Sunday, February 26, 2023

Snow and Snow Ratio

Fairbanks has seen a fair bit of snow this month, with measurable accumulation on 16 days and a total of 20.6" so far.  The record number of days with accumulating snow in February was set just last year, at 18 days.  The long-term median February snowfall is just 7 inches.  However, we're nowhere near the February 1966 record of 43".

It's interesting to note that this is now the 7th consecutive February with above-median snowfall, including 5 of 7 with over 20".  Remarkably, only 7 of the preceding 87 years (since 1930) had this much snow in February.  It used to be rare - fewer than 1 in 10 years - but is apparently now common.

Last Thursday's snowfall of just under 6" contained nearly half an inch of liquid equivalent, so it was fairly dense stuff by interior Alaska standards.  This got me thinking about snow ratios and the long-term trend thereof.  Is there any evidence that the snow ratio (total snowfall divided by liquid-equivalent precipitation) has changed over time?

Yes, absolutely: the last decade or so has seen generally lower snow ratios than in the previous six decades, and there have been no winters with a historically high ratio (e.g. over 20) since 2009-10.  Last winter had the lowest ratio on record for Fairbanks, only 10.5, and a large reason for that was of course the epic rain storm of Christmas 2021.

It's interesting to see that mostly low ratios also prevailed in the 1930s and 1940s, although I suppose there's a chance that different measuring practices could explain at least some of this.  However, we know there were some notable, indeed extreme, winter rain events in the 1930s, so a regime of low ratios is perhaps not unexpected.

The recent decrease in ratios is more noticeable for November-December than for January-March, although the absence of high ratios clearly stands out for both:

The low ratio in Nov-Dec 2010 reflects the exceptional rain event of that time - as documented here in the early days of this blog:

As for the low ratio in January-March 2016, that reflects the extraordinary rain at the end of March, which produced as much precipitation as the rest of the (very dry) 3-month period combined.

A decrease in snow ratio is what we would expect if the atmosphere is warming overall.  Snow tends to be more dense at higher temperatures, and of course increased warmth favors occasional winter rain, which crushes the total snow ratio.

The ERA5 global reanalysis (model) also shows signs of a change since 2010-11, with most years having a lower ratio of total snow water to total precipitation - see below.  This is not the same thing as snow ratio, as it's only affected by the rain vs snow partition, and not by snow density - so the ERA5 is just showing an increased frequency of significant winter rain.  It would be highly worthwhile to look at the spatial distribution of this change in the model data.


  1. Almost all the winter's snow is still sitting on trees at valley level. No wind in Fairbanks made it so. But being March, today we're finally seeing 14mph winds from the west. The trees are dispatching their load of snow plus a few attached branches. Spring has sprung!

    1. The March wind increase is interesting, because average winds aloft are already diminished from their seasonal peak in January; but of course the inversion is being defeated by the sun, allowing momentum to mix down to the valley floor. Also, the average N-S pressure gradient across the state is higher in March than in February. Another good topic for a post.