This pic. from the Chicago GLERL camera (from NOAA Coastwatch) at the Water Intake (the Harrison-Dever Crib shows a low cloud layer over Chicago. We’ve seen this layer of clouds more often than usual this month because of a persistent and humid east or northeast wind. When this pic.was taken the air temp. was 61.5, the dew point was 60.7 and the relative humidity was 97.1%. The wind was north-northeast coming down Lake Michigan.
The buildings of Chicago act like an elongated mountain. Air is forced to rise to get over the buildings. When it does that, it cools and the air becomes saturated and condenses out in the form of tiny water droplets…a cloud, or elevated fog. This cloud layer would not form to the north of south, where there are no skyscrapers.
It’s interesting because you can ride an elevator in a skyscraper and go from cloud above you at ground level…then you ride through the cloud (like being in fog) and then you rise out of the fog and into the sunshine. There is also a scattered layer of higher level clouds.