My husband is working in Mass. Tonight it is raining with the temp of 20 degrees. How can it be raining and not snowing/sleeting etc?
Posted January 10, 2009 12:22 p.m. EST
MIKE MOSS SAYS: Jane, You didn't mention what location in MA, but I checked observations for Boston (near the coast) and Worcester (middle of the state) for 7 and 10 pm that night. Boston had 33 degrees and a mix of light snow and drizzle at 7 pm, then 29 degrees and light freezing drizzle at 10 pm. Worcester had 30 degrees and light freezing rain at 7 pm, with 28 degrees and a mix of light freezing rain and snow at 10 pm. It doesn't seem to have been quite as cold as you were thinking, although it was subfreezing at both locations by later in the evening.
Rain can fall even at sub-freezing ground temperatures if there is an elevated layer of warmer air sufficiently thick to melt any snow falling from higher above. It then becomes raindrops which, in some cases, may fall into a very shallow layer of subfreezing air adjacent to the surface. When this happens, the droplets may be cooled to below freezing ("supercooled") without actually becoming frozen before they reach the ground. Usually, they will then freeze on contact to form an icy glaze, at least on elevated objects like tree limbs and power lines, etc. If temperatures were much warmer in the recent past, some of these objects and the ground may retain enough heat to prevent much glazing, at least for a while, so even though technically freezing rain is occuring, there may not be a lot of glaze forming. Perhaps this was the case when your husband reported just seeing "rain."
If the low level layer of subfreezing air become a little thicker, but still with a warm layer above it, sleet may occur instead of freezing rain. In this case the droplets fall through a deep enough layer to not only become supercooled but to actually freeze into nearly clear pellets of ice before they reach the surface. Of course, if the warmer layer of air above becomes shallow enough or drops to freezing or below, the melting in that layer will partially or completely cease, allowing for a mixture of precipitation types or a changeover to all snow at the ground.
These same complexities of vertical tempertaure structure, and the variation of those vertical profiles from one location to another nearby and over time, are a common feature of our local wintry weather events as well, and in many cases makes for a very challenging forecast.