Figure 1-12.—GOES visual image on left compared to GOES infrared

same as visible light (longer wavelength infrared energy is absorbed). Satellites normally carry sensors that measure energy levels in several specific bands of infrared wavelengths, and the radiation measured is directly related to the temperature of the different radiating surfaces. Since all surfaces radiate some amount of thermal (heat) energy, a major advantage of infrared satellite imagery is that it is available even when the earth is dark. Energy levels are measured much the same way that visual-range energy is measured. The individual measurements from each pixel, when composed into an image of the earth, form an infrared image. Most infrared satellite imagery is measured at wavelengths of 10.2 µm to 12.8 µm (far infrared). Some satellites are able to augment visual and far infrared imagery by also measuring near infrared (NIR) wavelengths (.74 µm to 2.0 µm). NIR imagery generally shows better land/water contrast and simplifies low-level feature identification, such as shorelines, snow/ice, and vegetation. The satellite receiver and processor control how the composed image will look. Normally, the energy measurements for infrared image pixels are assigned g-ray shades with low-energy readings appearing white and high-energy readings appearing black. The lighter the gray shade, the colder the object seen. With IR images, space surrounding the earth is white, and warm land or water masses are dark gray or black. Infrared imagery is an excellent tool for oceanographic analysis, such as evaluating sea surface temperatures and determining ocean front and eddy locations. It is also very helpful for identifying high clouds and upper-level wind flow, but less reliable for identifying low-level features. Look at figure 1-12. Notice how lower level clouds, very distinct in the visual image, are more difficult to determine in the Figure 1-12.—GOES visual image on left compared to GOES infrared (IR) image on right. Space and cold cloud tops appear white and warm water and land areas appear dark gray to black in an IR image. 1-12