Lakes, rivers, and the ocean appear much warmer than the cooler land. During the day water is notably colder than the land, and in the night it appears warmer than the land, because of its heat retention and the commonly larger drop in land surface temperatures. This plot may clarify this idea. This plot shows the assignments of gray tones as a function of radiant temperatures for the Day and Night images. In the Night-IR image, cities are slightly warmer than their surroundings.
Some valleys appear conspicuously dark, perhaps because of cold-air drainage from uplands.
In the visible and thermal images shown on this page, locate features from the test scenes and note their tonal patterns in the thermal images. Heavily forested areas in the Appalachians are dark, denoting low ATIs. Those soils in the Piedmont and Coastal Plains, where they are exposed better because of fewer trees, have somewhat higher ATIs. We can crudely separate the Piedmont rom the Plains by its lighter tones. The same Day-thermal image, which we used as partial input in making the following ATI image, reveals a prominent thermal pattern in the Atlantic ocean, when we assign colors to the different calibrated temperatures.