The rain-snow line is the boundary between places where precipitation reaches the ground as rain and places where it falls as snow. In mountain areas the same idea is often given as a snow level: the elevation above which snow falls and below which rain falls.
What sets the line
Snow that falls into air warmer than freezing starts to melt, so the line follows where the melting layer meets the ground. Studies cited by the National Weather Service (NWS) Western Region found the rain-snow line averaged about 700 feet below the melting level, and snow levels can sit roughly 500 to 1,500 feet below the 32°F (0°C) height.
The change is rarely sharp. A mix of rain and snow may be seen within a few hundred feet of the snow level.
How forecasters place it
Forecasters lean on wet-bulb temperature, which includes the cooling from evaporation as precipitation falls into drier air. The Western Region method, now standard in NWS software, uses the height where the wet-bulb temperature is 0.5°C (about 33°F) as its snow level.
Layer thickness gives a quick first guess, and a 1000-500 mb thickness of 5,400 meters is a common critical value. Weather Prediction Center guidance warns that this method will not resolve thin warm layers, and that partial thicknesses can still miss a very thin one.
Why it moves and why it is uncertain
Melting draws heat from the air, and the cooling is strongest where precipitation rates are high. Persistent melting can cool a layer to 32°F, letting snow fall farther before it melts.
Terrain bends the line too. Snow levels can dip toward higher terrain, so observed snow can reach lower than the free atmosphere alone suggests.
Related ideas
The freezing level and wet-bulb zero describe the melting layer aloft, and the rain-snow line is where that layer reaches the ground. On radar, the bright band marks the melting layer and can help show where the transition sits.