LOW & HIGH-ALTITUDE TURBULENCE: UNDERSTANDING THE CAUSES OF A ROUGH RIDE
For most air travelers, turbulence is a word that invokes at least a little dread and discomfort. However, in most cases such apprehension is at least partly due to fear of the unknown. Once we better understand the subject, our newfound knowledge can help allay unnecessary anxiety.
With this series of posts, we’ll examine the causes of turbulence and methods for avoiding and mitigating any encounters. In this first section, we’ll examine the types of low-level turbulence. Low-level turbulence includes all forms of turbulence found below an altitude of approximately 15,000 ft. Regardless of your flight’s eventual cruise altitude, all aircraft must pass through these lower levels after departure and prior to arrival at the destination airport. At times, certain low-level turbulence is unavoidable and is best handled when properly understood.
Roughly defined, turbulence is an irregular motion of the atmosphere. Mechanical turbulence is caused by terrain, buildings, or other structures disrupting the smooth airflow through a given area. This type of turbulence is most commonly encountered during takeoff and landing when hills, ridges, or infrastructure around the airport interfere with the flow of surface winds. Based on their surroundings, some airports are more susceptible to mechanical turbulence than others. When encountered, mechanical turbulence only lasts for a few minutes and will dissipate after landing or once climbing a few thousand feet upon takeoff.
Commonly referred to as thermals, convective turbulence is generally associated with warm weather. This turbulence is caused by solar heating reflecting upwards off the surface. The higher the temperature, the stronger and higher you can expect the thermals to be. When sufficient atmospheric moisture is present, cumulus clouds tend to denote the upper extent of thermals. As with mechanical turbulence, convective turbulence is limited to within a few thousand feet of the surface and will only be experienced during takeoff and landing.
As the name implies, frontal turbulence is typically found ahead of a fast-moving cold front. You can anticipate frontal turbulence by watching the weather before your planned departure. If a cold front is approaching your departure or arrival airport, you might encounter some frontal turbulence. En route, such encounters are generally not an issue, as frontal turbulence is most common at lower altitudes. However, if you’ll be cruising at low altitude (on commuter aircraft or short legs), it’s possible you’ll encounter this turbulence in cruise. In such cases, the pilots, dispatchers, and air traffic controllers can often coordinate to avoid the most probable turbulence areas.
Though categorized as a form of turbulence, wake turbulence is generated by aircraft, not the atmosphere. While all aircraft generate some turbulence as a result of lift, the strongest wake turbulence is produced by heavy aircraft during takeoff and landing. To combat wake turbulence, ATC maintains minimum separation distances between aircraft. On the ground, aircraft are sometimes required to hold for a few minutes to allow wake turbulence to dissipate. Additionally, pilots can request additional separation/hold time if they judge it’s in the interest of safety. Due to the wake turbulence avoidance procedures used by pilots and ATC, your chances of encountering wake turbulence are extremely rare.
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What can you do?
As a passenger, your best defense against low-level turbulence is knowledge of its characteristics. By referring to the information above or researching the topic on your own, you’ll be better prepared for possible low-level turbulence encounters. In many cases, your knowledge of when, where, and how long turbulence is likely can go a long way to relieving any anxiety about an encounter. During flight, try to remain seated with your seatbelt fastened. Secure your personal items to the extent possible and keep a hand on any foods and drinks, particularly hot items. If a trip to the restroom is necessary, try to go before entering or after exiting expected turbulence zones. Below, we’ll discuss other forms of turbulence and identification/planning/avoidance methods used by pilots and ATC. Don’t let a chance encounter with turbulence keep you from enjoying your time aloft. The more you understand its causes, the better you’ll enjoy your flight.
High-Altitude Turbulence: Bumpiness Aloft
In our prior look at turbulence, we discussed the various forms of low-level turbulence; bumpiness found below approximately 15,000 ft. With this section, 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.
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).
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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 Mountain Turbulence
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.
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