Graham Pashak, age 11, of Calgary 47, Alberta, Canada, for his question:

Does any water vapor escape from the atmosphere?

This is possible, though it happens on a very small scale. Far more water vapor is added to the normal quota in the atmosphere from deep underground. Some of this water had been trapped down there since the earth formed its rocky crust. Perhaps some is formed when hydrogen and oxygen combine in underground volcanic activity. This buried moisture continuously seeps and erupts to the surface. Such things as springs do not add extra water or vapor. They merely recycle groundwater collected from seeping rains.

The few molecules of vapor that escape must ascend many hundreds of miles to the upper limits of the atmosphere. There they must convert heat into enough speed to escape the pull of gravity. Escape velocity at such heights is much lower than at the surface, where it is about 25,000 miles per hour. Up there, it can be achieved by a few gaseous molecules, including perhaps a few molecules of water vapor.