Average Sunshine in Illinois for September
Number of Sunny Days
In the table below, the average number of Sunny Days for a city in Illinois is the total days this month when the sky is mostly clear. This includes the days when cloud covers up to 30% of the sky during daylight hours.
Partly Sunny Days have cloud covering from 40% to 70% of the sky during the daytime.
Total Days With Sun is a sum of the Sunny plus Partly Sunny days. The rest of the days are mainly overcast, with at least 80% cloud cover. All the numbers are annual averages, made from years of weather watching.
Amount of Sun
Here's another way to look at how much sunshine a city gets. The % Sun number measures the percentage of time between sunrise and sunset that sunshine reaches the ground. Again, these amounts are monthly averages based on many years of weather observations.
National Climatic Data Center. Comparative Climatic Data.