Weathercatch: There's more to winter precipitation than snowflakes and rain

<p><p>Not all precipitation that falls this time of year is snow. And the alternative isn’t always plain old rain. Sometimes we get ice – and it comes in several varieties.</p></p><p><p>Consider the weather event that transpired 12 years ago yesterday, when every form of wintry precipitation showed up across much of Eastern Washington and North Idaho.</p></p><p><p>On Dec. 15, 2009, falling snow turned into a mix of sleet, freezing rain and freezing drizzle. These forms of precipitation fell in some combination or changed back and forth throughout the day, depending on where you were located. Eventually, conditions caused an icy sheen to cover roads, sidewalks and power lines, resulting in dozens of vehicle accidents, including a four-car pile-up on Interstate 90, and thousands of customers without power.</p></p><p><p>On Jan. 11, 2018, similar, though less severe conditions in the Spokane region reduced visibility and produced slippery roads that snarled traffic and delayed the opening of schools.</p></p><p><p>Ice is never nice.</p></p><p><p>Sleet, frozen rain, frozen drizzle – what’s the difference, anyway?</p></p><p><p>It typically begins with snow. Regardless of the time of year, most precipitation originates as snowflakes high in the clouds. Whether they remain as snowflakes or change by the time they hit the ground depends on temperatures in the layers of air as they descend through the atmosphere. Snow will remain snow when temperatures stay cold all the way down.</p></p><p><p><strong>Sleet</strong> occurs when snowflakes partially melt in a pocket of warm air far above the ground but then refreeze in a cold layer of air above the surface. At this point, they become tiny ice pellets that bounce off cars, sidewalks and other hard surfaces. Sleet tends to run on the crunchy side, making it less treacherous than the icy sheen left by frozen rain or frozen drizzle. Even so, it can make travel tricky.</p></p><p><p><strong>Frozen rain</strong> falls through a thicker pocket of warm air than what sleet encounters with snowflakes melting into pure raindrops. Then, once they hit frozen ground, they freeze on impact into a clear glaze. Freezing rain can be challenging to predict because it’s hard to know if and when the subtle tango between pockets of cold and warm air will occur. It doesn’t take much to turn roadways and sidewalks into sheets of ice. The weight of accumulated frozen rain can also break tree branches and power lines.</p></p><p><p><strong>Frozen drizzle</strong> is similar to freezing rain except the droplets are tinier – less than half a millimeter in diameter, compared with the 1-5 millimeter size of a raindrop. Descending from clouds or thick fog, frozen drizzle appears mistlike. Although it’s unlikely to build up thickly on hard surfaces, its thin icy sheen can be deceptively dangerous to motorists and pedestrians alike.</p></p><p><p>Sleet, frozen rain and frozen drizzle (sometimes called frizzle) are especially hazardous when hidden beneath a layer of snow that falls over them. Not only is the ice layer invisible, but there’s little to no traction under the blanket of snow. As more winter storms parade through the Pacific Northwest, the possibility that multiple types of frozen precipitation will fall in the Inland Northwest goes up. As you go out and about, please, be alert for icy conditions. And keep in mind that bridges and overpasses are the first to freeze.</p></p><p><p>After all, winter doesn’t manifest itself by snow alone.</p></p><p><p>———</p></p><p><p><em>Nic Loyd is a meteorologist in Washington state. Linda Weiford is a writer in Moscow, Idaho, who’s also a weather geek. Contact: ldweiford@gmail.com</em></p></p>