The troposphere is the lowest shell of our atmosphere, bounded by the earth's surface (below) and the tropopause (above). It is best divided into two layers:
Information about the troposphere comes from radiosondes, which enable the plotting of tephigrams, and radars, particularly MST systems, which can reveal the height of the tropopause etc. The height of the tropopause is a limiting factor in tropo propagation. Further details are on other pages here, and can be illustrated by the effects of folds in the tropopause.
Folding of the tropopause, seen on the Aberystwyth MST Radar on 24 Oct 98
Prior to 1200, the tropopause appears to have been quite high, over 11 km above ground. At about 1200, a fold in the tropopause is seen to pass, with the lowest point of the fold (associated with a frontal system) at about 1500. The height of the tropopause then increases slowly, to about 8 km by midnight. The effects on VHF tropo propagation (say, for the 2 m band) would have been quite dramatic: prior to 1100, DX of around 700-800 km should have been quite feasible for suitably-equipped stations. From noon onwards, propagation would have deteriorated dramatically, so that the afternoon and evening would have presented very poor conditions, with operators struggling to make DX of 400-500 km. The worst DX would have been expected between 1200 and 2100 on paths with the midpoint (and thus common volume) above the MST radar. If you were active on VHF bands during 24th October 1998, and would be kind enough to send me details of stations worked, I will be very happy to summarise and post that information here, to correlate against these retrospective predictions.
The substantial changes in radio refractive index in the lower troposphere can support refractive modes of propagation, whilst the much smaller changes in the middle and upper troposphere are seldom able to support refraction, but more usually support scatter modes. Among the classic papers demonstrating these differences is:
Crain, C M (1955) Survey of airborne microwave refractometer measurements. Proc IRE October 1955, 1405-1411.
Above the tropopause, changes in temperature and water content are very small indeed, resulting in very little alteration in radio refractive index. Hence, there can be little scatter or refraction, and no real assistance to propagation until the E layer and meteor trails are attained, above about 90 km.
However, David Dunham, WA1CUH, has recently ("Ozone layer propagation: Pondering the possibility", CQ VHF January 1999, 32-38) proposed that some far DX could result from refraction in ionised ozone or the D layer in the stratosphere. He took as his example a series of 2000 km QSOs achieved on 2 m by W1LP in the period 13-15 May 1998, between Cape Code and stations in Florida and Cuba, which most would ascribe to ducting tropo. David has produced evidence that stability and low windspeeds in the troposphere and stratosphere could have resulted in very little scatter until the radio waves reached ionised ozone or the D layer. He is working on a more detailed account of his hypothesis, which we eagerly await.
Last updated 5 Apr 1999
Howard Oakley
Mail howard@quercus.demon.co.uk