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While asking questions about building my fantasy world, I keep hearing about wet/dry seasons. Obviously, one has rain and the other doesn't, and I've gathered that they occur with regularity on an annual basis, but what exactly are they? What causes them, and when/how often do they form?

Note: I originally asked this question on Earth Science here. I have gotten no replies. Being unable to move questions between sites, I am now asking it here.

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Vincents answer is pretty complete, but a bit technical...I'll try a less technical more discussion answer.

First to note...we don't fully understand why. These are patterns, we can observe them, but whats actually causing them isn't well known. The ocean heavily influences our climate and it has a few cycles that are multi-year cycles that impacts the wet/dry seasons...El Nino / La Nina are pacific ocean patterns and the El Nino is tied to the Indian monsoons, Caribbean rainfall, and California's dry season (it effects the Saharan dust layer as well...a not well understood pacific ocean cycle has world wide impacts). It's why your question ends up in world building and not Earth Science...we are still in semi-speculative territory with this. As a general pattern...if you don't experience much of a winter/summer seasonal change, odds are you'll see a dry/wet season instead.

If you want the simplest explanation...the wind patterns tend to form a 'rain belt' (defer to Vincents answer and ITCZ explanation). This rain belt moves over an area during it's rainy season and elsewhere for it's dry season. In the case of India, winds from the Southwest pick up rain clouds from the Indian ocean and move them inland creating the monsoons. During the dry season, these winds reverse and they come from the North East as relatively dry winds.

The reasons for this are significantly more complicated...but keeping simplistic: if you take a globe and view it from the North pole, then flip it over and view it from the south pole, you will notice there is a significantly greater amount of land in the Northern Hemisphere vs a much larger percentage of ocean in the southern hemisphere. The oceans heat differently than the land does...oceans almost entirely absorb the suns energy while the land tends to reflect it much more. The ocean sucks up the energy from the sun, but the majority of it goes to changing water from a liquid to a gas and not heat changes...while on land this energy tends to raise the temperature of the air instead. This seasonal variation in the amount of sunlight the Earth absorbs is heavily responsible for the wet/dry season. Using India as an example again...the Northern regions of India sees a high pressure system dominate during the winter which drives the winds from the North East and keeps the region dry. During the summer months, the Indian ocean greatly warms and it shifts to a low pressure system over the Thar desert/Northern and central India with the high pressure system over the Indian ocean. This pushes the majority of the humidity out of the Indian Ocean and over India instead.

You would have to give further descriptions to get much more in detail than this. I'd go with the general rule that any place considered tropical and between the tropic of Cancer and Capricorn would see a dry/wet season instead of a cold/hot season

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    $\begingroup$ To future viewers: I've marked this as the answer because I find it a lot easier to understand than Vincent's answer. That being said, Vincent has a lot more detail, and his answer should definitely be viewed as well. $\endgroup$ – Thomas Myron May 12 '15 at 16:20
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According to the Köppen climate classification a dry season is defined like this:

  • To have a dry season for temperate climates, oceanic and continental climates (C and D) need to receive at least 3 times as much rain in the 6 coldest months than in the rest of the year.

    But it is different if the climate is dry during the winter. These places need to receive at least 10 times more rain in the 6 hottest months than in the rest of the year

  • Semi-arid climates (BS) often have a wet and dry season but it is not a defining factor. When they do, the dry season occurs mostly during winter. It is generally caused by the monsoon; I will come back to it later.
  • For tropical climates (A) to have a dry season, they need to have at least one month with less than 60mm of precipitation.

What cause them ?

  • Dry summers: the dry summer climates are located above the tropics close to the 35th and 40th degrees of latitudes. Look at this image.

    That's the air circulation on the planet. Basically, hot air rises at the equator and eventually gets back down. This is the Hadley cell. Air rise where it's the hottest, at the equator in a zone called the Intertropical convergence zone (ITCZ). The rising air quickly cools off and lose all the water in once contained. This means that the other side (subtropical high) is really dry. However, this Hadley cell moves with the seasons. It goes north in summer as the continents are getting hotter. This is pushing the high pressure area toward places like North Africa, Argentina and South Africa, hence the dry summer.

  • Dry winters: there are 2 types of dry winters. One is close to the tropic and is also caused by the Hadley cell. In the northern winter, it moves to the south and these areas now become affected by the subtropical high.

    The other type occurs mostly in Asia and is caused by the Siberian high, a high pressure system. If we go back to the picture, we have the polar high at the top. Well, it's not exactly on the pole. there is a large high pressure area over Siberia for most of the winter. This, like the subtropical high, is pushing dry air toward much of Asia.

  • Wet summers: You might start to get the idea but again, it is caused by the movement of the Hadley cell. When it moves toward you, it brings a lot of precipitations. Larger continents like Asia will pull the Hadley cell much more than Europe.

When?

It depend where you are. The dry season is at is driest usually close to the hemisphere winter solstice (when it's a dry winter). And on the opposite, the wet season would be during the summer solstice. But again, the Hadley cell moves gradually, meaning that places closer to the equator might have their wet summer during the end of spring instead. See map More details here : Creating a realistic world map - Currents, Precipitation and Climate

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