Frog rain is a rare meteorological phenomenon in which frogs get swept up in a storm, travel miles, and then fall from the sky when the clouds release the water. ­­­­Frogs can weigh as little as a few ounces. But even the heavier ones are no match for a watery tornado, or a waterspout, as it's called when a whirlwind picks up water. The waterspout sucks up the lower-weight items in the body of water as it m­oves across it. Frogs are fairly lightweight. They end up in the vortex, which continues to move across the water with the high-pressure storm clouds. As the rain falls, the vortex eventually loses all the pressure that's keeping it going, and it releases whatever it has picked up in its travels. Sometimes, this cargo includes frogs.

Source: How Stuff Works