If raindrops could talk, they would tell an interesting story about where the water that forms them came from. Now, a NASA computer model can help tell some of that story. The new model simulates water movement in the atmosphere around the world, tracing it from where it evaporates to where it falls back to Earth as rain or snow.

So far, this model has only been tested in summer over the U.S. and India, two countries that typically have lots of moisture that time of year. Whereas rain in India often comes directly from the ocean, the model has shown that much of what falls on the U.S. in summer is "recycled" - that is, water from previous U.S. storms that evaporated from the ground and then fell again. With refinements, the model may eventually be able to dissect a classic East Coast nor'easter and tell how much of its water came from the Gulf of Mexico, the Atlantic or Pacific Oceans, or from water left on land by previous storms.

Such improvements will certainly lead to a better handle on tomorrow's or next week's weather forecast, but also, by understanding how geographic sources of moisture change from year to year, will yield a clearer picture of how climate changes in the long term.