215National Oceanic and Atmospheric Administration (NOAA) polar-orbiting satellites
provide both visible and infrared imagery of Earth in a low-resolution format
called Automatic Picture Transmission (APT). Some ground station software
is able to display both types of images side-by-side, greatly enhancing
the comparison process.
The visible images display see-able
topography, and are dependent
upon sunlight to illuminate features.
Consequently, it is productive to
acquire visible images only during day-
time. Only visible images contain shad-
ows. Those shadows fluctuate with the time of
day—shadows early and late in the day will be
more pronounced than shadows in mid-day images.
Infrared images display gradients in temperature, with the warmest temperature
appearing dark gray or black, and the coldest temperatures displayed as very pale gray
or white. Infrared images are not limited by daylight, and provide equally informative
images at noon or midnight. Large urban areas will appear on the image as a dark
spot — indicating the concentration of concrete and other building materials that
retain heat, as well as heat-producing inhabitants (people, cars, utilities, etc.). Such an
area is called a heat island and is distinguishable from less developed areas that more
quickly react to nature (heating up during daylight, cooling off at night, etc.).
The dominant feature in each image will be clouds.
- In the visible image, almost all the clouds will appear bright white—because
reflected light is being observed. - In the infrared image, the same clouds will appear as varying shades of gray
depending upon their temperature (determined by their altitude above the Earth). - Shadows appear only in visible images.
- Another distinguishing feature between two types of images is that shades of gray
may appear in infrared images where little or no contrast is seen in visible images.
For example, an image pair of the North Atlantic in winter will display far more
shading of the Gulf Stream meshing with the cold Atlantic in the infrared image,
than will the visible image.
BA C K G R O U N D: APT IM A G E RY