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The other common type of satellite imagery depicts the intensity of infrared radiation emitted from the Earth. Remember from the module on radiation and the diurnal temperature cycle that the Earth is continually emitting radiation into space: that's what causes the Earth to cool at night. The amount of radiation emitted by any object is a function of its temperature: the hotter the object, the stronger the radiation and the shorter the wavelength of the radiation. The sun is very hot; the Earth emits only a tiny fraction of the radiation emitted by the Sun, and the wavelength of the emitted radiation is in the infrared band rather than the visible band.
A "photograph" of the infrared radiation leaving the Earth's surface shows where the Earth is hot and where the Earth is cold. This is useful in forecasting temperatures, in locating areas of frost and freezes, and in determining the distribution of sea surface temperatures offshore. This example of an infrared satellite image from the East Coast clearly shows the pattern of sea surface temperatures. The Gulf Stream is prominent as a narrow band which glows brighter because it is warmer than the surrounding seas. Closer to shore, the ocean gets cooler; the shallow bases are especially dark. The coastline shows up clearly; the portion included in this image stretches from Cape Hatteras to Cape Cod.
Natural features aren't the only things visible from space. New York City shows up as slightly brighter than its surroundings. This is because the buildings, concrete, and asphalt of the city are excellent absorbers of solar radiation and become comparatively warm during the day. Philadelphia also shows up faintly in this image.
Because the emitted radiation is so much weaker in intensity than the visible radiation, the detector on the satellite must draw in radiation from a broader swath of the Earth in order to get enough radiation to detect and measure intensities with. Thus, infrared images are generally much poorer resolution than visible images.
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