Dear Tom,
Why is the shortest day of the year, Dec. 21, not near the midpoint of the coldest weather of the season; and why is the longest day of the year, June 21, not near the hottest weather of the season?
--Martin Vesole, Highland Park
Dear Martin,
The annual cycle of temperatures lags behind the annual cycle of daylight by about one month---the so-called lag of seasons. Chicago's temperature cycle bottoms in mid-January and peaks in mid-to-late July. Even though heat input from sunlight increases after Dec. 21, the Earth is still losing more heat to space than it receives and temperatures continue to fall. By mid-January, though, increasing heat gain overcomes the loss and temperatures begin to rise. In the summer, heat gain, declining after June 21, exceeds heat loss until mid-July.
Why is the shortest day of the year, Dec. 21, not near the midpoint of the coldest weather of the season; and why is the longest day of the year, June 21, not near the hottest weather of the season?
--Martin Vesole, Highland Park
Dear Martin,
The annual cycle of temperatures lags behind the annual cycle of daylight by about one month---the so-called lag of seasons. Chicago's temperature cycle bottoms in mid-January and peaks in mid-to-late July. Even though heat input from sunlight increases after Dec. 21, the Earth is still losing more heat to space than it receives and temperatures continue to fall. By mid-January, though, increasing heat gain overcomes the loss and temperatures begin to rise. In the summer, heat gain, declining after June 21, exceeds heat loss until mid-July.
