Who cares about weather in my fantasy world? I do! It’s going to affect my characters in what they wear, how and when they travel, what they eat — including whether they grow their food, hunt it, or buy it — and what kind of houses they build.
Today, with the conveniences of central heating, air conditioning, and Gore-Tex, it’s easy to ignore climate. We can spend Spring Break snow skiing or cruise in January to Bermuda’s pink sand. Mother Nature has to play rough to catch our attention. The recent volcanic eruption in Iceland taught many folks that elemental forces don’t care if travel is canceled.
Unless fantasy characters control their weather through magic, they are very much at its mercy. In a quasi-medieval setting, a knight huddles around his fire during the winter, sets his serfs plowing come spring thaw, and rides off to battle in summer.
If the setting is harsh desert, game and water will be scarce. If your characters aren’t carrying provisions, they’ll have to take time to hunt. Blazing heat and plate armor don’t mix well. It’s implausible that desert warriors would devise heavy, European-style armor, and if invaders are wearing it they’ll quickly be melting under their breastplates.
If the climate is cold and snowy, then visibility is poor, travel is hindered, and there’s a danger of characters freezing to death. I love Poul Anderson’s essay, “On Thud and Blunder.” It ridicules stories where bare-chested heroes clad in fur loincloths stride around in blizzards with nary a goose-pimple.
People who are uncomfortable, thirsty, and hungry are going to be short-tempered, which leads to conflict, which leads to story development.
Terrain, of course, has as much effect on characters and plot as climate. Is your story world going to be mountainous, heavily forested, desert, verdant valleys, islands surrounded by ocean, icy tundra, barren wasteland, volcanic lava fields, or somewhere under the sea?
Such factors will determine not only how the characters live but also how often they receive visitors. Their degree of isolation in turn affects how narrow or open minded they are.