World and Nation

A Spectrum of Precipitation

A Spectrum of Precipitation

This month Mother Nature has provided us with the full range of winter precipitation, including snow, rain, sleet, and freezing rain. And she’s not done yet. Another weak round of wintry mixed precipitation is expected Tuesday night, followed by a more intense storm Thursday night. High temperatures for the rest of the work week will be in the mid-30s to mid-40s, with lows in the mid-20s to mid-30s.

The different precipitation types are caused by the vertical temperature structure of the lower atmosphere. In the presence of fronts and storm systems, the vertical temperature profile can be complex. Consider a situation where the conditions are suitable for snow to form at relatively high altitudes. If the temperature remains cold all the way to the ground, this snow penetrates to the ground. The term flurries denotes very light snow. If, as the snow is falling, it encounters a layer of warm air aloft, it may temporarily melt into rain. Depending on the thickness of the subsequent layer of cold air near the ground, the rain may have time to refreeze into ice pellets (i.e. sleet) if the cold air layer is thick, or else will freeze on contact with the ground (i.e. freezing rain) if the cold air layer is shallow. If the air is above freezing all the way to the surface, the result is simply rain.

On a final note, according to the National Oceanic and Atmospheric Administration, the probability of Boston having a White Christmas (defined as a snow depth of one inch or greater on Christmas morning) is roughly 25 percent.

Extended Forecast

Tonight: Rain mixed with snow. Low 35°F (2°C).

Tomorrow: Cloudy early, then clearing. High 45°F (7°C).

Tomorrow night: Clear. Low 22°F (-6°C).

Thursday: Mostly cloudy. High 35°F (2°C).

Thursday night: Rain, sleet, and snow. Low 29°F (-2°C).

Friday: Mostly cloudy. High 41°F (5°C).