I thought I knew a bit about radio propagation, but I'd never heard about sub-refraction. A bit of digging (www.google.com is the world's finest search engine) revealed the Radar Meteorology Glossary at
http://www-paoc.mit.edu/Radar_Lab/Glossary.html, which says that sub-refraction is "A condition of atmospheric refraction when radar waves are bent less than normal. Sub-refraction occurs when dN/dh > -39.2 N-units/km.". Less than helpful, huh?
However, a few hits later I found a rather interesting document --
http://www.msc-smc.ec.gc.ca/armp/kin.../profbird.html -- about weather radar and birds, which included the following: "Probably what happened (and what typically happens) is the radar beam experienced super-refraction, or a bending of the beam back down towards the ground. The opposite is called sub-refraction.
The radar beam does not emanate from the radar dish in a straight line. It bends somewhat down toward the earth, but the curvature of the earth is greater than the curvature of the radar beam. Exactly how the radar beam bends is determined by the index of refraction.
The index of refraction of the radar beam in the atmosphere is affected by inversions of temperature and moisture."
Which makes sense. It's also got nothing to do with sporadic E (about which I do know a bit), which is occasionally enhanced reflectivity of the E layer of the ionosphere. As has been said above, this has the effect of causing much higher frequencies than normal to carry over the horizon -- although rarely at radar frequencies. Sporadic tends to happen at VHF and below, which makes it fun for radio hams but a bit of a nuisance for everyone else.
Although 'reflection' from the ionosphere is in fact a refraction phenomenon -- as is all reflection of EM waves, including light from mirrors -- from the above I don't think it has anything to do with sub-refraction. There may occasionally be correlations between weather conditions that cause sub-refraction because of moisture, etc, and sporadic E, but I'd doubt it.
Hope this helps. I learned something new -- viva Pprune! The web site about birds and weather radar is entertaining in its own right, with happy talk about insects showing up at 2GHz and 'bird explosions' caused by enormous flocks of cow birds taking off at the same time and heading in all directions.
R
[This message has been edited by Self Loading Freight (edited 03 November 2000).]