Identifying Fog Using VIS and IR GOES
Imagery
Dr. Brad Muller
Fog is a stratiform cloud and has little texture in visible
imagery. It tends to conform to surface terrain features, filling
up valleys and canyons, and sometimes taking on a dendritic appearance
in mountain valleys. It is reflective and shows up well in
visible imagery (below left) in contrast to less reflective land
surfaces. On the other hand, it often can
hardly
be distinguished in IR (below right)
because its temperature is close to that of the surface.
On the left is an early morning VIS image of New Mexico while the image
on the right is a simultaneous IR.
By comparing the visible image with the terrain map below, we see that
fog is generally found in the valleys, canyons, and lower lying
terrain, but not on the higher terrain.
The following surface map confirms that there was fog in New Mexico:
The following links show fog in the Appalachians and piedmont region
of North and South Carolina and northern Georgia. As the
sun comes up and heats up the ground areas of fog can often be
seen to shrink by burning
off
around the edges first. Note cumulus clouds forming over
the raised heat source of the mountains as the sun becomes higher,
warming up the mountain slopes. As the clear land surfaces
heat up while the fog temperature itself changes little, contrast
increases
in the IR
image and the fog becomes more apparent.
Note in the VIS imagery
the dendritic
patterns in some
locations in the Appalachians as the fog fills up the canyons and how
in some places it
lines up with the southwest-northeast oriented valleys and
ridges. The fog burns off during the course of the morning.
The fog barely shows up
in the
IR because their cloud-top temperature is very close to that of the
nearby land surfaces.
Four views of fog in California's Central Valley during nighttime and
during daylight hours; the value of Dr. Mosher's "Day-night Visible
(Vis++)" is evident at night: Visible
(GOES Channel 1), IR
(GOES Channel 4), Near
IR (GOES Channel 2), and "Day-night
Visible (also called VIS++)". The geography for this region can be
seen in the terrain map below.
Nighttime view:
VIS--useless at night!!
IR--not helpful for fog at night.
Near IR--a little better, but still not
clearcut. Mosher's
"Day-Night Visible (also called Vis++).
Daytime view:
VIS--good for fog in daytime.
IR--better in daytime, but not necessary.
Near IR--fog looks dark, cold ocean light.
Mosher's "Day-night Visible"--during daylight
just showing visible.
Terrain of California: