Illinois has certainly never been referred to as the sunshine state – but why is Chicago so cloudy, especially during the wintertime?

Data from the Illinois State Climatologist shows that in the span of an average year, Chicago typically sees mostly cloudy or cloudy skies on 50% of all days. The other half of the time, skies are split evenly between partly cloudy and mostly sunny conditions.

Winter is Chicago's cloudiest season, while summer is the sunniest. Chicago skies are cloudy 60% of the time in December – Chicago's cloudiest month – and only gray 33% of the time in July, our sunniest month.

There are several reasons that explain the persistent gray during the winter months: frequent storm systems, a temperature inversion, moisture from Lake Michigan, and a weak winter sun.

Frequent storms

The polar jet stream is typically over or near Illinois for much of the winter, driving repeated storm systems into our area that often bring clouds and precipitation.

Temperature inversion

Cold, shallow air masses frequently move into the Great Lakes during the winter months. As warmer, more moist air rides on top of the cold air at the surface, stratus clouds develop.

Moisture from the lake

As cold, dry winter air blows across Lake Michigan, it picks up moisture and heat, leading to clouds and lake-effect rain or snow. Even when no precipitation develops, cold winter winds tend to form low clouds on the downwind side of the lake. This is especially noticeable early in the winter when the lake is ice-free. Since Chicago's prevailing wind direction is offshore during the winter months, however, lake-enhanced clouds are not as common in the city as they are on the downwind shore in southwest Michigan.

Weak winter sun

Chicago's summer sun is three to four times stronger in June than it is in December, when the northern hemisphere is tilted away from its warming rays. This not only leads to colder temperatures overall during the winter months, but also means daytime heating can't warm the Earth's surface as efficiently and create rising currents of air capable of "mixing out" the low cloud cover.

If you don't want to wait until the summer months to see the sun, wait for a cold, dry winter airmass that brings northwest winds from Canada. This meteorological setup can lead to cold – but bright – winter days.