A common adage has it that if something comes out of the sky in Colorado during the winter, it's going to be snow.

As thousands of commuters who negotiated the perilous streets of Boulder County on Wednesday night and Thursday morning can attest, just because people say something, that doesn't necessarily make it true.

This week's icing event is the second such episode in just eight days, with Jan. 24 having produced similar conditions, triggering numerous crashes, including a four-vehicle collision that closed down a segment of U.S. 36 east of Boulder for three hours.

Again this week, freezing drizzle led to numerous wrecks, requiring both Boulder and Longmont to go on accident alert on Wednesday night.

"It's unusual, but not terribly rare," said Nezette Rydell, meteorologist in charge at the National Weather Service in Boulder. "We usually don't get freezing drizzle in February. More likely, in November or in March."

As for what it means — if anything — for Boulder County to see two such weather events in such a short span of time, Rydell said the answer is foggy.

"Is it a trend? It's hard to say, with two events, if it's a trend," Rydell said. "I'll leave it to statisticians to answer, better than I."

Boulder meteorologist Matt Kelsch has the statistics.

Dating back to the winter of 1989-1990, the record at the Boulder Climate Station shows such occasions typically occur once or twice each winter. The last winter season in which it did not happen at all was 2005-06. The greatest frequency on record in one winter is four occurrences, which was documented twice in successive winters, 1993-94 and 1994-95.

"Freezing drizzle, although it doesn't happen every winter, is really not all that uncommon along the Front Range," Kelsch said in an email. "You don't need a warm and moist oceanic air mass for freezing drizzle. You just need low upslope clouds that are too shallow to generate icy crystals in the high part of the cloud.

"Without those crystals we don't generate snowflakes. Instead, tiny droplets in the clouds get heavy enough to fall to the ground as drizzle and mist."

Colorado State Climatologist Nolan Doesken fielded some questions fresh off a session of scraping ice of his car windows.

"Cold air is heavier and denser than warm air," he explained, "and when we get the polar fronts coming down over the great plains, some of them are accompanied by cold air all the way up into the atmosphere.

"But some of them come in only with the dense, cold air near the surface, and with much warmer air" higher into the atmosphere, he said. "And that's the situation that causes these challenges. Things have to be quote, unquote just right to make that happen."

Kelsch also went a step further, to clarify some often-abused terminology.

"It's important to point out the common misunderstanding of what drizzle is," Kelsch said. "Drizzle is not light rain, it's small rain. It's the tiny droplets that don't make a sound when they hit the ground.

"Rain is larger drops that often start as hail or snow in the cloud and melt on the way to the ground (even in summer most of our rain starts as hail or snow). Winter rain is rare around here, but winter drizzle is not."

A weather glossary should not be in demand Friday.

After a forecast temperature dip overnight to about 19 degrees under mostly cloudy skies, Friday is forecast to be sunny and dry with a high of about 40.

Charlie Brennan: 303-473-1327, brennanc@dailycamera.com or twitter.com/chasbrennan

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