The usual four seasons, especially as defined by the equinoxes and solstices, don’t work very well for interior Alaska. Show cover is generally established by a month after the autumnal equinox, and stays on the ground until well after the vernal equinox. Rivers freeze a little later and remain frozen longer in the spring, and the only running water for six months of the year is in hot springs and indoors. But there is one season that everyone both longs for and dreads: Breakup.

Breakup is the time of year when snow melts and rivers thaw. The two are connected by more than sunshine and warmer weather. Melting snow makes mud (one of the reasons breakup is a time of some dread) but it also runs into rivers. If the water rises in the upper stretches of a river before lower reaches are thawed, as often happens in Alaska, the result can be ice jams and resultant flooding. I’ll talk about that some other time, but right now I want to discuss the simple process of melting snow.

Clean snow reflects most of the solar energy that strikes it. Some of the sun’s rays are absorbed within the snow pack, and cause internal melting and settling — but this is a slow process. Even clean snow, however, is a very good absorber in thermal infrared wavelengths. The sun doesn’t put out much energy in these wavelengths, but buildings, trees, and just about everything else except polished metal does. As a result, snow near the south side of a building melts much faster than snow out in the open. So does snow near tree trunks.

I see this every year. In addition to the photo of my road, which is rapidly turning into mud, I took two of the north and south yards of my house, minutes apart. Both areas got almost exactly the same amount of snow, and both have very similar exposure to sunlight. The snow stake still has a good 18” of snow. The ground around the birch is almost bare.

Why? Two reasons, actually, and the combination explains why open birch forest is usually the first natural area free of snow around here. First, birch trees hold their seeds through winter, and drop them shortly before breakup. As a result the seeds on the snow around the tree absorb the solar radiation and transfer that energy to the snow, speeding its melt. Natural selection? Quite possibly. It certainly seems likely that the enhanced snow melt, leading to earlier warming of the ground, would help the tree.

Second, the tree itself absorbs some solar energy, and then re-radiates it to the snow in the form of thermal infrared. Just about any object poking through the snow this time of year has a little depression around it. Spruce trees do an even better job of absorbing sunlight than do birches, but they also shade the ground and transfer much of the energy they absorb directly to the air. As a result spruce forest, while it probably does a better job of warming the air than birch forest, is among the last areas to have completely bare ground.

On a different note entirely, one of the fixtures of breakup in Fairbanks is the Beat Beethoven 5 km race, a fundraiser held today for and by the Fairbanks Symphony Orchestra. I won’t be running this year, though I did “run” with a cane once — and came in last. The idea is to cover the 5 km before the end of Beethoven’s 5th Symphony, about 30 minutes. I’m volunteering this year to park my car along the race route with the radio tuned to 91.5 (KSUA, the campus radio station) blaring out Beethoven’s 5th. I expect temperatures below 50°F and much of the course to be slippery or wet!

Added later (after the race.) This is definitely a family race. There were parents pushing their children in strollers, parents with children in backpacks or riding piggyback, dogs, and one contestant on crutches. (And she wasn’t at the end, either.) I did have a bit of a problem in that instruction to volunteers said if possible, not to have your car idling as the runner went by. I did. And needed a jump to start the car after the battery totally discharged itself.

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