Daily Archives: July 20, 2020

On more than one occasion in this column we have commented on the areal extent of air colder than 23 degrees about 1 mile above the 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, and the complete absence of such air lasts only a very few days. Continue reading →