WINTER SUB-FREEZING PERIODS AND SIGNIFICANT THAWS IN THE BOREAL FOREST REGION OF CENTRAL NORTH-AMERICA

Winter daily maximum temperatures were examined for 56 sites in northe rn portions of Ontario, Michigan, Wisconsin, and Minnesota over the pe riod 1960-88. The longest sub-freezing period of winter averaged 20-30 days in the southern portion of the region, 30-40 days around Lake Su perior, and 90-100 days in extreme northwestern Ontario. These are twi ce as long as sub-freezing periods at similar latitudes in eastern Can ada. The sub-freezing period is shortened by about one week along the shores of the Great Lakes. There is annual spatial correlation of the longest sub-freezing period, indicating regional synoptic-scale contro l. The late 1970s had the longest sub-freezing periods but no signific ant linear trend was found in lengths of sub-freezing periods. The ave rage date of the first significant thaw (> 10-degrees-C) ranged from e arly March in the south to late April in the north and is delayed 1-2 weeks along the shores of the Great Lakes.