The main reason commercial airliners encounter very little turbulence above 25,000 feet is primarily related to the nature of the jet stream and the atmospheric conditions at that altitude.
- The jet stream produces a much smoother flow of air: The jet stream, which is a fast-flowing air current found at high altitudes, tends to create more stable flying conditions. While there can be turbulence associated with or near the jet stream, at cruising altitudes above 25,000 feet, the air movement is often more laminar (smooth) compared to lower altitudes where thermal turbulence and weather systems can disturb the airflow.
Other factors mentioned have some relevance, but they are not the main contributors:
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There are no clouds above 25,000 feet: While it's true that many weather phenomena, including convective clouds, are less common at higher altitudes, there can still be clouds at high altitudes (like cirrus clouds) and turbulence associated with them.
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The air is so thin at altitudes above 25,000 feet that turbulence has much less effect: While thinner air does reduce drag and may affect some characteristics of turbulence, turbulence itself can still occur, and it can impact aircraft at high altitudes.
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Water vapor has mostly disappeared at colder elevations and its effect on weather turbulence: Although it's true that humidity decreases with altitude, and reduced moisture can lead to fewer cloud formations and related turbulence, this is not the primary reason airliners experience less turbulence above 25,000 feet.
Overall, the smoother air associated with the jet stream and other high-altitude atmospheric conditions is the key factor in the reduced turbulence experienced by commercial airliners at those altitudes.