Atmospheric turbulence is the chaotic, irregular movement of air characterized by unpredictable changes in wind speed and direction. When an aircraft encounters these disturbances, the sudden shifts in airflow cause the familiar bumps or jolts passengers feel. Turbulence is a normal condition of the atmosphere that aircraft are engineered to withstand safely.
How Terrain Creates Mechanical Turbulence
Mechanical turbulence arises when the smooth, horizontal flow of air is physically disrupted by obstacles on the Earth’s surface. This process begins with boundary layer friction, where the air closest to the ground slows down and becomes disorganized due to contact with the surface. The effect is greatly magnified when air encounters significant physical barriers such as mountains or large hill ranges.
When strong winds flow nearly perpendicular to a mountain range, the air is forced upward over the peak and then descends rapidly on the downwind side, known as the lee side. This flow disturbance often creates a series of standing waves that extend far downwind from the mountain crest, which are called mountain waves. These invisible oscillations can propagate high into the atmosphere, sometimes reaching into the stratosphere.
The most intense turbulence associated with mountain waves occurs in the rotor zone, forming beneath the crests of the lowest standing waves. Rotors consist of horizontal vortices that spin with great energy, creating severe and sometimes extreme turbulence that can cause rapid changes in an aircraft’s altitude and airspeed. Pilots are trained to be particularly wary of rotor clouds, which are ragged, turbulent clouds that visually indicate the presence of this intense, recirculating air.
Thermal Updrafts and Convective Turbulence
Convective turbulence, also known as thermal turbulence, is produced by vertical air movement resulting from the differential heating of the Earth’s surface. When solar radiation heats the ground unevenly, air parcels directly above warmer surfaces, like paved roads or dark fields, become warmer and less dense than the surrounding air. This buoyant air begins to rise in columns called thermals.
As these warm air pockets rise, they create updrafts that push an aircraft upward, followed by compensating downdrafts of cooler, denser air sinking back towards the surface. This continuous cycle of rising and sinking air creates a choppy, turbulent ride, particularly at lower altitudes where the heating is most direct. The intensity of convective turbulence is often highest during the late afternoon, following peak solar heating.
This type of disturbance is intensified when the rising air mass is moist enough to cool and condense, forming cumulus clouds. Intense updrafts and downdrafts associated with towering cumulonimbus clouds, or thunderstorms, generate severe turbulence. The chaotic mixing of air within and around these massive cloud structures requires pilots to maintain a safe distance.
Wind Shear and Clear Air Turbulence
Wind shear is defined as a rapid change in wind speed or direction over a short distance, either horizontally or vertically. This phenomenon is a primary cause of turbulence at high altitudes, especially in the form of Clear Air Turbulence (CAT). CAT is a significant concern for high-altitude air travel because it occurs outside of visible weather features like clouds or thunderstorms, making it nearly impossible for pilots to detect visually.
The most common source of high-altitude wind shear is the jet stream, a narrow band of fast-moving air found near the tropopause, the boundary between the troposphere and the stratosphere. Wind speeds within the jet stream core can be immense, and the air masses immediately adjacent to it move at vastly different velocities. This extreme velocity gradient along the edges of the jet stream generates strong shearing forces.
CAT is typically strongest on the cold air side of the jet stream, where the temperature gradient and wind shear are most pronounced. The turbulence is caused by the air forming invisible, chaotic eddies and waves at the boundary of fast-moving and slow-moving air masses. These shearing forces result directly from the atmospheric pressure and temperature gradients that define the jet stream’s location and intensity.
Since CAT lacks any visual signature, pilots and meteorologists rely on sophisticated computer models that forecast areas of high wind shear. Atmospheric factors, including sharp changes in temperature and pressure, are the primary indicators used in these prediction models. Despite these efforts, CAT remains difficult to predict precisely, often requiring pilots to rely on reports from other aircraft that have recently passed through the affected airspace.