Interpreting Visible and Infrared Satellite Imagery


I’d like to take a
few minutes to talk about the differences
between visible satellite imagery and infrared
satellite imagery and the types of information we
can get from each type of image because that’s a really
big part of this lesson. So I have a visible
satellite loop and a corresponding
infrared satellite loop. There’s the infrared loop
from January 30, 2014. We’ll see what
things we can tell from each particular
type of image. First, visible satellite
imagery works off of reflected visible light. So the more light that
an object reflects, the higher it’s albedo
and the brighter it will appear on
visible imagery. The less light it reflects,
the lower its albedo and it will appear darker. That often helps us tell things
about how thick an object is, how thick cloud cover
is, in a particular area. Infrared imagery though
works differently. It works off of emissions
of infrared radiation which is tied to temperature. The warmer an object
is, it will appear dark on infrared imagery, at least
traditional infrared imagery like this loop. And the colder an object
is, the brighter white it will appear on
traditional infrared imagery. So getting back to our
visible image here. As our loop goes on, we
can see a stationary area of white shading across
Alabama and Georgia. As the loop goes
on, it doesn’t move at all which means
it’s not cloud cover. The clouds are moving, as we
can see here over Florida. But this area of white
across Alabama and Georgia is actually snow cover. Skies are clear
that area, and so we see the high albedo
of snow cover across Alabama and Georgia. We can also see little dark
specks across northern Georgia. Those are unfrozen
bodies of water that appear dark
because they have much lower albedo than the
surrounding snow cover. We also note that
the entire image is fairly dark at the
beginning of the loop, and it brightens up
as we go on and we can see the features better. That’s because the loop starts
at 13Z which is about 8 o’clock in the morning, and so the
sun is still low in the sky. There’s not a lot of
reflected visible light off of anything at 13Z. But as the sun rises, we get
more reflected visible light and everything brightens
up on the image. Because infrared
satellite imagery doesn’t require visible light,
that progression of the day appears differently. We can notice it here though
in an area of clear skies across Alabama and Georgia. It’s not because of the
reflected visible light, but it’s because of changes
in temperature that occur. With skies being clear in
this area, as the sun rose throughout the day,
the ground warmed up. And we can tell that on
this infrared image because at the beginning of the loop,
it’s fairly faint gray colored. But as the loop goes on,
we go from that faint gray and it gets darker because the
ground is actually warming up as the loop goes on, and we
get that change in shading. Also, on the infrared image,
we can see some fairly bright looking clouds over in Arkansas
toward the end of the loop. The fact that they’re bright
means that they’re cold. And they must have fairly high
tops because they’re cold. On visible satellite
imagery though, those clouds look a bit differently
toward the end of the loop. We can see some clouds
here, but they’re fairly faint gray which
means that they’re not reflecting a lot
of visible light. They have a low albedo and so
they must be relatively thin. So we know from the visible
image that they are thin clouds and from the infrared
image, they’re high clouds so we
must be talking about cirrus clouds
over Arkansas. Now over the central
Gulf of Mexico, we have a kind of a speckled
appearance to the clouds. They must be reflecting a
decent amount of visible light. They’re brighter
than those cirrus clouds we just looked at. But when we look at those
clouds over the central Gulf on infrared imagery, they’re
actually pretty hard to see. You can very faintly see
those speckled clouds over the central
Gulf, but they’re very, very dark gray shaded
on the infrared imagery. That means that
they are warm which means they must have low tops. So we know that they’re- they
have low tops from the infrared imagery, but still at least
somewhat thick from the visible imagery. So we’re looking at some
sort of a low topped cumulus cloud across the central Gulf. As we look at the clouds
over Florida here, we can see they
have bright shading on the infrared imagery. And that means that
they have high tops, it’s a cold cloud top. And when we look at those
same areas of clouds on the visible loop, we can
see that those clouds are also quite bright. So they must be very thick. They have a high albedo. They’re reflecting
a lot of light. So we know from the visible
loop that they’re thick. And we know from the infrared
loop that they have high tops. So those are probably,
at least some of those clouds over Florida are
probably, cumulonimbus clouds. So hopefully this
gives you an idea to how to track
things like snow cover in the progression
of a day and how to tell cloud thickness from
visible imagery and cloud height from infrared imagery.

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