Turbulence — Page 113, Lesson 97

I want to walk you through the effects of mountain waves and how to recognise and avoid them. This is a critical topic for any pilot operating near high terrain. Let's start with how far these waves can travel. The resultant waves from mountain ranges can extend for hundreds of miles downwind of the range if suitable conditions prevail. That's a very significant distance. The waves may extend well above the tropopause — that's the boundary between the troposphere and the stratosphere — and the wave form itself may be seen in cirrus clouds high in the troposphere and also in noctilucent clouds, which occur at altitudes around 250,000 feet in the upper mesosphere. That gives you an idea of just how high these wave effects can reach. Now, the turbulence effects of mountain waves. The most severe turbulence can occur in what we call the Rotor Zone, which lies beneath the crests of lee waves — the waves on the downwind side of the mountain. This rotor zone is often marked by Roll Clouds. The most powerful rotor lies beneath the first wave crest, which is one wavelength downwind from the ridge. Flight in the waves themselves can be smooth, but severe turbulence may occur. Occasionally, violent turbulence will occur due to wave 'breaking' — that's when the wave structure collapses and creates extremely chaotic air. There's another important point here. Normal turbulence associated with flight across jet streams is frequently greatly increased when the jet passes over mountainous areas, particularly when mountain waves are present. So the combination of a jet stream and mountain waves is especially hazardous. It has also been found that turbulence caused in the troposphere due to mountain waves may continue well into the stratosphere. An aircraft flying close to its ceiling on these occasions might find itself in serious difficulty. If you're already at high altitude near your aircraft's performance limit, encountering severe turbulence can be extremely dangerous because you may not have the power or airspeed margin to recover. Let's move to visual recognition features of mountain waves. Provided there is sufficient moisture in the atmosphere, distinctive clouds are formed with mountain waves, and these provide useful warning of the presence of such waves. There are three types to know. First, lenticular clouds — these are lens-shaped clouds that form on the crests of the waves. They may appear above the mountain tops and in the crests of the waves downwind. They may be found up to, and possibly above, the tropopause. Ragged edges on these clouds indicate turbulence. Second, rotor clouds or roll clouds occur under the crests of strong waves downwind of the ridge. The strongest rotor is normally formed in the first wave downwind and will be level or slightly above the ridge crest. Third, cap clouds form on the ridge itself, and strong winds may sweep the cloud down the lee slopes — that's the downwind side of the mountain. Now, a very important note: the characteristic clouds I just described may be obscured by other clouds, and the presence of standing waves may thus not be evidenced. So you cannot rely solely on seeing these clouds. Also, if the air is dry, clouds may not form at all, even though mountain waves are present. That means you can have severe mountain wave turbulence with absolutely no visual warning. Finally, let's cover the action to avoid the worst effects of mountain waves. There are five key points. First, read the meteorological forecast — that's your primary source of warning. Second, arrange to cross mountain ranges at 90 degrees — that means perpendicular to the ridge line, not parallel to it. Third, fly at the recommended turbulence penetration speed for your aircraft. Fourth, do not fly parallel to and just downwind of the range at any altitude. And fifth, avoid flight through or near the rotor zone. That figure shows rotor streaming, which gives you a visual picture of what we've been discussing.