The maximum extent of ice coverage on Lake Michigan each winter varies considerably. The maximum extent, expressed in percentage, depends on air temperatures, internal water currents, and wind speed/direction. Usually, the maximum extent occurs in February, but can happen in January or even early March. On occasions, Lake Michigan has been considered "frozen-over."
The task of determing the ice coverage on the Great Lakes was assigned to the Great Lakes Environmental Research Laboratory (GLERL)in Michigan. They maintain a NOAA Great Lakes Ice Atlas, going back to the 1972-73 winter utilizing satellite imagery.
Below are links for ice coverages on Lake Michigan and Lake Superior.
Below is a simple graph showing the year-to-year variations. The maximum ice coverage of 93% occurred in the 1978-79 winter, followed by 90% in the 1976-1977 and 1993-94 winter seasons.
We did some digging into various sources, including a book entitled "Wisconsin's Weather & Climate" - 2002 by Moran and Hopkins, and came up with a list of when there was a good probability that Lake Michigan was considered to be "frozen-over." Below is the list:
The winters of.......
1888-1889 (Feb 1889)
1903-1904 (Feb 1904)
1935-1936 (Feb 22, 1936)
1962-1963 (Feb 1963)
Note that the winters of 1976-77 and 1978-79 are listed, but the data on the GLERL web site suggest that these two winters had a maximum ice coverage of 90% and 93%. Consequently, it may be that an ice coverage of about 90% or more qualifies for a "frozen-over" designation. Following is a depiction of the ice coverage in Feb 1979, from GLERL, when Lake Michigan was considered likely "frozen-over".
We'll be working with the GLERL folks in the next week to get a better understanding of what all of this means, and update this story and graph above as we obtain more information.
Our counterparts at the Marquette, Michigan, NWS office looked at the ice coverage maps for Lake Michigan back to 1973, and came up with the following listing of winter seasons when Lake Superior had at least a maximum of 90% ice coverage (would be considered as "frozen-over"):
1976-1977, 1977-1978, 1978-1979, 1985-86, 1990-91, 1993-94, 1995-96, 2002-03
...Rusty Kapela, WCM, & Penny Zabel, Met Intern, WFO Milwaukee, and Matt Zitka, WCM, WFO Marquette