The snake is old

The snake is old and very cold. There is only one — or, at least, I've never seen more than one at the same time, or tracks of two different sizes left on the same day. The snake changes size through the year. He grows larger as the days grow shorter and the weather colder, and attains his greatest size after the solstice. Thereafter he starts shrinking. When the spring comes and the snow melts, he vanishes. I don't know where he goes. At late autumn when the ground is only occasionally white and early spring when the sun burns bright white and you can hear the murmur and gurgle of melting water as it runs under the snow-banks he is no larger than an ordinary viper. Only his colour distinguishes him, for he is dirty blue-white like thick ice on top of a deep lake. At his largest, I have seen him slither down the slope of the hills in the distance, the end of his tail still hidden by the trees and his head already down in the valley. I know he has been through a field when I see a streak of blackened, bruised, and dead plants where the field otherwise has not known the bite of freeze. Anything that touches him must become as cold as him. He has never bothered me, and I avoid him. I don't know what he eats. He has been here for a long time.