Briefly, the AVHRR sensor on the NOAA Polar Orbiters scans and transmits back to earth a narrow strip across its ground track 6 times each second. Each strip is about 1 km wide and 1462 km long (909 miles). These scans are received as long as the satellite is in view of the ground station and all the strips together form an image of the earth below. The along track size of the image depends on how long the satellite is visible from the ground station. This may be a very short time if the satellite appears to just skim the horizon, or up to about 15 or 16 minutes if it passes directly overhead. The AVHRR image data has 5 channels or wavelength bands, ranging from the visible to the far infrared. Each band shows somewhat different features as discussed in the next section.
General information about satellite imagery
Nick Kew and co-authors have put together
The Satellite Imagery FAQ which gives a lot of details
about this topic and pointers to other webs ites.
Single channel images
These are the most basic images but the most rare on this site.
They may be of any of the 5 channels but are most commonly channel
2 or channel 3. Some comments on what each channel shows follows.
These comments will also help interpret the color composite images.
All the channels show clouds, land, and water, but each has their
special strengths.
(Detailed spectral response curves are available:
NOAA-12,
NOAA-14).
Water Temperature images
These images show the surface temperature of water bodies such as
the ocean and larger lakes. These may be day or night images and
typically have land and cloud areas filled in from a single channel
image, usually channel 2 (day only) or channel 3 (the land/clouds
channel is indicated on the image). A temperature scale in degrees
C is given at in the lower right corner of the image.
Color composite images: channels 2, 1, and 3 displayed as red, green,
and blue
This color combination was selected to try to match the commonly used
Landsat infrared color images (which typically uses Landsat bands 4, 3,
and 1 as red, green, and blue (wavelengths not the same as AVHRR)). The
match is not perfect but the images are similar. Vegetation areas show
as red or some variations such as orange or purple. Muddy water or shallow
water shows as green. Clear water shows as black if cold or blue if warm.
Clouds appear fairly white if warm and yellow if cold. This usually gives
a good indication of the relative cloud heights, low clouds are warm and
high clouds cold (and yellow). An unfortunate side effect of this color
combination is that cold snow appears yellow. Perhaps the best way to
learn what the colors in the image mean is to check what time of year the
image was taken and look at a detailed road map or other atlas. Urban
areas often look pale blue or blue-gray. However barren regions also
sometimes show a similar color. Agricultural areas show as a bright red
or somewhat orange. Barren areas with some vegetation may appear somewhat
purple from the mixture of red and blue. Mountainous areas such as in
Colorado show altitude related color bands, from the low barren areas that
appear blue, to somehwat more vegetated darker areas, to a brighter red
where more vegetation occurs to a darker red typical of pine trees, to
a dark yellow-gray of areas above the timberline (and perhaps some brighter
yellow areas of snow). Although clear views are the most sought after
it is not always possible to avoid some clouds or variations in
atmospheric clarity. High thin cirrus cloud areas may appear as areas
that are a bit more yellow than nearby areas, perhaps even faint shadows
may be seen. This makes interpretation of any single image harder.
Also the time of day makes a difference in the appearance of an image.
NOAA-14 daytime views usually have a high sun angle, especially in
summer, so relief is not well seen. NOAA-12 images sometimes show
very nice shadowing making mountainous areas very apparent. Unfortunately
NOAA-12 passes occur in darkness in the winter, useful for water temperature
and fires but not for landform images in general.
Color composite images: channels 1, 2, and 4 displayed as red, green,
and blue
This type of image is often really 1, 2, and -4 which means channel 4 has
bright and dark reversed. The labeling on the image will indicate if this
is the case. This channel combination is mainly used to show clouds for
daytime images. Since clouds are often cold they appear dark on channel 4
but bright if channel 4 is reversed. This combination makes most clouds
appear pretty much white except for warm low clouds which will appear yellow.
Also since channel 2 is colored green vegetation will appear green since this
channel shows the strong near infrared reflection from vegetation.
Cold water will appear bright blue for the reversed channel 4 case.