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
