LAKE SUPERIOR CLIMATOLOGY

Lake Superior Overview

Located along the U.S. Canadian border, Lake Superior is the northernmost of the five Great Lakes, connecting to Lake Huron through the St. Marys River. Lake Superior is the largest lake in the system by volume and surface area and is surrounded by several wildlife areas and national parks. Lake Superior is also the deepest of the Great Lakes, based on its bathymetry, and includes inflows from the Long Lac and Ogoki water diversions.

Over-lake precipitation varies seasonally, with higher totals occurring in the late spring, summer, and early fall months (Figure 5). This seasonality aligns with the seasonal variability of lake levels, with the highest levels occurring at the same time as the greatest total precipitation. Precipitation totals vary annually as well as seasonally, and do not always follow the same pattern year to year.

Runoff rates follow the same general monthly pattern, but can vary in magnitude from year to year (Figure 6).  Peak runoff occurs in late spring and early summer, primarily due to snowpack melt.  Runoff amounts are also influenced by impervious surfaces such as concrete, that prevent water from entering the soil below, and agricultural practices within the basin.

Evaporation rates follow the same general monthly pattern from year to year. Maximum evaporation occurs in the late fall and winter months with minimum evaporation occurring in the late spring and early summer months (Figure 7).  Because water has a higher heat capacity than air, the lake stays warmer in the fall as air temperature falls. This leads to increased evaporation due to the greater difference in temperature between the air and water. Evaporation is also closely related to high and low ice years. When evaporation rates are high in the fall, signifying more heat loss, it creates conditions for a year with high ice cover. Cold lake surface temperatures and high ice cover then reduce evaporation. The same is true for lower evaporation rates in the fall, signifying less heat loss, and low ice cover years. These conditions can then lead to increased evaporation in the following months (see Ice Cover Overview page for more information).

Precipitation, evaporation, and runoff are combined in total net basin supply for Lake Superior (Figure 8). 

There are multiple factors that lead to the differing levels of ice cover between the different lakes including depth, surface area, and latitude. Lake Superior is the northernmost of all the Great Lakes, exposing it to colder temperatures. However, it is also very deep, which allows a higher capacity for heat storage (Figure 13). The large surface area of the lake allows for loss of this stored heat, and therefore more favorable conditions for the formation of ice.

During the winter months, Lake Superior experiences varying levels of ice coverage. On average, higher ice coverage is observed closer to the coasts and channels due to more shallow water and protection from winds and currents (Figure 14). Below average ice cover was observed during most years in the past decade for most of the lake surface area. Though there were individual years with high ice cover, such as 2014 and 2019, the overall decadal average was low.

When considering annual maximum or average ice cover in the Great Lakes, it is common to treat each “ice year” as the period between December (of the previous year) and May (of the current year), as this is when freezing events occur in the region. Figure 15 demonstrates this, with peak ice cover between February and March almost no ice cover in the beginning of December and the middle of May.

There is variability in Lake Superior ice cover throughout the ice year as well as between different years. Natural modes of climate variability can influence ice coverage. Years with strong El Niño Southern Oscillation (ENSO) events contribute to lower maximum ice coverage, while years with polar vortex intrusions generally lead to higher maximum ice coverage (Figure 16). 

Surface temperatures of Lake Superior follow a monthly pattern with the warmest temperatures occurring in late summer and early fall and the coolest temperatures occurring in late winter and early spring (Figure 17). Overall, the surface temperature of Lake Superior has increased. Between 1979 and 2006, an increase of 4.5° fahrenheit was observed in summer surface temperature. This increase is twice that of the air temperature in the same period. ¹

Despite year to year variability, an overall warming trend in air temperature has been observed in the Lake Superior basin from 1948 to 2014 (Figure 18).

Despite an overall warming trend, very warm or very cold years still occur due to annual variability (Figure 19). There are extreme high and low average temperatures throughout the period of record, but recent average temperatures are generally higher than in the past.