A sea surface is rough in presence of waves generated by wind, and is smooth in absence of wind. This is because it is composed by 3 images taken at 3 different days. Let us inspect the image with the Red, Green and Blue lenses. If your browser supports JAVA, you can view the applet that permits you to see the individual channels and the map. Otherwise you can simply view the map .
Exercises Match the dates to the corresponding tide.
As if it is the sea bottom. But this is not possible, since the Radar-waves are microwaves and will not penetrate at all the water. What we see is just the sea surface. Due to the tidal current that flows in and out the estuaries and moves also through the North Sea, there is a huge water flow across the Dover Strait. This flow is disturbed by hidden sandbanks, and this is exactly what you see in the image.