The images above are from the Advanced Microwave Sounding Unit (AMSU) on board NASA’s Aqua spacecraft. The left image was taken on September 12, about two weeks prior to the landfall of Tropical Storm Isidore along the Louisiana coast, while the image in the middle was taken on September 28, about a day after Isidore made landfall. The difference in the two microwave images in the vicinity of Mississippi is primarily due to flooding caused by Tropical Storm Isidore. The image from September 28 shows heavy flooding along the Mississippi River, especially in the states of Mississippi and Tennessee. Since water has a very low surface emissivity at the microwave wavelength being used for these images, surface water appears to be very cold. Land appears relatively warm (well above freezing even at night as seen in these images), but in locations with standing water the apparent temperature drops precipitously. The spatial resolution of the AMSU instrument is relatively large (each measurement spot is about 40.5 km, or 25 miles in diameter at the center of the swath), but the enormous thermal contrast in the microwave between land and water makes even small flooded areas stand out.
The Aqua spacecraft has an exact 16-day repeat cycle, hence by using images 16-days apart, the two images have exactly the same spatial coverage, which makes it convenient to obtain the difference between the September 28 and September 12 images. In the difference image (shown at the far right), white indicates no difference, green indicates very little difference, and blue/purple indicates primarily heavy flooding. Red indicates warming likely due to higher air temperatures. (The straight lines on the right and left edges of the difference image are caused by slight differences between the two repeat passes of Aqua.)
The images and an earlier version of the text were provided by the AIRS Science Team, which is the focus for the science being done with the three Aqua sounders: the Atmospheric Infrared Sounder (AIRS), the Humidity Sounder for Brazil (HSB), and the AMSU.