MISR images of the southeast portion of Georgian Bay in Ontario,
Canada, acquired on March 6, 2000, during Terra orbit 1155. The color
image is from the nadir (vertical) camera, and highlights a cloud to the
southwest of Christian Island. In this view, the shadow cast by the cloud
on the water is visible just north of the cloud itself. Bright areas in the
image are either cloud or ice; an example of the latter is the frozen Lake
Simcoe.
The eight monochrome images are red band data from the off-nadir
cameras. Starting with the one in the upper right and moving
counterclockwise, the images progress from the most forward-viewing to
the most aftward-viewing camera. Thus, the top (bottom) row of
monochrome images are views acquired forward (aftward) of vertical.
The apparent displacement of the cloud from south to north as the view
progresses from forward to aftward is primarily a geometric parallax
effect due to the cloud's elevation above the surface.
In each image in the top row, a fainter feature with the same shape
as the cloud is visible within Georgian Bay. The feature and the cloud
itself approach one another as the view angle becomes less oblique. The
feature is present only in the water, and disappears over the land surface
of Christian Island. What is it?
We are observing reflections of the cloud in the water. Their positions
are dictated by the law of reflection, which states that the angle relative
to the vertical of the reflected rays is the same as the angle of the incident
rays. Therefore, the apparent location of a reflection relative to the cloud
changes as a function of camera view angle. Unlike water, land does not
act as a good mirror. Also, in the aftward views the reflections are less
visible because they are blocked by the southern extension of the cloud.
Reflections of this sort are not visible in conventional vertical imagery
because in that case they lie directly underneath the cloud, and are
consequently obscured.
Image Credit: NASA/GSFC/JPL, MISR Science Team
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