For those who don't know, a vault toilet is a glorified outhouse. It has a toilet seat over an underground chamber (no plumbing between your bottom and the vault) and a chimney to vent the odors.
Sometimes, though, the wind will blow down the chimney and up at your butt. When the wind is warm, it can be like those expensive, high tech Japanese toilets with dryers built in. But when the wind is cold, it'll wake you right up and make some things pucker.