"Freezing Degree Days" (FDD) are defined as the temperature below freezing. Here FDD use 0°C as the freezing point like the Polar Science Center. Other institutions use 1.8°C as the freezing point.
Example: 1. Day: average Temp: 3°C 2. Day: average Temp: 5°C 3. Day: average Temp: 7°C Total: 15°C
ERA40 is a reanalysis data set from ECMWF that has been applied to calculate daily mean temperatures (2m) for the period from 1958 to 2002.
Data source: Danish Meteorological Institute:
Orginal project by Andrew Slater: http://cires1.colorado.edu/~aslater/ARCTIC_TAIR/index_80_t2m.html
This graph shows Freezing Degree days over the whole freezing season. The sea ice thickness can be estimated based on the following formula (Lebedev 1938): Thickness (cm) = 1.33 * FDD (°C)^{0.58}
It is important to note that the calculated thickness using this formula only accounts for the freezing of sea water and not ice growth from snowfall, freezing rain or ridging. By using 0°C as the freezing point the formula matches measured ice thickness better.
More info at: https://nsidc.org/cryosphere/seaice/processes/thermodynamic_growth.html
