What I've discovered since moving to Germany is that culture is more dependent on climate than I'd ever suspected. Example: the dorf I live in features adorable steepled houses and grassy, overgrown walls of unhewn stones. For a long time, I considered these to be cultural affectations. People had built things that way because they found it aesthetic, right?
Or at least, mostly-wrongo. It is aesthetic, but it's also a direct result of geography and geology. The soil here is rocky. You can't dig more than a handsbreadth into the ground without hitting stone; some of them very large stones. Anytime someone clears a field for agriculture, or digs a foundation for a house, or clears ground for an attractive lawn, they're going to end up digging up piles of rock. They could cart the rocks away, of course, but it's aesthetic and convenient to just pile them along the border of the property. Saves the trouble of building a fence, and all.
Combine the prevelence of rocks with lots of rainfall, and you get cobblestone roads, which are also pretty common around here. Asphalt's recently taken over as the road material of choice, but for centuries, if you wanted to walk around without getting your shoes muddy, cobblestone was the way to go.
Steepled roofs? Let's just say I dare you to find a culture anywhere, at any period of history, that constructed flat-roofed buildings in areas with heavy snowfall.
Climate doesn't just affect architecture. Consider customs like taking a Siesta at noon and then partying or working long into the night. Map out regions of the world where these customs are common, and you'll see they occur primarily in regions that are extremely hot over the day; too hot to work or party effectively at noon.
How about language? Have you ever been out in freezing cold weather for hours, and discovered that it was difficult to talk afterward? Certain phonemes, especially the American 'r' and 'th' sounds, are extremely difficult to pronounce when your face is numb. Now I'm no linguist, but I bet if you tracked it, you'd find that languages from Scandinavia and northern Europe have dropped precisely those sounds that are difficult to make in sub-zero weather.*
Ever heard the observation that the closer you get to the equator, the more open and friendly the culture becomes? Yeah. Try smiling when your face is frozen. Cultures that developed in areas with harsh winters would need to rely on social cues other than facial expressions.
Ever wondered why so many Renaissance painters came from Italy and France? I could be way off-base on this, but think about the weather for a moment. Italy's warm. It's easy for an artist to work year-round on, oh, say, a mural that covers the ceiling of a chapel, or a free-standing sculpture. It must be nice to not have your paints thicken up unless you stand near the fire during winter, too.
Is all of this based on conjecture? You betcha. And I may be wrong about some of the details. But my point is that climate affects culture far more than we generally realize. And as writers, we can use this knowlege to add vibrancy to our worlds.
Let's look at one more example: cooking.
I grew up in northern California, basically a desert. My Mom (and primary cooking instructor) grew up in Utah. Also a desert. We baked our potatoes in the oven. We baked a lot of things in the oven, in fact, and tended to steam or sautee vegetables rather than boiling them.
When I came to Germany, I thought it was the weirdest thing to boil potatoes in water, but that's the way everyone did it. They cook a lot of things in water here, in fact, and crock pots and casseroles are practically unheard of.
Cultural? Maybe. But it's also environmental. Because if you lived in the desert in the days before indoor plumbing, the last thing you wanted to do was waste water. Why boil your potatoes when you could bake them instead?
Okay, I'm throwing this one out to the peanut gallery now. What other customs appear at first glance to be cultural, but are actually influenced by environment?
*Several people have contested the 'th' thing in the comments. I haven't seen enough evidence to be convinced that I'm wrong, but the veracity of the statement is definitely up for dispute.