Shown are daily surface areas on the 450 K isentrope derived from ECMWF data for 1 September 1957 onward.
They are based on 2.5x2.5 degree daily (12 h UT) analyses on pressure levels, either from the ERA40 reanalysis (September 1957-August 2002) or from operational analyses after 2002.
The first 15 years of ERA40 are yellow, the second 15 years from ERA40 are lightblue, and the third 15 years form ERA40 are orange. The black lines give minimum, maximum and mean temperature over ERA40. The red line is the current year (2011).
| PV > 28 PVU | T < -78 C | |
|---|---|---|
| Northern Hemisphere | 28 PVU area | -78 C area |
| Southern Hemisphere | 28 PVU area | -78 C area |
