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Once geological processes and erosion have created a landscape this landscape will in turn alter the planet. Ocean currents and trade winds will tend to form and certain weather patterns will emerge. To keep this reasonably scoped we are just going to look at the currents, wind patterns and the resulting precipitation and climate effects. Landmass formation and erosion has been covered in previous questions and the resulting effects on life and biomes will be covered in a follow-on question.

  • What are the processes that drive weather and ocean patterns?

  • How do these processes shape the weather and the climate?

  • How can those processes be easily drawn upon to create realistic looking maps?

There are already good climate classification systems such as the Köppen Climate Classification. We do not need to redefine those or list the climates. Instead we are looking to described the processes that result in these climates and use that to inform the creation of a map that has a realistic climate distribution.


Note:

This is part of a series of questions that tries to break down the process of creating a world from initial creation of the landmass through to erosion, weather patterns, biomes and every other related topics. Please restrict answers to this specific topic rather than branching on into other areas as other subjects will be covered by other questions.

These questions all assume an earth-like spherical world in orbit in the habitable band.


See the other questions in this series here : Creating a realistic world Series

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    $\begingroup$ the dwarf fortress map creation will do most of that (it won't use winds though IIRC) $\endgroup$ – ratchet freak Oct 9 '14 at 13:04
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    $\begingroup$ I still think that this need to be addressed in separate questions. The climate at least need a specific question. It's not a bad thing if it increase our number of questions per day. $\endgroup$ – Vincent Oct 9 '14 at 16:28
  • $\begingroup$ @Vincent I did consider this and even split it in the first version of the question, the sections of it are so inter-dependant though that it's hard to separate them. $\endgroup$ – Tim B Oct 9 '14 at 16:56
  • $\begingroup$ I see, I understand what you mean. $\endgroup$ – Vincent Oct 9 '14 at 22:51
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    $\begingroup$ This what-if.xkcd can be used as a tutorial. $\endgroup$ – Cephalopod Oct 13 '14 at 13:54
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*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 and south.

    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 the Westerlies.

  • 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 zone.

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 the equator.

  • 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.

    For example: http://3.bp.blogspot.com/-rZPe2PJhFKE/UuNaB8HxEeI/AAAAAAAAk08/Hviu1TrSk-4/s1600/Screen+Shot+2014-01-25+at+12.30.08+AM.png

    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 sometimes.

  • 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 of text.

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 the temperature. *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.

enter image description here 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.

  1. 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.
  2. 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.
  3. 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 desert (BWk).
  4. 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 mountains.

    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 influence.

    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 continents

  5. 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.

  6. This last area is particular for her low temperature. It is the most notable trait, since it encompasses almost all precipitation patterns.
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  • $\begingroup$ The message still need editing but I will continue tomorrow. Apparently, I have reached the limit of 30 000 characters... $\endgroup$ – Vincent Oct 10 '14 at 4:20
  • $\begingroup$ this is part of a document I did not so long ago. $\endgroup$ – Vincent Oct 10 '14 at 4:23
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    $\begingroup$ Wow, this is a very detailed descriptions of the weather and climate systems! I'm wondering if the presentation of the classifications can be simplified somehow though? Maybe placed into a table or diagram. That will reduce the word count and also might make it easier for people to understand the relationship between the climate types. (Or if you want I can ask a separate question about climate classifications and we can put that part of it into an answer there and link to that answer from this one?) $\endgroup$ – Tim B Oct 10 '14 at 8:37
  • $\begingroup$ both ideas are good actually. I just need to figure out the best way to display the information. $\endgroup$ – Vincent Oct 10 '14 at 16:20
  • $\begingroup$ Ok, cool. If you want me to ask the second question just let me know $\endgroup$ – Tim B Oct 10 '14 at 18:17
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Global Circulation, Precipitation and Climate

Hadley cells

In meteorology, there is a very useful concept of Hadley cell. The biggest insolation is at the equator and from there, the heat has to be transported to the poles. In this process, hot air rises at the equator, because it is lighter than the cold air. The hot air cools down as it expands, which decreases its capacity to hold water and consequently, there is always lot of rain around the equator. The dried air then moves polewards. During this process, it turns eastwards because of the Coriolis force. Here, the rotation speed of the planet starts to be important. When the air completely turns eastward, the Hadley cell is disrupted and the air start going down. It becomes hotter and therefore dry, as is contains not much water. At the end of the first Hadley cell, there are usually deserts. For Earth, this happens around latitude 30°. Slowly rotating planets, like Venus, will have its Hadley cell uninterupted and this will not happen until the poles. Quickly rotating planets would have deserts and eastward winds much closer to equator - maybe there would not be big deserts at all, since they would merge with the rainy equator region completely.

Overview

Especially for non-earthlike planets, these are rather plausible guesses than hard facts. Use with caution. Temperature differences are for the 1 atm atmosphere. For denser atmospheres, they will be smaller, for thinner, they will be larger. Great references can be found here and here, and for a visual, you can look here.

Earthlike planet

  • Equator area will be rainy. (If there is enough oceans around the equator.) Prevailing winds will be mildly westward, compensating the strong eastward jets at higher latitudes
  • Around lattitude 30°, there generally will be deserts, although some special conditions can prevent it. (Sahara was rain forest once.)
  • Between latitude 30° - 60°, prevailing wind direction will be eastward.
  • Difference between average temperatures of equator and poles will be approximately 40 K.

Quick rotation (~5 hours)

  • Equator area will be rainy. (If there is enough oceans around the equator.)
  • Heat transfer is greatly reduced as the Hadley cells are disrupted, temperature differences between poles and equator might be 80 K or more.
  • There will probably be no sharp region of deserts, since the Hadley cells will be small.
  • There will probably not be very pronounced areas with eastward or westward directions of winds, again because of small Hadley cells.

Slow rotation (but still faster than 1 day = 1 year)

  • One big Hadley cell. Due to lack of strong Coriolis force, it might be quite weak. It is therefore probable that atmosphere will superrotate, like on Venus. (Prevailing winds are eastward everywhere, or westward everywhere. Depends how it started, it keeps the original direction most of the time.)
  • No distinct zone of deserts.
  • Great temperature differences between day and night.
  • Land will get much hotter during the day.
  • Difference between average temperatures of equator and poles might be approximately 40 K, heat transfer should work.

Tidally locked or pole towards its star

  • Equator and poles do not play usual role of cold and dry vs hot and humid. Instead, there is a hot substellar point and a warm insulated side, and a cold dark side.
  • Temperature differences between substellar point and could be around 80 K.
  • Atmosphere might superrotate, see the previous case.
  • No distinct desert zones, probably lot of precipitation on the insulated side.
  • Temperature and wind maps would generally be similar to this pattern.

Local features

Mountains

Apart from the global circulation of the atmosphere, there is lot of local features, that can create deserts, for example. In my opinion, one should roughly imagine where the clouds are coming from and where do they go. Clouds appear above oceans, the warmer the region, the more clouds appear. Then, they follow the prevailing direction of winds. (See the discussion above.) If there are high mountains between, there will be a lot of precipitation on the side from where the clouds go, and dry region, possibly desert, after that.

Continents

The water slowly rains away above the land. Hundreds or thousands of kilometers in the middle of large continent, there will be a dry region with big temperature differences between summer and winter. (Or even within one day for slowly rotating planets.) This might go to extremes for large continents.

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  • $\begingroup$ I do not understand the ocean currents too much. Hopefully, someone will fill this gap. They contribute on the circulation of heat between equator and poles and they are shaped by the landmasses present. They might be very important for quickly rotating planets with reduced atmospheric heat transfer, but this is only a speculation. This is all I can say. $\endgroup$ – Irigi Oct 9 '14 at 14:21
  • $\begingroup$ Ocean currents are quite important for Earth too. It seems that when Antarctica was cut off from all the other landmasses, a current formed all the way around it, causing Antarctica to become the frozen hell-hole it is today, and allowing the Gulf stream to really start going. Parroting Earth might work well - upload.wikimedia.org/wikipedia/commons/0/06/… seems simple enough to copy. Very similar to air currents, except that it's shaped by the continents much more. $\endgroup$ – Luaan Oct 10 '14 at 8:07
  • $\begingroup$ From the figure, it seems they follow similar pattern as the wind: Travel from equator, turn eastwards around lattitude 30°-60°, turn back, travel westwards at equator to compensate. The only difference is, that the continents break the flow into many small "cells". (And that the wind travels back to the equator at lower altitudes, not in the equator westward-tropic eastward loop). $\endgroup$ – Irigi Oct 10 '14 at 9:53
  • $\begingroup$ @Irigi With such a detailed answer, it's clear that you know your stuff. I was wondering, therefore, if you'd be qualified to help me with a world I've been building. $\endgroup$ – JohnWDailey Dec 7 '18 at 0:27
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There are already some very good, detailed answers posted, but I'll happily throw in my two cents worth.

Our oceans and atmosphere absorb energy from two different sources :

  • The rotational energy of the earth
  • The thermal and other available spectrum energy radiated by the sun, and to a lesser extend the earth's core and the various organic and geological processes taking place on the planet.

This energy is managed and transferred according to the laws of thermo-dynamics. Thermo-dynamic flows are characterised by ergodicity.

An ocean's currents are the mechanism by which its energy is managed and released into the atmosphere or, to a lesser extent, into organic and geological materials located in and around the oceans. Currents are ergodic flows that are the agents of entropy, distributing energy about the ocean's constituent molecules while at the same time radiating energy (and hydrogen, oxygen, etc) into the atmosphere.

The climate, atmospheric conditions, and weather are the mechanism by which atmospheric energy is managed and released into space or into the environment (i.e., the oceans, other atmospheric phenomena, or the earth's surface). Again, the ergodic flows of atmospheric phenomena are the agents of entropy, distributing the energy about the atmosphere. For example, each time thermal energy causes a rain drop to form, energy is release and an $ H_2O $ molecule is formed - actually, a rain drop consists of many molecules. That raindrop can then be absorbed by a plant, an animal, the oceans, etc.

The ergodic nature of these thermo-dynamic flows are what shape our weather and climate. For example, as atmospheric energy increases, winds increase in speed in a desperate attempt to distribute and release excess energy, often featuring dramatic effects with powerful discharges of energy.

These processes create characteristic patters of erosion (by wind or water) that we see on our coast lines and geology. The laws of therm-dynamic are assumed to be universal (via the Copernican doctrine) and so these same patterns should be present in any similar habitable alien environment, subject to applicability to local geology.

I hope I haven't misunderstood your question. It's late!

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