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Freezing Drizzle AND Snow?

You may wonder, “How can it snow & drizzle at the same time?”

Short answer: when clouds aren’t cold enough to produce snow (exclusively or at all) drizzle can form.

Longer answer: If the air is sufficiently cold and wet, only snowfall is possible. But when precipitating cloud top temperatures are above -10° C (14° F) but still below freezing, the ice nucleation process requires help. Unstable, rising air can induce the formation of snowfall. Failing that, with only slightly below freezing temperatures, snow crystallization needs smoke, dust, pollution or any other tiny airborne surface upon which to bond. Otherwise precipitation can fall as drizzle/mist.

Why today? We’ve had a shallow layer of moist, sub-freezing air (< 5,000 ft) “capped” by warmer, drier air above since Sunday evening (1/26/20). Cloud tops have been just below -10° C. The moist air from sublimating snow/ice and weak low pressure has combined with onshore, upslope winds to induce some lift in the lower atmosphere. This, along with whatever particulates were aloft, produced snowfall. The remaining moisture has fallen as drizzle which froze upon contact depending on the surface composition. Most surfaces weren’t cold enough to allow much, if any, ice accumulation. A light glaze has been observed in places around our property — just enough to increase stopping distances on untreated pavement.

Normally this time of year clouds are much colder. But we have been running well above normal throughout the atmosphere recently. That’s why we’ve had several wet snow events in December & January.

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