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Coastwatch users:
Due to technical issues, this site is no longer able to produce updated surface water temperature contour maps. Please check back in July for more information about future resources.
Daily maps of observed surface temperatures can be found at these NOAA links:
Lake Superior:
Lake Michigan & Lake Huron:
Lake Erie:
Lake Ontario:

Top Ten Most Frequently Asked Questions

Key terms are identified and defined in detail below the map.

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Key Terms and Definitions

Isotherm 1. An isotherm is a graphic representation of temperature gradients. "Iso" (similar) "therms" (temperatures) are connected points forming lines that look to some people like the elevation lines on a topographic map. On the CoastWatch "port site" images, each line indicates a change in temperature of 1 degree fahrenheit.
Temperature 2. Fahrenheit temperatures presented to the nearest one-tenth degree. The grid pattern you see supplements the isothermic representation for ease of use. Learn more about water temperatures and thermal bars.
Latitude and Longitude 3. The latitude and longitude grid provides both degrees (0) and minutes (') distance north and west of the equator and prime meridian. The example on the left locates you at 79 degrees, 30 minutes west (of the prime meridian).
Cloud 4. Cloud covered areas of the lake surface cannot be read accurately by NOAA satellites so the computer program colors them black. Areas near the black masked cloud cover may not be as precisely reported as those some distance from the clouds. CoastWatch images are generally representative of surface temperature conditions, they are not as accurate as your road atlas.
Probably Cloud 5. Some areas are masked with grey color because the computer program has indicated the temperatures presented are probably cloud temperatures. The air temperature at the tops of clouds can sometimes be very nearly the same as the surface water temperature of the lakes below. The grey-scale masking indicates that the accuracy of temperature reporting is lower at this time. Watching CoastWatch temperature patterns over the days, weeks and seasons provides useful information on lake surface temperatures.
Area of Coverage 6. Each area of coverage has a unique name. This is a "regional view" of Western Lake Ontario. CoastWatch also provides close-up "port views."
Image Date and Time 7. The image date and time in this example is April 30, 1998 at 6:57 p.m. Images are drawn from satellite overflights that occur several times per day. We refer to these overflight images as "early evening" and "evening" images in our archives. The best available (usually the least cloudy) images are selected for presentation on CoastWatch, so when the clouds persist for several days, the image you see may be several days old.
Most questions can be answered by your Sea Grant Agent or on our CoastWatch Help Pages. Please send comments or bug reports to Please include the name of the location or file that's giving you trouble, as well as the type of software you are running. This is a cooperative project between the NOAA CoastWatch Great Lakes Regional Node located at the NOAA Great Lakes Environmental Research Laboratory in Ann Arbor and the Sea Grant Network.
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