Climate impacts all life and is important for understanding how much rainfall occurs in various areas and the resulting vegetation, or lack thereof. It’s an easy way to distinguish one location from another in stories. A humid region will impact the style of clothes and even customs of inhabitants. Maybe it’s so hot and humid in midday that people work in the morning and late afternoon but take a long noon break. Those in northern climates might make the most of noon heat.
Climate is a long-term weather pattern, rather than day to day changes. Many factors influence climate and can be daunting to consider. Terrain, altitude, nearby bodies of water and their current, and latitude (distance from the equator) all impact climate. The ratio of land to water, how distant that water is, and the location of mountain ranges also have an impact. Another factor is the density and type of vegetation, which increase heat retention and rainfall. How far inland we are also changes things.
These variables cause the changes in rainfall amounts, temperature, and humidity. This might seem trivial, but areas can be well known for their climate. Examples include India’s monsoon season, the humidity of the southeastern United States, and the dryness of the southwestern United States. As the latter two demonstrate, we can’t rely on latitude to explain climate, as both areas are equidistant from the equator but one is hot and dry and the other is hot and wet. The prevailing winds and rain shadows from mountain ranges will be partly responsible for where forests and deserts lie.
There are different climate rating systems, but the Köppen climate classification is the one we’re familiar with (even if we’ve never heard of it) because it has terms like subtropical and rainforest, so we’ll use that one. In this system, climates are divided into five main groups, each with subtypes or subgroups. Much of the information in this section comes from Wikipedia’s Köppen Classification Summary at http://www.artofworldbuilding.com/Koppen.
Some excellent color images online can give a quick peek into just how variable climate is on even one continent. One is below, but the rest of Earth can be viewed online at this link: http://www.artofworldbuilding.com/KoppenMap