Why do the seasons lag the daylight amount?
The shortest day (least daylight) is Dec 21st (1st day of winter), but coldest is usually late Jan or early Feb. The same goes for June 21st (1st day of summer), where it's generally warmer in the following months. Shouldn't Dec 21st be the coldest day and June 21st the warmest day in the northern hemisphere? The answer is "no" .
This is called the seasonal lag in temperatures. For example, Lincoln, NE:
June 21, 2010: Sunrise 0555 AM, Sunset 9:02 PM (15 hours and 7 minutes of daylight)
June 21, 2010: Sunrise 0613 AM, Sunset 8:53 PM (14 hours and 40 minutes of daylight)
Lincoln Average high temperature June 21 is 87 F; July 21 is 90 F
Although the amount of daylight is decreasing after June 21, it still exceeds the loss of heat to space and the temperatures continue to rise for 4-6 weeks. Think of the analogy of stopping a car. Hitting the brakes, it takes some time for the vehicle to stop going forward.