Ice Cover
For most of the nineteenth century Lake Champlain froze over almost every winter, and somebody wrote down the day it happened. The record runs unbroken from 1816 — the year without a summer — to this past February, which makes it one of the longest continuous observations of anything in the northeast. It is kept as a plain table on a weather-service page.
Watch a winter
The bed of the lake, north to the right, colored by depth — palest in the shallows, deepest blue down the four-hundred-foot (122 m) trench. Ice forms where the water is shallow and forms last where it is deep, which is why the two ends and the bays freeze first and the broad lake off Burlington freezes last, or not at all. Pick a winter and run it.
What “closed” means
Closed means more than that there was ice — there is ice most years, in the bays. It means the lake shut: historically, that commercial navigation stopped, and later that ice could be seen running continuously from Burlington west across to Port Kent. Since 2009 it has been called from satellite imagery. That is three different instruments over two centuries, and the join between a shore observer and a satellite is the one real weakness in the series.
The chart shows a change far larger than a change of method could explain: the difference between a lake that closed every winter for fifty years running and one that has closed nine times in twenty-six. The median closing date moved eighteen days later over the same stretch.
By half-century
| years | closed | of | median date |
|---|
Two and a half weeks later, and two thirds fewer winters in which it happens at all. Both changes are almost entirely after 1950.
Can the closing be predicted from freezing degree days?
Bates, by way of Myer and Gruendling, gives a threshold for this lake: about 240 to 280 freezing degree days for a permanent freeze in Shelburne Bay. A model built on that idea is rebuilt here on every data run, fitted on the record above and scored on winters it was not fitted on. It does not meet the skill threshold set for it, so no forecast is shown.