‘Supercooled’ water the culprit with freezing rain and drizzle

Here comes winter! We’ve already had an early taste of arctic air, and at least for a day or two, snow blanketed the Inland Northwest.

Though Mother Nature has brought in some milder air and rainfall in the days leading up to Christmas, the winter solstice will begin Monday at 9:47 a.m. PST. With the shortest days upon us, cold temperatures, and the beginning of the winter season, you might have had the notion that the earth must be at its farthest distance from the sun right about now.

Interestingly enough, the opposite is true! The earth is actually at perihelion (the closest distance from the sun) each year in early January, and at aphelion (the farthest distance from the sun) in early July. The exact dates of aphelion and perihelion differ slightly from year to year. Of course it is not the distance from the sun that causes our seasons here on earth (remember that the Southern Hemisphere is gearing up for summer), but the earth’s tilt on its axis. During our winter solstice, the most direct rays of the sun are focused on the Tropic of Capricorn, located at 23.5 degrees south latitude and passing through Brazil, South Africa and Australia.

But enough about the folks who’ll be sweating out the months of December and January. Across Eastern Washington and North Idaho, we’ve been dealing with a potpourri of precipitation that has included snow, freezing rain and rain. Back on the 15th, after 2 to 4 inches of snow coated the Spokane and Coeur d’Alene areas, temperatures actually started to rise during the afternoon and into the evening hours.

Before climbing above freezing, however, an icy glaze of freezing rain and drizzle topped the snow and area roads. How is it that liquid precipitation (drizzle and rain) can fall when temperatures are below freezing? Contrary to the general teaching that water freezes below 32 degrees, it is actually possible to have liquid water which is colder than that. It is referred to as “supercooled” water. In the case of freezing drizzle, supercooled water droplets precipitate out of a low stratus cloud deck. Sub-freezing temperatures may exist from the ground all the way up to the clouds.

Drizzle differs from rain in terms of drop size, with drizzle drops measuring less than .02 inches. Anything larger is considered a rain drop. With freezing rain, the temperature profile often includes a layer of above freezing temperatures above a shallow sub-freezing layer near the ground. This situation often occurs in valley locations with a departing Arctic airmass, as the coldest air in the low lying areas is the last to get scoured out.