C. Ben Bates of Joppa writes to ask how the presence of water vapor in the atmosphere relates to the boiling point of water, where liquid water becomes a gas. "It is hard to imagine that liquid water is `boiling' up there in the clouds," he says. It's not, of course. Water vapor is the gaseous phase of water, but no boiling is necessary to produce it. Water vapor can and does enter the atmosphere directly, from "evaporation" (liquid to gas) or "sublimation" (ice to gas).