Illinois has never been considered the Sunshine State. However, cities in Illinois are considered in the middle ranks of U.S. cities in terms of cloudiness. They are neither as sunny as Las Vegas nor as cloudy as Seattle.
Here is a breakdown of the number days of clear, partly cloudy, and cloudy days for the few sites in Illinois with long-term observations. Hourly weather observations of cloud types and coverage (in tenths) are combined into daily values. The daily average sky cover is assigned to three classes. Daily average sky cover ranging from 0 to 3 tenths is considered “clear.” Daily average sky cover from 4 to 6 tenths is considered “partly cloudy”, and from 7 to 10 tenths is considered “cloudy.”
You can also visit the National Climatic Data Center for more information on cloudiness of U.S. cities.