In our last look at turbulence, we discussed the various forms of low-level turbulence; bumpiness found below approximately 15,000 ft. With this post, we’ll cover causes of high-altitude roughness. As an airline passenger, most of your travel time will be spent in high-altitude cruise. Turbulence encountered during this stage of flight will likely last longer than most low-level turbulence experienced after takeoff and on the landing approach. We’ll also discuss turbulence common in mountainous areas. Once you’re familiar with the characteristics of these forms of turbulence, you’ll be better able to understand when, where, and how long such turbulence is likely to last.
Clear Air Turbulence (CAT)
While clear air turbulence can occur at any altitude, it is most common above 15,000 ft. CAT is the result of airflow from different directions and/or of differing speeds converging. At higher altitudes, CAT is often found in or near the jet stream. Depending on the route & altitude of flight, CAT can persist for hundreds of miles.
Detecting it: Probable areas of clear air turbulence can often be determined before the flight takes off. Pilots and dispatchers refer to winds & temperatures aloft forecasts and reports to determine wind speed & direction at various altitudes. If wind direction and speed change significantly over small changes in altitude, that’s a sure sign of possible turbulence. The greater the variation, the stronger likelihood of turbulence.
Additionally, airline crews examine Pilot Reports (PIREPS) filed by other pilots. PIREPS are possibly the most valuable resource for locating turbulence, as other planes are reporting the exact location, altitude, and intensity of roughness aloft. With weather forecasts, the best pilots can do is guesstimate where/when/how strong turbulence might be found. PIREPS are available via aviation weather sources, or they can be relayed directly to airliners by air traffic controllers (ATC).
Avoiding CAT: As you might imagine, no one wants to fly through turbulence if it’s avoidable. Pilots and dispatchers consider weather reports/forecasts and PIREPS when determining the route & altitude for flight. In some cases, changing altitude or varying the route will avoid the most turbulent areas. At other times, CAT is so pervasive that the only option is to ride out the bumps.
Mountain Wave Turbulence
As the name implies, mountain wave turbulence is found near mountainous terrain. This form of turbulence results from air being disrupted as it flows over (perpendicular to) mountains. Think of it as mechanical turbulence (see previous post) on steroids. The disrupting peaks cause the air to undulate on the downwind side of the range(s). With strong winds, these oscillations can persist for over 100 miles and cause severe turbulence.
Detecting it: For flights traversing mountainous areas, pilots and dispatchers will look at wind reports and forecasts for the upwind side of the range(s). Speeds above 40 knots are most conducive to mountain waves. Below 25 knots, this possibility diminishes. With widespread light & variable winds, mountain wave turbulence is a nonissue.
In addition, mountain wave turbulence can often be located by noting unique cloud formations. Rotor clouds and lenticular clouds (Google them for some great images) can form below and above the wave crests respectively when sufficient moisture is present. Cap clouds, which form above mountain peaks, can also signify mountain waves.
Avoiding Mountain Waves: If you fly over flatlands, mountain wave turbulence is a nonissue. For flights near high terrain, staying at least 3,000-5,000 ft. above the peaks is generally effective to overfly any hazards. Most commercial flights will be well above these altitudes. For takeoffs & landings at mountainous airports, crews monitor weather reports/forecasts, PIREPS, and ATC updates. They also plan climbs & descents to avoid the most hazardous areas. In extreme cases, they’ll delay/cancel the flight in the interest of safety.
When it comes to turbulence, airline crews go to great lengths to avoid or minimize bumpiness for their passengers. If safety is an issue, pilots have no trouble postponing the flight to await favorable conditions. As an airline passenger, any turbulence you experience is likely unavoidable and not considered hazardous. Though annoying, consider in-flight turbulence a small price to pay to arrive safely at your destination.