On the night of April 26, 1977, the residents of Grand Haven looked westward across the relatively cold waters of Lake Michigan and saw city lights and a flashing red beacon. But the nearest urban area westward from them was Milwaukee, Wisconsin, 75 miles away, well below the geometric horizon and thus normally not visible. Their sightings were later confirmed to have been Milwaukee when a Grand Haven resident timed the blink rate of the flashing red light and linked it to the Milwaukee Harbor entrance beacon. US Weather Service records also confirm that strong inversion conditions were indeed present that night. The unseeable had indeed briefly become visible.
The arctic mirage generally forms under conditions of a uniform and widespread temperature inversion. When the temperature rises at a rate of 6 degrees Fahrenheit per 100 feet, the Earth’s horizon will appear flat. If the inversion becomes stronger, the horizon will then appear to rise vertically from the flat position. Thus, when the inversion gradient reaches 10 degrees Fahrenheit per 100 ft, the horizon appears to turn upward to the observer.
For more information about Arctic Mirages, please visit: http://www.islandnet.com/~see/weather/elements/supmrge.htm