Weathercatch: Logical rime, reason for early 2009 ice event

Twelve years ago this month, more and more white covered the Spokane area each day, but it wasn’t from snowfall.

From Jan. 13-23, 2009, persistent fog and episodes of freezing drizzle caused thick ice to build up on everything from roads and lawns to power lines and tree branches. But this wasn’t just any ice; it was rime – a winter phenomenon of white ice crystals that form when water droplets in fog rapidly freeze to a cold surface.

People often confuse rime with the more commonly-seen hoarfrost. Although both can create beautiful displays of ice crystals, they are formed under different meteorological conditions. Rime is created in dense fog or light freezing drizzle (colloquially known as “frizzle”), as was the case in January 2009. Hoarfrost, however, forms on clear cold nights when moist air is trapped near the ground. Also, rime is formed by minute liquid drops in fog when both the air and a solid surface are below freezing. Conversely, hoarfrost’s source of moisture is water vapor, not droplets, located in above-freezing air, rather than subfreezing air.

Rime is typically denser and harder than hoarfrost. When it builds up, it gets heavy, whereas hoarfrost remains light and therefore easier to remove from a surface.

So when dense fog and frizzle enveloped parts of the Inland Northwest for 10 straight days during the 2009 event, it lead to an unusually thick accumulation of rime across landscapes. Think about it – all those tiny droplets floating in the fog encountering solid, cold objects such as utility poles and freezing upon contact. As more fog developed, rime kept piling up on rime.

Although the extensive crystalline coating was stunning to see, it was also dangerous, with tree branches and utility lines collapsing under the weight of the accreted rime. Widespread power outages occurred and fallen tree limbs damaged several parked cars. Hardest hit were communities west of Spokane, including Reardan, Harrington and Davenport.

This January is a far cry from 2009. While we’ve seen fog – some of it dense – it’s been fairly localized and short-lived, often scoured out by winds or mild, sunny intervals. We also received some snow this week.

As the month draws to an end, our biggest opportunity for some sunshine is Friday. Beyond that, models are trending toward a light wintry mix of precipitation over our region. There’s a chance of some combination of snow, rain and a bit of freezing rain into early February.