National Geographic : 1969 Jan
Water Colors give clues to water conditions in an infrared photograph taken from a mile above Brownsville, Texas, right, and Matamoros, Mexico. Pallid blue of the Rio Grande, beyond a silvery patch of reflected sun light, reveals the presence of silt (see also page 66). Contrasting hues of an oxbow, once part of the river, and an adjacent black lagoon stem from variations in IR reflectiv ity, suggesting differences in water clarity. In the topsy-turvy world of infrared, hedge rows and citrus trees ap pear as red stripes and polka dots against green plowed ground. Golf course greens and agri cultural test plots show up red at lower right. temperature is high enough, part of the radia tion becomes visible as light. At lower tem peratures the huge bulk of the radiation falls in the infrared (below the red) portion of the spectrum, just below visible light, with much smaller amounts still lower, in the microwave radio region (diagram, pages 48-9). Though invisible, these radiations may be detected and measured. Just as the eye distinguishes a green object from a blue one, so infrared de tectors can note subtle temperature changes. Crystals Keep Watch for Forest Fires The rattlesnake has no difficulty perceiv ing temperature differences of a few tenths of a degree a foot away; one of man's more sensitive devices can feel the heat from an ice cube a mile away! Obviously, infrared detectors can be used to spot forest fires (pages 68-9). A crystal of indium antimonide in a plane at 20,000 feet can see a tiny fire only one foot across, long before even a wisp of smoke shows. Since 1965 the U. S. Forest Service has been using such a device in hopes of reducing the Na tion's losses from 110,000 forest fires a year.* "With our detector, we have spotted forest fires which were so well hidden that aerial observers were never able to find them, even though they knew where to look," says Stan ley N. Hirsch of the Forest Service's North ern Forest Fire Laboratory in Missoula, Montana. If infrared can detect fires, it can also sur vey hot spots in the earth's crust. Infrared studies made by the U. S. Geological Survey of Kilauea volcano, on the island of Hawaii, and of the new island of Surtsey that exploded from the sea off the coast of Iceland in 1963, reveal subterranean lava channels not visible on the surface.t Such readings help tell if un derground temperature is rising-whether, in effect, the volcano is "running a fever"-and thus may make possible better warnings of impending eruptions. Surtsey, incidentally, enjoys the distinction of being the first erupting volcano to be moni tored almost simultaneously by infrared in struments on the ground, in aircraft, and in a spacecraft (the weather satellite Nimbus II, in August 1966). *See "Forest Fire: The Devil's Picnic," by Stuart E. Jones and Jay Johnston, GEOGRAPHIC, July 1968. tSee "Fountain of Fire in Hawaii," by Frederick Sim pich, Jr., GEOGRAPHIC, March 1960, and "Surtsey: Island Born of Fire," by Sigurdur Thorarinsson, GEOGRAPHIC, May 1965.