The Thaw

While there is some warming of the surface of the earth by the hot core of the planet, it is tiny and constant, and does not enter this process. The temperature of the air controls the process. As cold weather continues, the thickness of the frozen dirt increases. We do not notice this because the dirt was solid to begin with, and remains solid as the ice thickens, and we have a good ride. In the central Midwest, frost depth can reach a foot or more, and 18 inches is considered safe for burying water pipes, to see they do not freeze. In the late 70s here, the winters were very cold, and the frost occasionally was more than 18 inches. In Minnesota, the frost can be several feet deep, and in Alaska the frost line is very deep and can be permanent, “permafrost”.

The trouble for our trails begins on warm, especially sunny days, in late winter or spring. The surface reaches 32 and the ice begins to melt, at first just making the trail slick. Much more worrisome is when the thaw continues, melting the water to a depth that does not yet reach the bottom of the frozen dirt. The remaining frozen layer is an impervious blockage to the melted water above, which in warm, unfrozen dirt would percolate down from the surface, as it would in a summer rain, keeping the dirt from becoming plastic and too soft. When the last of the frozen dirt does thaw, percolation can take place, the water descends from the surface, and the trail dries and becomes ridable. There can be several freeze/thaws over a winter, making a rider very frustrated.