Chicago has a reputation for frequent cloudiness, prompting many to seek an objective measure of the city’s sky conditions. To move past anecdotes and local lore, it is necessary to examine meteorological data to determine the frequency of cloudy days. Long-term climate records provide a clear answer to how often the sun is obscured.
How Meteorologists Define Cloud Cover
The classification of sky conditions relies on a quantitative system to measure the portion of the sky covered by opaque clouds. Meteorologists use a scale that divides the visible sky into tenths or eighths, known as oktas, to determine cloud cover. For long-term climate analysis, the daily average sky cover is categorized into three classes based on a tenths scale.
A day is considered “clear” if the average sky cover ranges from zero to three-tenths, meaning most of the sky is visible. A “partly cloudy” day falls into the middle range (four to six-tenths). The designation of a “cloudy” day is applied when seven to ten-tenths of the sky is covered by clouds, indicating a near or total overcast condition. This standardized approach allows for accurate comparison of cloudiness across different regions.
Chicago’s Annual Cloud Cover Statistics
Chicago averages 176 days each year officially classified as “cloudy,” meaning the sky is more than 70% obscured (seven-to-ten-tenths cover). The city also experiences an average of 105 “partly cloudy” days and 84 “clear” days annually. This places Chicago in the middle ranks of U.S. cities for cloudiness, significantly sunnier than places like Seattle but not as clear as the southwestern states.
The incidence of cloudiness is not evenly distributed throughout the year, showing a strong seasonal fluctuation. The winter months experience the highest frequency of overcast skies. December is typically one of the cloudiest months, with the sky being overcast approximately 67% of the time. January also ranks high, with the sky being mostly cloudy or overcast roughly 59% of the time.
Conversely, late summer and early fall bring the clearest skies to the region. August is statistically the clearest month of the year, with the sky being clear, mostly clear, or partly cloudy about 67% of the time. This change highlights the strong climatic controls influencing the city’s sky conditions.
The Meteorological Causes of Chicago’s Cloudiness
The primary meteorological factor contributing to Chicago’s cloud cover is the continental climate, featuring frequent short-term weather fluctuations due to passing storm systems. The polar jet stream often travels over or near Illinois during the fall, winter, and spring. This jet stream is instrumental in forming and moving low-pressure systems, which are characterized by extensive cloud cover and precipitation.
Lake Michigan also exerts a significant influence on local cloud formation, particularly during colder seasons. When cold air masses move across the warmer lake water, moisture evaporates. This moisture cools and condenses, fueling the formation of low-level clouds, known as lake-effect cloudiness. This process is most noticeable in late fall and early winter before the lake freezes.
The lake’s effect on cloudiness is reversed in the summer, when the water remains cooler than the surrounding land. This cooler surface stabilizes the air directly above the lake, suppressing the development of convective clouds and reducing cloud cover near the lakefront. The city’s cloudiness statistics reflect a complex interplay between large-scale weather patterns and the localized thermal effects of the Great Lake.