Today’s Image of the Day from NASA Earth Observatory features a dark patch of water located east of the Delmarva Peninsula. The image was captured on May 6, 2023 by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite.
“The bright, silvery appearance of much of the water is due to sunglint – an optical phenomenon that occurs when sunlight reflects off the surface of the water at the same angle that a satellite sensor views it. The result is mirror-like specular reflection that shows up as long, bright areas in MODIS data,” explains NASA.
“The ‘roughness’ of the water surface can influence how sunglint areas appear in MODIS images – sometimes creating distinctive patterns within them. Most of the time, winds create enough waves and choppiness in the water that light gets reflected in a variety of directions (diffuse reflection) as it hits the sea surface.”
With weaker winds, the sea surface is smooth. This was the case at the time the photograph was captured along the U.S. East Coast.
Scott Bachmeier is a research meteorologist with the Cooperative Institute for Meteorological Satellite Studies at the University of Wisconsin-Madison.
“Within a region of light winds, the appearance of the rather flat underlying water surface – either darker than adjacent water, or extremely bright – depends on the sun-satellite geometry,” explains Bachmeier.
Chuanmin Hu is a professor of Optical Oceanography at the University of South Florida.
“In this case, the dark features were likely areas with very weak winds,” says Professor Hu. ”But other things that change the ‘roughness’ of the water can lead to changes in sunglint patterns as well. These include oil slicks, freshwater slicks, ship wakes, ocean circulation, internal waves, and ocean circulation.”
“Some of the narrow dark features inside Delaware Bay and Chesapeake Bay may be partly due to freshwater, but freshwater slicks are not generally as large as the main dark patch visible east of the Delmarva Peninsula.”
Image Credit: NASA Earth Observatory