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.
MISR was built and is managed by NASA's Jet Propulsion Laboratory,
Pasadena, CA, for NASA's Office of Earth Science, Washington, DC. The
Terra satellite is managed by NASA's Goddard Space Flight Center,
Greenbelt, MD. JPL is a division of the California Institute of
Technology.
For more information: http://www-misr.jpl.nasa.gov
Image by NASA/GSFC/JPL, MISR Science Team