Daily Archives: December 4, 2023

Each winter we keep track of the areal extent of air colder than 23 degrees Fahrenheit at the 850 mb pressure level (about 1 mile above sea-level) around the entire Northern Hemisphere. This measure allows us to characterize the intensity of the winter season with respect to the lower tropospheric temperature.

Over the past 75 seasons there has been a systematic decrease in the December-January-February, or DJF, average areal extent of about 4.6%, and this is an unequivocal sign of global warming, measured in the winter season. Continue reading →