Satellite
Temperature
Observations
for the Great Lakes
Images
courtesy of the National Oceanic and Atmospheric Administration (NOAA)
Polar
orbiting satellites pass over the Great Lakes several times per day,
taking pictures of surface temperature using an AVHRR (Advanced Very
High Resolution Radiometer) instrument. This measurement is restricted
to the top 100mm (4/1000 of an inch) or so of the water column, giving
a truly surface temperature, and doesn't work where there are clouds.
The examples below are from dates of coastal Lake Michigan cruises
for the NSF Aquanauts Program at the UWM Center for Great Lakes Studies.
WINTER:
Water throughout the Great Lakes region is very cold (blue colors
being less than 4ºC) and several of the Great Lakes are still
covered with ice (gray to black). As long as the water is 4ºC
or less, almost any amount of wind will mix the lake from top to
bottom. Both shallower (Lake Erie, Lake Huron) and more northern
lakes (Lake Superior) have ice through March in many years.SPRING:
Shallower waters are fastest to warm in spring, because there is
less water to heat. Green colors indicate water warmer than 4ºC
and are particularly obvious in Green Bay, far southern Lake Michigan,
and all of shallow Lake Erie. Other nearshore areas are also starting
to warm. This is the time that plankton are very vigorous, because
they have nutrients from winter but the warmth of spring. The spring
bloom lasts several weeks during April and May.
EARLY
SUMMER: June shows southern Lake Michigan warming to above 10ºC,
though colder areas still exist in the middle of the lake, especially
to the north. Both Green Bay and Lake Erie are becoming warm enough
for swimming with a wetsuit (15ºC). Lake Superior (deep) and
much of Lake Huron (somewhat deep and northerly) are still very cold.
MID-SUMMER:
By mid-August the maximum amount of warming has usually occurred.
Invigorating swimming temperatures in the mid-20ºC range are
common in Lake Michigan and Lake Huron. The shallower Lake Erie is
very warm now. Even deep Lake Superior has areas of warm water. Lake
Michigan surface temperatures are nearly uniform lakewide. Stratification
is pronounced at this time, with the warm water lying over very cold
water starting at 10-15m depths.
LATE
FALL: Rapidly cooling lakewaters have dropped to about 10ºC
by November. Note the fairly uniform cooling of all the lakes compared
to the localized warming seen in spring. Within a few weeks these
maps will look like "winter" above. When the surface temperature
drops to 4ºC, water from the bottom of the lake can mix back
to the surface for the first time in about 6 months.
Here is a year-long movie of the satellite surface map, generated by
NOAA. Click here for
a large version.
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