http://neven1.typepad.com/blog/2016/04/winter-analysis-addendum.html#more

Another way to express how warm or not-cold the Arctic has been this winter, is through so-called freezing degree days. The NSIDC explains it as follows in the All About Sea Ice segment of their website:

The relationship between thermodynamics and sea ice thickness can be thought of most simply in terms of freezing degree days (FDD), which is essentially a measure of how cold it has been for how long. The cumulative FDD is simply daily degrees below freezing summed over the total number of days the temperature was below freezing.

In other words, the sum of the number of days below 0 °C multiplied by the temperature for each day. Here’s a graph from the +80N 2m Tair page on Dr. Slater’s website that shows this year’s FDD compared to those of other years:

In spite of the catastrophic collapse of this statistic, caused by the warmest year since records have been taken, this does not necessarily mean this summer’s ice low will be a record, that is, break the 2012 drop in ice. Ice cover in summer is poorly correlated with winter extent because of variability in weather and current factors. Still, here it is…

On my monitor, the colors and text come out wonky, so here is the direct link to the image:

http://neven1.typepad.com/.a/6a0133f03a1e37970b01b8d1b7816e970c-pi