What causes big differences in solar radiation in Chicago throughout the year?

Dear Tom,
Several years ago, the Tribune weather page compared the amount of solar radiation, measured in langleys, on the winter and summer solstices. I don’t remember the numbers, but I recall being struck by the large contrast.
— Virginia
Dear Virginia,
The change in incoming solar radiation is quite large in Chicago. Variance increases with latitude from the equator and is generally reflected in the climatological change in temperature between January and July.
Using 42 degrees N for the latitude of Chicago, about 300 langleys of radiation per day are received on the winter solstice, versus about 950 on the summer solstice. Maxima and minima in average temperature lag the solstices by 3-4 weeks. Also, on the winter solstice, the peak elevation of the sun is 24.5 degrees above the horizon, versus 71.5 degrees on June 21.
Variance increases with latitude from the equator and is generally reflected in the climatological change in temperature between January and July.