Air turbulence is the irregular, chaotic movement of air that causes an aircraft to experience bumps or jolts during flight. When a plane flies through rain, many people assume the falling water is the source of the shaking. The question is whether individual raindrops cause the disturbance, or if the turbulent conditions merely produce the rain as a byproduct.
Precipitation vs. Atmospheric Dynamics
Individual raindrops do not possess the mass or force to significantly disrupt the flight path of a commercial airliner. Turbulence is a macro-scale atmospheric phenomenon, an effect caused by large, sudden shifts in air pressure and wind patterns, not micro-scale impacts from water droplets.
The air movement that causes an aircraft to be jostled involves energy transfers across a broad area, often miles wide. For an aircraft to experience a noticeable bump, it must encounter a change in airflow strong enough to displace its wings or airframe. This requires substantial changes in wind velocity or direction over a short distance, a condition that precipitation cannot create. Therefore, the water falling from the sky is a symptom of unstable weather, not the source of the instability itself.
Convective Activity: The Real Source of Weather-Related Turbulence
The most severe weather-related turbulence is linked to atmospheric instability, a process meteorologists call convection. Convective activity occurs when warm, moist air near the ground rises rapidly through cooler air above it, forming towering clouds like cumulonimbus. The rain is a result of this vertical motion, where rising air cools and condenses water vapor into droplets and ice.
Within these massive cloud structures, powerful vertical air currents create chaotic conditions for aircraft. Strong updrafts can propel an aircraft upward, while adjacent downdrafts can pull it down, causing severe turbulence. The presence of rain or hail indicates that the convective process has reached the precipitation stage, confirming the existence of the intense vertical currents that are the true cause of the shaking.
Rain that falls steadily from a layered cloud (stratiform precipitation) is associated with stable air and results in little turbulence. In contrast, the heavy, showery rain of a thunderstorm cell signals the presence of unstable, rapidly moving air that generates intense turbulence. Pilots avoid these areas because the chaotic vertical movements pose a significant hazard to the aircraft and its occupants.
Other Atmospheric Factors That Cause Turbulence
Turbulence is a general atmospheric phenomenon caused by several other factors independent of precipitation. One common cause is wind shear, which is the rapid change in wind speed or direction over a short vertical or horizontal distance. An aircraft flying through a boundary where a fast-moving air current meets a slower one can experience an abrupt jolt.
Clear Air Turbulence (CAT)
Another type of disruption is Clear Air Turbulence (CAT), which occurs outside of clouds and is visually impossible to predict. CAT is frequently encountered at high altitudes near the jet stream, where horizontal wind shear develops between the fast-moving river of air and the surrounding atmosphere. This turbulence is concerning because radar cannot detect it, giving pilots little warning.
Mechanical Turbulence
Airflow is also disrupted by terrain, leading to mechanical turbulence, particularly at lower altitudes. When strong winds pass over obstacles like mountain ranges, the air is forced upward and then cascades down the leeward side in complex, wave-like patterns. These mountain waves can cause significant turbulence, sometimes extending many miles downwind of the terrain.