54 years of Great Lakes ice cover

NOAA has recorded daily ice cover on each Great Lake since the 1973 ice year. That record is what makes it possible to say whether this winter is actually unusual, rather than just feeling like it.

What the record actually contains

The GLERL ice climatology gives daily average ice cover as a percentage for each lake, from November through early June, for every ice year since 1973. An ice year is labeled by the January calendar year, so the 1975 column holds November and December of 1974 along with the early months of 1975.

One detail in that dataset is worth appreciating, because it is a small model of the honesty problem this whole site deals with. The record distinguishes a recorded zero from a day where ice cover simply was not measured. A zero means someone looked and there was no ice. A gap means nobody looked. Conflating the two would quietly manufacture data that does not exist.

When Michigan ice peaks

Averaged across the full record, Lake Huron ice cover peaks in the third week of February, at roughly half the lake surface. That average conceals enormous year to year variation, and the variation is the real story: on the same mid February date the record contains years under 10 percent cover and years above 95 percent.

For an ice angler that spread is the whole point. A mid February date is not a season, it is a coin flip weighted by that particular winter. Which is exactly why accumulated cold for the current season matters more than a calendar.

Lake by lake, and why Michigan waters differ

Ice behaves differently on each lake for reasons that come down mostly to depth and shape.

• Lake Erie is shallow and freezes most completely, often approaching full cover in a cold winter.
• Lake Huron carries Saginaw Bay, which locks up well ahead of the open lake because it is shallow and protected relative to the main basin.
• Lake Michigan is deep in the middle and rarely freezes across, but its northern end and protected bays like Little Bay de Noc make reliable ice.
• Lake Superior is deep and cold, and its ice varies dramatically between mild and severe winters.
• Lake St. Clair is shallow and small enough to freeze readily, but it has current running through it constantly, which is a different problem entirely.

A lake wide percentage is a blunt instrument for a specific bay. Grand Traverse Bay can be wide open while Lake Michigan as a whole reports meaningful cover, because that cover is concentrated in the far north and in shallow water elsewhere. The region pages here note where the lake wide number is and is not informative.

Using history to read the current season

The useful comparison is not this year against last year, it is this year against the distribution. If today sits above the 54 year average for this date, ice is running ahead of a normal season. If it sits in the bottom quarter of years, no amount of one cold week is going to catch it up quickly, because the square root relationship means late cold adds less thickness than early cold does.

That is the comparison the front page makes automatically, and it is the reason this site pulls the whole historical file rather than just today.