Cold spot returning on path to winter

Steve Ackerman and Jonathan Martin University of Wisconsin

Question: How far has summer beaten back the areal extent of cold air?

Answer: On more than one occasion in this column we have commented on the areal extent of air colder than 23 degrees Fahrenheit at 850 hPa – about 1 mile above ground – as a measure of the extremity of winter.

In the middle of January, about 68 million square kilometers of the Northern Hemisphere are covered by air that cold at that level. Between about July 5 and July 20 that area shrinks to zero, but the complete absence of such air lasts only a very few days. At the beginning of August we are just past the point in summer that the area is beginning to increase again. Thus, despite the fact that we went through a couple of really hot and humid weeks to end July, the return of winter has already begun in terms of that measure.

That's largely because the number of daylight hours has already begun to decrease. At every latitude north of 66.5 degrees, the sun was out for 24 hours on the day of the summer solstice – June 21. Ever since the specter of nighttime has been creeping poleward, with daylight decreasing ever so slightly just above the Arctic Circle. The associated night-time cooling is greater during a longer night, resulting in the gradual increase in the amount of cold air covering the Northern Hemisphere.

So if the past couple of weeks have been a bit too much heat, perhaps some solace can be found in the fact that the tide is already, however imperceptibly, beginning to turn.