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In order for precipitation to form several ingredients are necessary.
- A source of moisture. The primary moisture sources in the U.S. are the Atlantic and Pacific Oceans as well as the Gulf of Mexico. Winds around high and low pressure systems transport this moisture inland
- Clouds need to form. The most effective way to do this is by lifting the air. This is caused by forcing air to rise near fronts and low pressure areas or by forcing the air up and over mountains or, more commonly,
- Coalescence, cloud droplets and/or ice crystals are too small and too light to fall to the ground as precipitation. So there must be a process(es) for the cloud water, or ice, to grow large enough to fall as precipitation.Coalescence is the collision between cloud droplets of varying size, sticking together to form larger drops.