Climate Watch

The transition from summer to winter activities and travel over the land in western Alaska sometimes happens in short order, with snow and sustained freezing temperatures coming and staying, and other times it’s a protracted affair, with repeated freeze and thaw cycles with snow then rain and back again.

Has the timing of this “land freeze-up” changed over the past century?

One simple climate tool that we’ve used with some success in estimating how far along land freezeup is called the “Net Freezing Degree Days”. Bethel Search and Rescue has maintained detailed records for a number of years on the safety of Kuskokwim River for travel at various dates, and the freezing degree day concept has proved useful in giving a rough idea of the state of ice development on the lower Kuskokwim River.

So, what’s a “freeze degree day”? It’s actually a simple concept: just take a day’s average temperature and subtract it from 32°F. If today’s average temperature were 30°F, then we would have two freezing degree days. If today’s average temperature were 34°F, then we would have -2F freezing degree days. If we keep a running tally of this in the autumn, we get a measure of the “accumulated cold.”

Here we’ve extracted the date when the running total of the net freeze degree days gets to 100, and we’ll use this as a rough but consistent guess for when the land would be freezing up (for reference, five days with an average temperature of 20°F would come to 100 freezing degree days). When we do this, several interesting facts emerge. For one thing, this corresponds with what we already know: there has been a lot of year-to-year variability on when the land freezes up. Secondly, while there have always been individual years with very late freeze-up (1926, 1979, 1998), the typical “land freezeup” came in late October for most of the 20th century but that has changed and can now be expected in the second week of November.