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In a solar eclipse, the moon shields a limited region of the earth's atmosphere frSm the heating effect of the solar radiation. This shadow travels through the earth's lower atmosphere at supersonic velocity, causing the neutral gas to emit internal gravity waves that form a bow wave about the shadow region. Tentative estimates of the amplitude of this wave indicate that it will be detectable well outside the area where the eclipse can be observed directly.

Atmospheric gravity waves are subject to horizontal ducting in a stably stratified atmosphere that contains levels of wind maximum or minimum, quite apart from any superimposed consequences of vertical variation of stability and quite apart from the existence or otherwise of critical levels. The process, “Doppler ducting”, is examined here for two prototype cases (an isolated wind maximum and a wind minimum at ground level) and for two more realistic model cases (a strong tropospheric jet and a nocturnal boundary layer jet). The wave structure, modal behavior, and dispersion curves of all four are very similar. It is argued that Doppler ducting is available to a part of the gravity wave spectrum in all real circumstances and that it should be considered as a possible candidate whenever observations indicate that some ducting process is operative. One particular wave event is discussed in some detail, and it is shown that Doppler ducting in its simplest form provides a plausible alternative to a more complicated ducting process (which involved critical levels and energy leakage) previously inferred. Doppler ducting is available equally to gravity waves in the oceans when background currents vary with depth.

The wind‐shear theory of the formation of temperate‐zone sporadic E layers is extended to take into account effects associated with a neutral wind profile having a descending phase velocity. It is shown that layers of long‐lived ions (presumably of meteoritic origin) that are formed at higher altitudes are carried down with the wind profile to a region roughly 85–95 km in altitude, where they are dumped. Above about 95‐km altitude the layers are trapped in portions of the neutral wind profile having a shear of the correct sign according to the simple (equilibrium) form of the wind‐shear theory. Below about 95 km the layers are not trapped in this manner, and their position shows no obvious correlation with the sign of the neutral wind shear. The effects of diffusion are discussed, and an expression for the equilibrium width of a sporadic E layer is obtained.

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