Teachable moments

Winter is here and in full effect...the salmon run has all but stopped for the season, and with it the fieldtrips to Carkeek Park.  I took my last group of middle-schoolers on a romp around the woods this afternoon in search of the PACNW iconic fish.  We saw several carcasses, their eye-balls excavated by well fed crows and gulls, and two tails of two live ones peeking out from beneath a log, gently sweeping away the sand and silt from what seemed to be a perfect nesting site.   Good luck little fellas. 

The salmon sighting however, although really cool and very fortunate, was a side note for both me and the kids.  What really had us talking was the wintery wonder land that has descended on Seattle over the past week.  Carkeek, being set in the valley carved out by Piper's Creek, is a low spot on the map.  The sun's shallow winter track barely allows for any direct light in some parts, and as a result, the hoarfrost that my fellow naturalist, Stephanie, calls "ephemeral formations" isn't all that ephemeral.  It sticks around and grows through nightly accretion and produces some of the craziest looking miniature ice kingdoms.  If you've ever grown a crystal on a string, or made rock candy, you can picture exactly what I'm talking about. 

So in honor of my students today who toughed it out, frozen toes and all - a brief post on that magical stuff that meteorologists call hoarfrost:

Hoarfrost is also called radiation frost because it forms when heat radiates from the ground and is lost into the atmosphere.  This has been happening a lot around here lately because we've had such clear nights.  Without any clouds to trap that surface heat that collected throughout the day, the air quickly cools as the sun goes to bed and the ground quickly freezes.

Cold air has less ability to hold water vapor than does warm air.  So as air cools, any water vapor present is forced to condense, and becomes liquid.  The point at which this happens is the dew point (or in this case, the frost point).  The colder the air, the lower the dew point, so even mostly dry air will deposit frost if it gets cold enough.  For example, tonight's forecast has the dew point at 11°F.  Now even though the air temperature will only reach about 20°F, those low lying areas (like Carkeek Park) where cold dense air will sink and collect will be much colder at ground level and hoarfrost is sure to form.

The good news from a gardeners standpoint is that it won't be very soggy for the next few days.  The bad news or coarse is that the ground is impenetrable.  I can't believe I'm saying it, but I could go for a little cloud cover soon.  If this cold snap lasts any longer, Clara and I may have to get another dog to keep us cozy.