Until I get to populating the world with some people and setting up their societies and their myths and histories, this is possibly my favourite part of building a world – figuring out the climate. I think, if you’re trying to make something that feels plausibly earth-like, this is one of the hardest parts to get right. It’s pretty abstract, but also very definitely driven by hard physical rules and the topography of the world.
I start with the prevailing winds. Earth, prevailing winds are largely tied to latitude and there’s a pleasantly simple map of them. Without turning into a climatologist, the most important thing to keep in mind at his stage of building a new world is how water is distributed from the sea to the land, the basic rule being that the prevailing winds generally pick up water from the sea and carry it with them to the land and the flatter the land is, the further it will get spread about to fall as rain. So if the prevailing wind hits a fairly gentle expanse of lowlands rain gets spread over a wide area with a bias towards coastal regions (Western Europe is a reasonable exampel of this), while if the prevailing wind hits a mountain range, nearly all the water picked up from the sea will fall on the mountains and on the seaward side, resulting in a very wet climate on one side and a very dry climate on the other (South Island New Zealand for example).This is sort of explained in detail in the Wikipedia article on prevailing winds.
This is all very glib generalisation, but it’s probably good enough to work with two rules of thumb:
1. The prevailing winds carry the bulk of an area’s rainfall. The first mountain range they hit will steal that rain. The land beyond will likely be dry.
2. The further from the sea a place is, the more extreme the temperature change will be between summer and winter (particularly true for coastal regions in the path of the prevailing winds, less so for those where the prevailing wind blows from land to sea).
By far the easierst thing to do is simply find a part of the real world with a similar latitude and a similar topography and then steal its climate
Also note that where the rain falls doesn’t necessarily equate to where the people will be.
Next up: rivers.