Siting Jungles and Deserts
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
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
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!