Aqua MODIS Ocean Color Progression during Hurricane Katrina

The Aqua satellite orbits the Earth every 99 minutes in a polar, sun-synchronous orbit. The MODIS instrument on Aqua observes reflected light from the Earth in 36 spectral frequencies. These observations can be processed to show many properties of the Earth's surface, from temperature and phytoplankton measurements near the surface of the ocean to fire occurrences and land cover characteristics on the land surface.
This animation shows MODIS ocean color data from about 4 days of individual Aqua orbits. Ocean color is a measurement of the amount of chlorophyll in ocean phytoplankton and is therefore a direct measurement of the amount of life in the ocean. It can only be measured in ocean regions that are free of both clouds and sun glint, the bright band of specular reflection in the center of each granule. For this animation the data is accumulated and so builds up a complete picture of the surface of the Earth except around the South Pole, which is in darkness during the entire 4-day period.

See http://science.hq.nasa.gov/oceans/living/color.html