Overview

Rain and snow mixed describes precipitation that contains both liquid raindrops and partially melted snowflakes falling together. It is a common wintertime phenomenon when atmospheric temperature profiles are close to the freezing point. The shorthand RASN is used in meteorological reports to indicate a mixed rain-and-snow condition. Several languages and forecasting services use different common names for this type of precipitation; for example, the term sleet is used in many countries to mean a rain–snow mixture, while in parts of North America that same word refers to ice pellets (North American usage).

How it forms

Mixed rain and snow occur when snowflakes fall through a shallow layer of above-freezing air and partially melt, producing wet flakes or a combination of drops and flakes. If the melting layer is shallow and the surface air is cold enough, some snow survives to the ground along with liquid drops. If the melted drops refreeze into solid pellets while falling or upon impact, the phenomenon is classified differently (see distinctions below). Meteorological instruments and observers use METAR and forecast codes such as RASN to record the occurrence.

Impacts and safety

Because mixed precipitation contains liquid, it can accumulate as a slushy layer on roads and sidewalks, reducing traction and increasing the risk of accidents. Visibility is often reduced, and wet snow can cling to power lines and trees, increasing the chance of outages. Travelers are advised to expect rapidly changing road conditions and to follow local weather guidance (safety advice).

Terminology and distinctions

Different forms of winter precipitation are sometimes confused. Useful distinctions include:

  • Rain and snow mixed (RASN): both liquid drops and snowflakes reach the surface.
  • Ice pellets (sleet, North America): drops melt then refreeze into small, hard pellets before reaching the ground; see ice pellet.
  • Freezing rain: liquid drops freeze on contact with cold surfaces, forming glaze; see freezing rain.
  • Graupel: soft, rime-coated snow pellets distinct from a rain–snow mix; see graupel.

Observation and forecasting

Forecasters look at temperature profiles, humidity, and radar returns to predict where mixed precipitation will occur. Automated surface observations, pilot reports, and human spotters help verify conditions. When forecasts indicate temperatures around 0 °C (32 °F) through a deep or shallow layer, agencies often warn of a wintry mix that can change quickly to all snow or all rain depending on small temperature shifts.