The atmosphere or tone of your community is hard to measure. But achieving a positive tone is critically important to your community success. Like a massive black hole you can't see it directly, but a negative tone warps everything around it and pulls it down. That's why community management and moderation are so important: effective community leadership in your terms and guidelines, the welcome messages and content you create, and in every interaction online with your members influences the overall mood of your community. Each is an opportunity for you to positively engage your members to elevate your community tone.
Don't let negativity sap the energy and light from your community:
Be transparent: Establish guidelines for member behavior and cite them when taking action.
Be positive: Don't rise to the bait when a member is frustrated, and be twice as nice online as you are in person. Be respectful: Warn your members politely and in private, not in public. There's no need to embarrass anyone.
Be consistent: Document the actions you take with members (and be sure to read the notes of others).
What are you doing to keep your community a warm and sunny place?