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How do you predict how much rain will fall in a storm before the storm hits? Is it the same for snow fall?


There are several factors that determine how much precipitation will occur, including the amount of precipitable water that is available (how much water vapor is in the layer of the atmosphere where the precipitation will form), how quickly the air is being lifted (as air is lifted it cools and water vapor condenses into water droplets or ice crystals, which can fall as it is formed or coalesce into larger raindrops or snow), how quickly the weather system is moving (which will determine how long precipitation will last), etc.

Nowadays, these factors are calculated by massive computer models (about 11 models are run on a routine basis) and the precipitation for a particular location is determined.

The same technique is used for snow as is used for rain. The computer models also determine what the temperature will be in the various layers of the atmosphere and at the Earth's surface and therefore whether the precipitation will form as snow or rain and whether snow may melt as it falls,thereby ending up as rain at the Earth's surface. Many of the very light rains that occur in wintertime are actually the result of snow melting as it falls through a warm layer of the atmosphere just above the surface.

David R. Cook
Climate Research Section
Environmental Science Division
Argonne National Laboratory

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