I'll break this up into two, distinctive parts. In creating the atmosphere, tone, mood, etc. of a world, the very first thing I focus on is the introduction a reader, roleplayer and even I will later have in the world. The first few posts, the description in the OP, so on and so forth. Each new area should have its own atmosphere and tone that can either be connected to or totally different from the primary one. How the world is explained and how its plot is explained as it unravels are two entirely different things. Describing a series of events might lead to an entirely different tone for a world than describing the places and their history. I think it's important to be very clear, at least in your own mind, as to what you're attempting to capture and accomplish, then make an anchor point in the roleplay for that. Even if a player starts later, chapters later even, they will start somewhere and its important that somewhere give the desired tone and create the desired atmosphere for them to pick up on it. With a good, solid start, preservation itself is relatively easy. In terms of how one can create a mood, word choice and emphasis are two aspects I prefer to use. The same thing can be described a thousand different ways and its important to pick just which way you want illustrating the world you are bringing to life. It's not about "getting the job done" or "getting the post wrote"; you're creating a long-standing resource for a world, and it's important to get it done correctly. Rewrite it a dozen times until you believe that everyone who reads it will be able to interpret it in a way that you find acceptable for your world.
Then comes culture. Culture and atmosphere go hand-in-hand in some respects, but they are truly different entities. A culture is a group of people brought together and bred by a world and its history. There can be different cultures. Each one will evolve. Each people will be different. And, in sculpting a world through culture, the biggest question to ask is "why?" Why would a human - a group of people - behave this way? What is the reason behind it? How did they grow, bond and adapt to survive? How did they react? Once you figure out the what's and how's, figuring out the why's is how you make others relate. And, if the subject isn't human, then the why might be different. But, if it's different, it should be consistent. There are universal traits in all humanity, and just like that, there are universal traits in any race you come up with, unless of course there actually aren't. In either case, it's important to preserve the consistency of why things happen as they do because as long as that is consistent, a whole group of people can be built on that foundation and you will always be able to return to it. If new events happen, you can predict how a culture will adapt because you know why they will do it. If a bunch of cultures react differently, chances are they're reacting on the same principle of "why?" and if a whole culture gets wiped out, then is it because they strayed too far from their basic, primal nature, or was their "how" just not sufficient despite having the correct reasoning? Culture can help maintain an atmosphere, but atmosphere itself does not create culture. More like, atmosphere is the culmination of almost every surface-level element in a world while a culture combines what is on the surface with what is at root, and in that way the two do affect each other, often extensively.
If there is an atmosphere I want to create, there is definitely a reason I want it created. As long as that reason is good enough, it will seep deep into the boundless depths of a roleplay to make itself present wherever I want it to be, whether its in dialogue, nonverbal interaction, the setting, the world, a series of events, so on and so forth.
