Chicago, IL UV Index Today
What is the UV index in Chicago right now?
0.8 Low
EPA describes low UV as needing no protection for most people.
Today's peak is 6 (High) at about 1pm local time.
When is UV strongest in Chicago today?
UV in Chicago peaks at 6 (High) around 1pm local time (GMT-5). It stays at 6 or above — the high band — for 1 hour, roughly 1pm to 2pm. UV is 3 or above for 6 hours.
Chicago today, in numbers
- Latitude
- 41.9°N — 33 of 121 from the north
- Today's peak
- 6 (High) — 81 of 121 nationally
- Peak arrives
- 1pm local (GMT-5)
- At or above 6
- 1 hour, 1pm to 2pm
- At or above 3
- 6 hours, 10am to 4pm
The window from 1pm to 2pm is the practical answer for Chicago today. Nationally today the spread runs 11.4 in Hilo (20°N) to 1.3 in Fargo (47°N). Why latitude and the clock decide this →
The lake clears summer sky and builds winter cloud
Lake Michigan does two opposite things to Chicago’s sky, on opposite halves of the year. In summer the water is colder than the land, and the lake breeze pushes that cool, stable air a few miles inland. Stable air resists the rising motion that builds afternoon cumulus, so on a lake-breeze day the clearing shows up as a boundary running roughly parallel to the shore, the lakefront under open sky and ground a few miles west under cloud.
Late fall and early winter reverse the temperature difference. The lake is then warmer than the air crossing it and feeds moisture into it. The Illinois State Climatologist’s account of Chicago’s climate has the lake increasing cloudiness over the area and enhancing winter precipitation through lake-effect snow, which reaches the city on winds out of the north or northeast rather than the prevailing westerlies. Those are the weeks when the sun has least to give anyway.
Sun angle sets the outer limit either way. At 41.9°N the noon sun reaches about 25 degrees at the December solstice and roughly 72 in late June, a swing of some 47 degrees. The lake widens both ends of it. It takes cloud off the shoreline in the season when the sun is already high, and puts cloud back in the season when the sun has nothing left. Most of the country gets those two effects from unrelated weather. Here they come from the same body of water, six months apart.
Nearest metros we model
What does Chicago’s UV index mean today?
0.8 is Low. EPA describes low UV as needing no protection for most people. Today Chicago also reaches High at its peak — a different band from the one it is in right now, which is why the hour matters as much as the day.
Modelled, not measured
The UV figures for Chicago are computed by the Copernicus atmosphere model, not read off an instrument here — unlike the AQI on our air-quality pages, which comes from a regulatory monitor. Cloud is where a UV model is weakest. What that means in practice →
Questions about UV in Chicago
What is the UV index in Chicago today?
When is UV strongest in Chicago today?
Is this measured or modelled?
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