MISR Sees a Cloud’s Reflection

MISR Sees a Cloud’s Reflection

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