World-Building #2:


Siting Jungles and Deserts

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There's a rough and ready rule of thumb on what you will find on a northern hemisphere continent:

Southwest is desert, southeast is tropical storms and regular rain, northwest is very wet, northeast is well-watered, and north-south corridors have tornadoes.

This is a vast oversimplification, but it's a good chant to remember. In a southern hemisphere, swap north and south, but keep east and west the same.

Climate is controlled by the movement of air between the poles and the equator, over land or water, and by the Coriolis-effect swirls of air and ocean currents, as shaped by surrounding continents. Climate builds up certain ways in certain areas, and to change this for your particular planet, so as to have abnormal climate zones, would require so much magic that water is likely to run uphill and gravity reverse itself in places. So don't let's do that.

Once you have roughed in your continents, you have to figure where the equator is, where the lines of the tropics fall, and look at the sea-coasts. Ocean currents always run in a clockwise direction in the northern hemisphere, counter-clockwise in the south. Look at the direction water swirls down a drain, and you can tell which hemisphere you're in: it's that pervasive. North is the direction on your left when you face sunrise.

Please, don't say "Well, on this planet the sun rises in the west." There is no absolute up and down in the universe. All planetary directions exist only relative to the course of the sun. By definition, east is "the general direction of sunrise." So the sun can't rise in the west, unless the sun is rising in what used to be the west, and the direction names have shifted after some cataclysm. One of the magnitude to reverse planetary rotation that quickly is guaranteed to rip the planet into an asteroid belt so, again, it is not in the realm of plausibility. (We'll leave "astronomical" north, south, east and west out of this because that's planetary chauvinism, applying the poles of one planet to all the solar system's or galaxy's rotations. It's thoroughly unhelpful to planet building.)

The line of the tropics is harder to define. It is the line farthest poleward from the equator where the sun will be dead overhead on midsummer day. "The tropics" lie between these two tropic lines, a band that straddles the equator. Fast fix: all spheres will be proportional, so put it the same relative distance as lies between the Tropic of Cancer and the Tropic of Capricorn on this planet. Of course, for that you have to have not only the equator and continent of interest, but an idea of how far it is to the pole. That also assumes the axis of the planet tilts about the same as ours. The more the planet tilts, the more extreme its seasons and the narrower the band of the tropics.

Now, down in the tropics you will have a tropical climate. Duh! But you would be surprised how many world-builders try to have a hemisphere-spanning continent with the whole thing a temperate climate, because that's the only climate they think in. Can't be. In the tropics, you have to go to high altitudes to get snow. Which does happen: Kilimanjaro sits on the equator, and has a glacier. There is a snow-skiing resort in Hawai'i, on the highest peak in the islands. The astronomers at the observatory there have friends Mainland send them snow chains for their cars.

Smack on the equator is a wet zone, tropical rainforest usually. This is a band of some width. Get out your world climate map or globe, and look at it. However, it does not encompass the whole of the tropics. It will extend to the oceanic tropics, your classic tropical islands in the sea. Continents are what shove the zone around by distorting ocean currents.

Back to ocean currents.

Let's discuss the northern hemisphere for simplicity.

On the west coast of a continent, the currents will be pulling arctic cold water down the coast, like the Humboldt current does past California. Swimming off LA can be like stepping into liquid ice. On the other hand, the water is warmer off New England: this area is dominated by the current carrying tropical warm water poleward, the Gulf Stream. The Black Current of Japan acts the same way. Air currents ride the same path, and are affected by the temperature of the ocean current below them. A warm current makes for moisture-laden air. This starts dropping out as rain when the air is cooled as the current cools, or when the air strikes land, especially mountains.

Because of this, tropical cyclonic storms are carried west to the continent, then northward up it, in a regular summer series. That is why the Caribbean and the eastern US suffer from hurricanes, and eastern Asia from typhoons. Different names, same kind of storm.

In the southern hemisphere, the currents are sweeping tropics to pole on the eastern side of land masses, in those ocean basins, which is why the northeast of South America and India have hurricane/typhoon seasons. The storm coasts will be plenty damp enough throughout the year to not ever be deserts. A cyclonic storm area is one place you just cannot put a desert.

This moist, warm current actually holds all around the bend of the basin, so the northeast of the basin, the northwest of the next land mass, is also quite damp, though cooling: the British Isles, the Pacific Northwest, Burma, for examples. But after it has dumped its last load of moisture there, it dries out and becomes only a fitful bearer of rain. Well north of the tropics its path will cause areas with a "Mediterranean climate" -- rain for a short season each year, dry the rest, and warmish because of the short distance from the tropics. You see this in southwest Europe (Spain) and on over the African coast, and North America (southern California and northwest Mexico).

In the southern hemisphere, the Thaar Desert of western India is located right opposite the typhoon zone, just as the Mojave is opposite the hurricane zone. Look at Australia for where its deserts and rainy areas lie: it proves out, too, in mirror image.

Additionally, climate gets drier inland. There is a "rain shadow" (that's the technical term) to mountains that can't be ignored. The Sahara is in the rain shadow of the Atlas and other mountains. The southwest deserts of North America are the result of the rain shadow of the western mountain ranges. India creates a rain shadow for most of the Mideast and part of eastern Africa.

If your continent spreads greatly east to west, its interior will resemble Asia, with an eastern rain shadow, too. There is a limit to how far inland the pole-bound current can send rainclouds, and if there are mountains which force it to dump even more of its rain load, you will wind up with rain shadow deserts like the Gobi. It could be said that the western deserts of the US are a meeting of eastern and western rain shadows. Rain shadow deserts, because they are not caused by the distance from the pole, may be quite northerly and miserably cold much of the year, like the Gobi or the central Eurasian deserts. Think of how dry eastern Washington and Oregon can be. Even regular deserts turn remarkably cold at night, especially in winter. In summer, the night coolth may be a savior. This rapid temperature shift is due to the fact that the dry air doesn't hold heat very long after the heat lamp called the sun sets.

This double rain shadow reached an extreme when the Earth had only one large continent. The interior was all Sahara or Gobi, with only specialized small animals that could stand the constant drought. Remember this when you are thinking of making one giant continent for your world. The interior will not really be habitable by humans, and completely unfit for agriculature. Consider, instead, two to four continents separated only by narrow seaways. It makes a difference.

Mountains have another effect: they can block or funnel polar air moving towards the tropics over the land. If you look at Eurasia, many high mountain ranges run east to west, blocking the mix of tropical and polar air. This is a second reason Central Eurasia is so dry. On the other hand, in North America the major ranges run north and south, creating a channel for the violent intermix of hot and cold continental air masses. Guess who has the annual swarms of tornadoes? Such whirlwinds were highly unusual in the Old World, real divine-wrath rare. They were most likely to strike in the Mideast, though, where the Black Sea creates a gap for polar air to pass down. In the Midwest, they are positively common in tornado season. At any time in the Tornado Alley states, low dark clouds need to be watched for funnels reaching down.

On the other hand, a tornado alley allows moist tropical air to spread north and bring rain. This is why the Midwest, while prone to droughts, is much wetter than Central Asia, blocked by mountains to the south.

These conditions can be ameliorated or shifted to a certain degree. A lot of desertification in the Mideast is due to salinization of the soil from irrigation, until it can't support heavy vegetation. Otherwise, you should remember that the area was once so fruitful that Herodotos said Greek readers would find the yield per acre literally beyond belief. That was in an era when salinization was so advanced that barley had to be grown in place of wheat! This is a desert created by millenia of agriculture, not by wind and rain.

If your world is just coming out of an ice age, deserts may cease to exist. In the tropics, ice ages are marked by "pluvials" -- rain ages. This means that while the sub-polar areas are under continental glaciers, the tropics will be thoroughly soggy, and the well-watered zones will spread outwards. This is why the Sahara was once green.

As well, when the world is coming out of an ice age, the climate may be drier and warmer, but there's still a tremendous amount of ice locked up in mountain glaciers. This may take centuries or millenia to melt. While this goes on, the path of the meltwater may green areas which, by the rules of rain patterns, ought to be desert. This happened in the Indus Valley. The Sarasvati River, which now doesn't even run all year long, became the channel for glacial melt. Many streams which now feed into the Ganges to the east fed west into the Sarasvati, which in some places farther down was three miles wide. That's no minor creek! This rainless water supply allowed the Vedic Indus Valley civilization (known from sites like Harappa and Mohenjo Daro) to grow and flourish where now the living is strictly desert hard scrabble. Just remember that in this stage of de-glaciation much of the north will still be under continental glaciers, and the northern climate zones will press somewhat farther south.

How do you extend deserts if you want more in your world? You have to make the last ice age a long time ago, and follow the curves of global warming and drying. This is not a steady line, but more like a roller-coaster. The present panic about global warming seems a bit silly, when you realize we are coming out of a miserable cold period called The Little Ice Age, full of famines and even one Year Without a Summer (1816) when it snowed in high summer in a number of temperate places like Massachusetts. We are nowhere near the Big Climatic Optimum or Little Climatic Optimum, which were the warmest, driest periods of climate. In short, you start with the natural deserts and all over the planet you extend them outward. You can't do it just on one continent and not on the others.

One last point: on a planet whose atmosphere permits it to hold water, the water never just disappears. In warm eras, there is less rain but also less glacial ice: the water will create higher sea levels. In ice ages/pluvials, the sea level drops, because so much water is sitting above water level as glacial ice. So if you are dealing in this planet down the road, or up the road, or any planet over time, remember: you have to balance ice level and sea level. You can't have a warmer, drier climate with the seas evaporating: this is a closed system, and the vapor isn't going outside and disappearing. Mars obviously suffered quite a cataclysm to lose its water. It doesn't happen as a "steady state" evolution!



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