*This answer is for an Earth-Like planet, rotating in the same direction.
First question: Where does the wind blow? It depends on the pressure.
Movement of air masses: Hot air rises and cold air descend, it’s a convection movement like the one you can observe when boiling water. The air flows from the high pressure zone to the low pressure zone. The hot air expands and rises in the atmosphere. This is drawing the air towards the hot areas that are in fact, low pressure. Cold air contracts and will eventually sink. When the air is descending on the land, it means that it’s a high pressure zone.
Precipitations occur when the air is rising. Other factors also generate precipitations but this is the most important. As the hot air rises over the land it cools off while ascending in the atmosphere. As the air gets colder, it cannot contain as much humidity compared to hot air and this water will fall down. On the opposite of the convection movement, the cold sinking air is always dry since it already got rid of most if not all the humidity it once had.
4 Areas of low and high pressure: As a general rule, these ‘’areas’’ move closer to the North Pole during the northern summer and closer to the South Pole during the southern summer. The movement is more pronounced over land than over the oceans, especially if the landmasses are located over the 30th parallel. This is because the temperature over the land has a larger variation throughout the year than the temperature over the oceans.
- ITCZ: Inter tropical convergence zone: This is a low pressure area
located loosely around the equator because it is the hottest place on
the planet and very hot air means very low pressure. The ITCZ gets
dragged over land if there is a large landmass in higher latitudes
during the hot season. Over the ocean, the ITCZ stays at the same
spot all year long.
- Subtropical ridge (also known as the Horse latitudes): it is located
around the 30° north and south of the equator. This is a high
pressure zone despite the relatively hot temperature. (More
information on this in the ‘’Movement of the air’’ section below)
Most deserts are found here but not all of this area is made of
deserts. You also need to consider the direction of the winds.
- Polar front: This is a loosely defined area with a relatively low
pressure in the mid latitudes (40° to 60°). The weather under the
Polar front is considered unstable or prone for rapid and often
unpredicted changes in the weather. The hot air from the tropics
encounters the cold air from the poles. Remember, here we have rising
air and precipitations. On the opposite, the subtropical ridge is dry
because it is a high pressure area with sinking air and
precipitations occur when the air is rising. (Mostly but not always)
- Poles: lastly, the poles are the coldest places on Earth and so, it’s
a very high pressure area.
Movement of the air: http://en.wikipedia.org/wiki/File:AtmosphCirc2.png
Coriolis Effect: if the planet was not rotating, the winds would go straight poleward. But since the planet is rotating, the winds are
deflected. The winds are deflected in a clockwise manner in the
northern hemisphere and counter clockwise in the southern hemisphere.
By itself, Coriolis doesn't make the currents, it really just
deflects them. http://en.wikipedia.org/wiki/Coriolis_effect
*The Coriolis Effect also apply on the water currents.
1-Hadley cell: Between the ITCZ and the subtropical ridge Following a convectional movement, hot air rises and the surrounding
air masses converge there to fill the gap. The air rise and then
moves toward the poles. It cool off with time and eventually the
increase in pressure will drag the air mass down near the 30° north
The surface winds are moving toward the equator because of the
pressure and the Coriolis effect is directing them toward the west at
the same time. The dominant winds are blowing east to west and are
called the north /south trade winds.
2-Ferrel cell: Between the subtropical ridge and the Polar front: The dynamic of this cell is mostly imposed by the other 2 cells and
it just follows a logical continuation of the same convection
movement. The rising air converges at the Polar front. At the
subtropical ridge, the air is sinking. So, you have the cold dry
sinking air at the subtropical ridge. This air will warm up until it
reaches the Polar Font and then it will rise again.
The surface winds are moving toward the poles, to the low pressure
area that is the Polar front. The Coriolis effect deflects them
toward the east. The dominants winds are west to east and are called
3-Polar cell: Between the Polar front and the pole Here, the very cold air creates a high pressure area. The air sinks and then moves
toward the equator. Getting closer to the equator, the air starts to
get warmer until it reaches the 60° latitude. At that latitude, the
air has become hot enough and start ascending in the atmosphere.
The surface winds are moving equatorward. Here, I think they are
deflected toward the west but I’m not 100% sure. So, the dominant
winds are moving east to west and are called the Easterlies.
- Bonus: Doldrums: This is an area near the equator where the winds are usually very weak. They are in the middle of a large low pressure
Second question: Where does the water go?: I will just cover the surface currents. Not the deep currents or the counter currents.
The ocean currents are influenced by 3 things: the winds, the Coriolis Effect and the landmasses.
Starting from the equator, the trade winds are pushing the water
toward the east. Then, when it reaches the coasts of the continent,
the water will be deflected toward each pole because of the winds and
the Coriolis force. It flows towards the pole until it reaches the
Ferrel cell over 30th of latitude. There, the water is pushed toward
the east by the Westerlies. The Coriolis force curves the shape of
the current that is in fact not really toward the east but also
toward the north a little. Eventually the water will reach another
continent. Winds are probably still pushing it toward the land so the
water current usually splits here. Some of the water will go north
and the rest will go to the south. The northern current should
continue its course following the established rules. The southern
current will stay close to the coast until it closes the loop near
If you don’t have a continent, there is likely nothing to stop the
movement of the water as long as the winds are pushing this water.
This is why the currents of the South Seas are spinning around
Antarctica. Antarctica is almost cut off from the ocean circulation.
It is surrounded by water and by a strong current that goes across
the whole planet. This current limits the heat exchange and is
keeping the continent colder. If we were to close the Magellan strait
between Antarctica and South America, it would cut this cold current
belt and Antarctica would be less cold since the polar waters would
mix with the rest a lot more than they do right now. This would also
make it possible for ice sheets to form. Strong currents are
preventing the formation of ice sheets.
The oceans play an important role in lowering the temperature
differences between the different regions of the planet. The currents
are taking the hot water from the equator and mix them with the cold
waters. It is something important to consider in a fantasy world.
Without this heat exchange, the equator would be much hotter. On
Earth, we have north-south oceans (Pacific and Atlantic) and this is
good for the heat exchange. Heat exchanges would not be the same if
America was an east-west continent because it would prevent this
mixing of hot and cold waters. The impact could he huge unless the
continent was located right on the equator. It that cases the impacts
would be smaller.
These are just general guidelines to set the ocean currents. The land is a really big factor influencing them. Here’s a good map for reference: http://upload.wikimedia.org/wikipedia/commons/6/67/Ocean_currents_1943_%28borderless%293.png
Third question: Where does it rain?
- Where the air is rising: near the Polar front and near the ITCZ.
Under the polar front, the precipitations are also caused by an alternation of hot and cold air masses. The limit between the Ferrel
cell and Polar cell has a shape similar to a wave.
The temperature of Chicago is colder than the temperature of
Anchorage, Alaska, even if Anchorage is closer to the pole. It’s
because Chicago is affected by the cold Polar cell and Anchorage is
under the hot Ferrel cell. The Polar cell is moving to the east so
Anchorage should expect rain (or snow most likely) in the following
days. As the air is getting cooler, it starts to rain. Here, the
winds are not always relevant. As long as there is moisture in the
air, you can have precipitations even in places with high pressure
- Most importantly, moisture will go where the winds go. The Sahara is a high pressure area but it pushes the surface winds toward Europe
and the Sahel, therefore all this air is dry and the Sahara receives
little rain. Libya is very dry despite being so close to the sea
because the winds are blowing offshore.
- Moisture traveling overland: Moisture in the air comes from evaporation. The evaporation is more significant when it’s hot and
over the water. The evaporation is still large overland but the
quantity of water is smaller. Forested areas like the Amazon basin
keeps a lot of moisture and this moisture make some areas more humid
than they would be without the forest. The Winds will carry the
moisture overland but not over the mountains. The moisture can travel
very far over the flatlands.
- Orographic lift: it is well know that air is getting colder at higher altitudes. This means more rain. Northern India (the state of
Maghalaya) is a good example of this effect. This is why, even if the
air masses could reach the other side of the mountains, the air would
be dry anyway.
- Mountains: The impact of mountains is really important. They prevent the precipitation in places that are in the rain shadow. If
the dominants winds are from the west, places located east of the
mountains will usually be dry.
Part four: the climates:
• Don’t forget that when it’s summer in the north it’s winter in the south. (Unless if you are in Walvis Bay, there is only one season there.)
Definition from Wikipedia:
Climate is a measure of the average pattern of variation in
temperature, humidity, atmospheric pressure, wind, precipitation,
atmospheric particle count and other meteorological variables in a
given region over long periods of time. Climate is different from
weather, in that weather only describes the short-term conditions of
these variables in a given region.
- Now we should have everything needed expect for the atmospheric
particle thing (don’t know what it has to do with climate, climate
changes maybe?) and we haven’t talked about humidity much yet. But we
will come back to this very soon.
- So, now from what I understand, a climate is a mixture of many
ingredients. It is pretty confusing to make sense of that. I need to
use a climate classification to simplify things and add another wall
Climate classification: There are a couple of classifications available for climates. We have at least 2 major systems worth to talk about:
Holdridge: http://en.wikipedia.org/wiki/Holdridge_life_zones It’s nice but in order to use it in world building, you will need to find
how to calculate the potential evotraspiration. That requires a lot
of information that we can only guess.
Köppen: http://en.wikipedia.org/wiki/K%C3%B6ppen_climate_classification This
is one of the most commonly use in the world. It is not perfect but
will do just fine in world building. To use it, you need the
temperature and the precipitation of the world. In fact, we are using
an improved version of this classification called the Köppen–Geiger
climate classification system.
Glenn Thomas Trewartha added several things including the Universal Thermal Scale. We will use that scale as a reference for
*Now, I’m not going to explain what the climates are, it would be too long. I will just talk about where to place them and the conditions
required to have them in a specific spot. Csa on the east coast, I
don’t think so!
The Köppen climate classification scheme
- The scheme is made of a maximum of 4 letters. Real climatologists
might use up to 4 letters but to make things less complicated, we are
not going to use more than 3. The first letter separates the climates
in five large categories. Each letter can be combined with a second
letter, and some can also be combined with a third letter. The
combinations are in the last part.
Click here for full size version
Now, I will try to explain starting from the equator and going to the pole. Keep in mind that it is a simplistic explanation.
- The ITCZ moves north during the northern summer and south during the
southern summer, influenced by large landmasses. Areas always
affected by the ITCZ, or close to it, will be in the A climates. Af
is the closest and has no dry season. Am is less affected by it and
does have a dry season. Aw has the driest dry season and is only
slightly affected by the ITCZ during the cold season. The climate
becomes progressively driest as we get closer to the tropics.
- Areas affected only by the summer ITZC will have very dry winters
and usually falls into the BSh climate, the hot steppe. This is true
only inside the tropics. Outside the tropics, the temperature is
colder, there is less evaporation and the land stays humid. This
creates the monsoonal climate Cwa. Cwb and Cwc are colder than Cwa
and appear at higher altitudes. Sometimes, as in Angola, the Cw
climates come before the hot steppe because the altitude is higher
and reduces the temperature and the evaporation.
- Moving a step further, we are now almost at the tropics. This is
usually where the Hadley and Ferrel cells meet. Unlike in the
precedent paragraph, there is no low pressure system here, it is
always dry. Here, the hot desert (BWh) is the most common climate.
But outside of the tropics, the lower temperature makes it a cold
As we move away from the tropics, the climates gradually become more
humid as the influence of the polar front starts to increase. The
hot season is dry under the subtropical ridge but winters are wet
under the polar front. Deserts become colder and are often bordered
by another band of steppes. As they are colder, they usually fall
into the cold steppe climate (BSk) but hot steppes are still
possible. Here, the desert can be affected by low pressure systems
but are too far from the ocean or the rain is blocked by the
Eventually, the humidity increases and we arrive at the
Mediterranean climates: Cs, Ds. The Mediterranean climates are only
found on the west coasts. They are more humid than the steppes
because they are usually close from the sea and/or at higher
latitudes. This means they are colder and are more affected by the
polar front. The steppes are almost at the limit of the front’s
Cfa: One exception to points 3 and 4 is the Cfa
climate. Unlike the other areas near the tropics, it is always under
a low pressure system: ITCZ + polar front or always under the polar
front. It is usually close to the tropics on the eastern side of
Mid latitudes are always under the polar front: Cf, Df. This is
usually what we refer to as the typical temperate climates. Although
poleward latitude might be outside the influence of the winter Polar
front, they are very cold so they have very low evaporation rate.
Latitudes close to the equator will feature mild winters with
temperatures above the freezing point even in the coldest month. The
temperatures on the west coasts are a little hotter and more
temperate than on the east coasts because they are affected by hot
oceanic currents. The east is affected by cold currents at these
latitudes. The temperature gets more extreme as we move closer to
the pole and away from the ocean.
Eastern Siberia: This is the place on Earth with the most extreme
climates. Unlike point 5, it’s drier because they are directly under
the Polar cell. This place is usually affected by the polar front
only in summer partially.
Dw climates: are another exception. Beijing should have a similar
climate to New York but it’s not the case. It’s specific to Asia,
or large landmasses. These places are affected by the Polar front
only during the summer. The winter is characterized by the high
pressure system around Mongolia and Siberia. Without this Siberian
anticyclone, the climate would be like in North America. If it make
things easier, it’s like in point 2 but it’s colder and we replace
the ITCZ by the Polar front.
As you see, there are no large steppes or desert in eastern China.
This is because the pressure is so low in summer that the gap
between the ITCZ and the polar front is small.
- This last area is particular for her low temperature. It is the most
notable trait, since it encompasses almost all precipitation