[Jimbuna]

A slightly differing set of information:

Finally, allied radar was still sensitive enough to be able to detect an object as small as a snorkel head. Although this was much more difficult compared to a surfaced U-boat, allied reports indicated that a surfaced U-boat could be detected up to a distance of 10km, whereas a snorkel head could be detected at only about 5km. Later, snorkel heads were coated with radar absorbing materials known as Tarnmatte. It was claimed that this reduced radar signature by up to 90 percent, although there were no definitive tests.

http://www.uboataces.com/snorkel.shtml