Longer days, more sunshine and temperatures

Dear Tom,
Sunshine has a big impact on temperatures, but our temperatures continue to fall for another month after the shortest day of the year, which is Dec. 21. Why?
Tim Standerall, Chicago
Dear Tim,
Sunlight is the source of the atmosphere’s heat, so it is reasonable to expect that Chicago’s temperatures should begin to rise after Dec. 21 as heat input from the sun increases. However, temperatures continue to decline into mid January because, even though heat input is increasing after Dec. 21, it is still less than the amount of heat being lost into space. Snow cover is also a factor that contributes to a decline in temperatures after Dec. 21. Snow, an effective radiator of heat, increases in coverage across North America well into January. Consequently, air masses arriving here from the northwest grow colder as snow cover increases.

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