From January 21-January 29, 2017, Chicago Illinois experienced 8 consecutive days of overcast conditions and no sunlight. This string of 8 consecutive cloudy days is special because it is the cloudiest spell Chicago has experienced in over 25 years. The last time 8 or more consecutive days of overcast skies occurred was back in late December of 1991 stretching into the second week of January, a full 12 day stretch, which also happens to be the cloudiest stretch Chicago has ever experienced in recorded history. This period was from December 29, 1991 – January 9, 1992. January 2017 has also been designated as the second cloudiest January in recorded history.
Why do such long stretches of continuous cloudiness occur?
The winter season is well known to be very cloudy in Chicago, especially from November through January, so it is no surprise that this impressive stretch of overcast skies occurred in January. Cloudier skies in the winter are largely due to the angle that the sun is in the sky and the heat that is absorbed by the ground. Due to lower angles of the sun during the winter months, less heat (energy) is transferred overall, therefore less energy is absorbed by the ground. When grounds are cooler, less heat is released into the nearby atmosphere resulting in cooler air. With cooler air near the surface, and warmer air above, this creates a much more stable column of air. When the atmosphere is in a state of stable conditions, vertical motions are more restricted due to a lack of buoyancy. Buoyancy is essentially the property of an object that allows it to “ascend through and remain freely suspended in a compressible fluid such as the atmosphere.” (American Meteorological Society, 2012) Condensation and clouds exists in more horizontally stratified states, so when the atmosphere is stable, it is allowing for these cloudy conditions to persist for extended periods of time. It takes some forcing in the atmosphere such as a system moving through to disrupt this stability and end cloudier spells.
A glimpse at the Mid-West United States
Recent synoptic patterns have resulted in cloudy conditions for a lot of the midwest as well.
For more information visit: https://weather.com/news/news/midwest-cloudy-streaks-january-2017
(Weatherchannel.com, Jan 30, 2017)
“Chicago’s longest strings of cloudy weather:
1. 12 days: December 29, 1991 – January 9, 1992
2. 10 days: December 16-25, 1895
3. 9 days: February 4-12, 1897
4. 8 days: January 22-29, 2017 (current)” (WGNtv.com, 2017)
WGNtv.com, Weatherchannel.com, American Meteorological Society