Rosalia Salas, age 11, of Loma Linda, California, for her question:

How much water evaporates from the earth?

On a ship out in the ocean, you can see an immense amount of water in every direction. It would be very difficult to figure the amount    but at least this much water has evaporated from the earth during your lifetime. To get a more accurate estimate of the process, it is necessary to figure the rate of evaporation. And this equation requires a time factor. We need to know how much of the Earth's water evaporates in a certain amount of time. Of course, the amount varies from place to place, with day and night and the changing seasons. However, experts have estimated the average amount on a global basis.

During an average year, the amount of water that evaporates from the entire surface of our earth is estimated to be 95,000 cubic miles. A cubic mile of water, of course, would fill a square tank one mile high and one mile along each side.

Only 15,000 of these cubic miles come from the land and the rest comes from the oceans. And sooner or later, all of it falls as rain or snow meanwhile the sun is replenishing the water cycle by evaporating more water from the land and sea.