Freezing degree-days (FDD) or thawing degree-days (TDD) are defined as departures of air temperature from 0oC.  When temperatures fall below 0oC, it gives a positive value, whereas it becomes negative when temperatures rise above 0oC. This index is a measure of both duration and magnitude of below-freezing (or above-thawing) temperatures during a specified period. Therefore, the cumulative values of FDD or TDD for a given winter season or summer tell how cold or warm it has been for how long.  As such, both FDD and TDD have been used to describe weather patterns and climate warming or cooling over time, as well as a proxy for the state of melting or freezing of arctic sea ice.

The following four plots show examples of cumulative FDD (top two) and TDD for just the winters and summers of 1949 and 2009. The values were computed with the NCEP daily mean air temperatures on a 25-km resolution at a 361 x 361 grid. The daily values of FDD at each grid cell were integrated from September 1 of each year to May 31 of the following year.  The daily values of TDD at each grid cell were integrated from May 1 to September 1 of each year.

A comparison of FDD between 1949 and 2009 shows a widely spread warming trend over the whole Arctic, whereas the comparison of TDD between 1949 and 2009 shows significant warming over the eastern Arctic and the Chukchi region. The overall picture is that the Arctic is warming up since 1949. This warming has been shown to have widely spread impacts on arctic sea ice and the whole ecosystem as well as the indigenous communities.

For any questions or requests for further information on the NCEP daily mean air temperatures, please contact Jim Maslanik at For any other inquiries regarding the computed FDD and TDD, please contact Yanling Yu at

Click each image for larger versions.






Site developed and maintained by Polar Science Center. Copyright © 2010, Seattle, Washington, USA
This project is supported by the Nation Science Foundation, Arctic Natural Science Program.