Felix wrote: Maybe I misread your description of the events, but it sounds like you were riding the primary ridge lift as opposed to the downstream waves that result from the overshooting of a stable layer of air after being lifted by the ridge. I think you would have had to move downstream a full wavelength to confirm the presence of an actual wave. Nonetheless, with the increasing stability of a evening atmosphere, the presence of a moving layer, and the other observations you described it would not surprise me if you were in wave conditions.

WAVE LIFT
The air is a light fluid and like all fluids it can experience waves. In fact, if you wish to see an object lesson on atmospheric waves, go to your nearest friendly stream and watch what happens downstream of a submerged rock or log. You'll see lift in front of the submerged object which corresponds to ridge lift, while behind it you'll see a series of ripples or waves. These waves can be quite large in a fast moving, deep stream.

Waves in the atmosphere are produced by a similar disturbance. Simply replace the rock or log with a mountain or ridge and you have the required setup. However, only certain atmospheric conditions produce waves. If we look at figure 147 we see the effect of wind blowing over a ridge in unstable, neutral and stable conditions. Note that only the stable situation tends to create an undulating pattern. This is because a lifted stable layer tends to return to its original level once it passes the mountain. However its downward momentum causes it to overshoot its preferred level so its stability brings it back up. Again it overshoots and continues this process downwind to oscillate up and down as if it were on a big soft spring. Thus we have our first requirement: a stable layer.

The next thing we need is ample wind. We find that generally waves require an average wind speed of at least 15 knots (26 km/h) at the mountain top. In addition, the wind must be fairly perpendicular to the ridge, not change direction with altitude and should show a general increase from the surface to the tropopause. These requirements are summarized in figure 148. Note that the lapse rate indicates a layer of stable air lying above the mountain. This is the ideal case, for an unstable layer below and above the stable layer create what can be described as a springboard for the stable layer to bounce on once the mountain begins the oscillation.

The shape of the wave-producing mountain is a factor in wave strength. The ideal mountain is shown in the figure. Basically the upwind side is concave, the back is steep and the mountain size is about that of the first wave.

A long ridge or mountain is the best wave producer. Short ridges and hills allow the air to flow around their sides and interfere with wave formation. The length of a ridge for optimum formation should be a minimum of one wavelength. Wave can be produced behind isolated hills as shown in figure 149. However, the wave will be small and die out quickly downwind. A perfect wave generator can produce a series of waves that extend for hundreds of miles downwind.

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Model sailplane pilots have been known to soar their small craft in waves produced by buildings, fences, small bumps and drop offs in the terrain. The process on such a small scale appears to take place in lighter winds than required for larger waves.