not fixed. They migrate with the seasons – southward during the northern hemisphere's winter and northward during it's summer. This accounts for how during the summer, India enjoys the monsoon, receiving tropical levels of moisture during the summer but becoming drier under the high pressure in the winter.
So what do these things mean for your world? Well, how are your continents placed? You can expect a band of rain forest across the interior and deserts in the sub-tropics, unless you have some mitigating body of water as a source of moisture. We haven't even discussed the effects of topography; imagine there was a massive mountain range along the Gulf Coast of the United States. If there was, much of the southeast US would be very arid. The mountains would simply squeeze the moisture out, as happens in the Cascades in the Pacific Northwest. Coastal Washington and Oregon are quite green and lush. The eastern parts of the state? Not so much. This is something to think of when placing deciduous forests and lush green farmland. Without a steady supply of rain, especially if you are writing about a medieval-ish agrarian society (with primitive irrigation), these things don't exist. Pine forests are a little different matter. If you have had millennia of steady patterns of rainfall over some area, you would have the forests of eastern North America, western Europe and southern Brazil.
Remember, nothing about this is supposed to be ironclad, just to get you thinking about how you set your world up.
Next time, we'll tackle that heat capacity of water and how that affects things on a big and small scale, fronts, and some other details that shape your land.