How can it rain at 20 degrees Fahrenheit?

Answer 1

there would have to be a layer of above freezing air on top of the ground so the rain would start as snow and melt in the above freezing air and become supercooled and fall as freezing rain (rain that freezes on contact with anything).

Answer 2

Rain at 20 degrees Fahrenheit is possible if the raindrops start off as liquid in warmer layers of the atmosphere, then freeze as they pass through a layer of colder air near the ground. This can happen in certain weather conditions where there is a mix of warm and cold air masses.