south pole accumulated deviation from average freezing days. How do I interpret this? Is the temperature on the south pole since September on average 8-10 deg higher now??
A definition of FDD
. The fact that January accumulated about -250 FDD suggests that the temperature was, on average, about +8°C on an absolute scale (not a relative scale).
In theory, FDD should be a simple proxy for the amount of freezing and thawing going on because the amount of heat transfer is proportional to the temperature differential multiplied by time (obviously, this ignores the effect of wind and sun). If there's five days at -1° C, that's +5 freezing-degree days which would induce a certain amount of freezing of any liquid water. If the following day averages +5°, that's -5 freezing-degree days which would undo the previous effect of five days of light freeze.
That this year's accumulated FDD from Sept to Feb has been -1250 but the usual FDD is around -560 suggests: 1) there's been more than twice as much thawing occurring; 2) temperatures have averaged about (-560 - -1250)/150 = + 4.6° C warmer than those of previous years.