|CLOUD DETECTION (IR vs. VIS)
METEOROLOGIST JEFF HABY
Visible (VIS) satellite imagery and infrared (IR) imagery have different ways of detecting clouds. Visible imagery
is produced by the sun's rays reflecting off of clouds. Infrared is produced by sensing the emitted
radiation coming off of clouds. The temperature of the cloud will determine the wavelength of
radiation emitted from the cloud. Some advantages of visible imagery is that it has a higher resolution,
shows cloud texture better, and can be used to get a good idea of the thickness of the cloud. Some
advantages of infrared imagery is that is can be used day and night, it can be used to
determine the temperature of cloud tops and earth surface features, and it can be used to get a general
idea of how high clouds are.
Below are on example of a visible image and an infrared image. Below the images I will discuss characteristics that
can be determined from the images and satellite images in general.
Visible Image of SE USA
Infrared Image of SE USA
Notice the visible image has a higher resolution of the clouds. On visible it is easier to see the
individual cloud elements. There is more texture on visible also. In general you will be able
to see vertical development on visible better than on infrared imagery. Brighter white
on visible represents thicker cloud. Where there is no cloud you can see the land surface. On visible it
is also easier to see land features. Notice the Mississippi River Delta and the tributary river
systems flowing into the Mississippi River on the visible image. Since different land surfaces have a
different amount of reflection (different albedo), areas with more reflection will show whiter and
less reflection will be darker. On infrared the whiter land surfaces are colder and the darker
land surfaces are relatively warmer. This applies to clouds also which will be discussed next.
A cloud that is very white of infrared imagery is a cloud that is very cold. Since temperature
tends to decrease with height in the troposphere, upper level clouds will be very white while
clouds closer to the surface will not be as white. If the clouds near the surface are the
same temperature as the land surface it can be difficult to distinguish the clouds from land. In the
example infrared image above the clouds are much whiter than the land. From this we know the
clouds are colder than the land. Since the clouds are not bright white though this suggest these
clouds are low and middle level clouds and not high clouds. The lumpy appearance of the clouds
on visible suggests the clouds are cumulus and stratocumulus type. Since the clouds are
not wispy and not very bright white like thunderstorm clouds, this suggests the clouds are
not very thick and are thus within the low and middle levels of the troposphere.
Below are some general rules to determine cloud characteristics when comparing visible and infrared
1. If the cloud is bright white on infrared then it is a high cloud or has a cloud top that is
developed high into the troposphere.
2. If a cloud is bright white on visible but is not bright on infrared then it is likely this is
a cloud that is close to the earth's surface. This can happen when there is a thick layer of
fog or stratus near the surface.
3. If cloud is seen on visible but very hard to see on infrared then it could be a layer of fog or
shallow stratus near the surface.
4. Thunderstorms will show bright white on both visible and infrared. A thick cloud will be
bright white on visible and cold cloud tops will show bright white on infrared. Look for
other features also to make sure it is a thunderstorm such as anvil blowoff, overshooting top and
extremely textured on visible imagery.
5. If a cloud is not very white on visible then it is likely a thin cloud. If a cloud is not
very white on infrared then it is likely a cloud near the surface or it is a very thin cloud.
6. When the sun is close to setting, clouds will not show up as white on visible imagery due
to less reflection.
7. Wispy looking clouds on visible that are very white on infrared are likely high level
clouds such as cirrus or anvil blowoff.
8. Cumulus clouds have a lumpy texture. Stratus clouds have a flat texture especially on
infrared. Cirrus clouds tend to be thin and show up white on infrared.
Below is a direct comparison between a VIS and IR image with some important notes about the
comparison. Notice much of the cloud is bright white on VIS in Texas but a much darker shade
on IR. This indicates low clouds. There are thunderstorms in eastern Tennessee. Notice those
cloud areas are bright white on both VIS and IR. There is light rain falling in
western Mississippi. These clouds are not as white as the thunderstorm clouds but are
more white than the low clouds in much of Texas on IR.