This is no joke. It will be enforced for May 19 to May 21.
The flight advisory was issued by the Federal Aviation Administration. The advisory bans non-commercial aircraft from flying within 10 nautical miles of downtown Chicago at altitudes below 18,000 feet.
A nautical mile is about one minute of arc of latitude along any meridian. It amounts to 1,852 meters, or about 1.15078 standard miles.
In addition, the restrictions include an outer ring from 10 to 30 miles around downtown Chicago. Only planes arriving or departing at local airports will be allowed within that outer ring. Flight training, seaplanes, and other types of flights will be banned from the outer ring.