Graupel in Montreal
January 30, 2013
Shown above are snow pellets on the top of my car from a snow squall in Montreal, Quebec on January 20, 2013. Instead of snowflakes, however, snow pellets (graupel) were falling from the sky – some of the biggest pellets I had ever seen. Graupel forms when snow falls through a layer of supercooled water droplets. These droplets freeze instantly when they contact the snow crystals coating the crystals in a shroud of ice.
January 20 was a day of unusual transitional weather in Montreal. Temperatures were well above normal (39 F or 4 C) early in the day but dropped steadily, bottoming out at -4 F (-20 C). During the morning hours, with the temperature still above freezing, an odd winter thunderstorm brought downpours of rain mixed with sleet. Afterwards, it was alternately clear and stormy, with occasional whiteout blizzard conditions and wind gusts approaching 40 mph (64 km/h). At one point in the morning, when the temperature was hovering near the freezing point, a brief squall passed, and when I got to my car I found it covered with these 1/8 to 1/4 in (0.3 to 0.6 cm) pellets.