Airline Transport Pilot Written Knowledge Practice Test

A cold air mass moving over a warm surface tends to become unstable due to the significant temperature difference between the cold air mass and the underlying warm surface. When the cold air is heated from below, it becomes less dense and begins to rise, which can lead to the development of vertical motion. This rising motion is conducive to the formation of cumuliform clouds, which are characterized by their puffy, cotton-like appearance. As the vertical motion continues, turbulence may also develop due to the rapid updrafts and downdrafts associated with this instability.

When assessing the other options, they do not align with the characteristics of a cold air mass over a warm surface. Stable air is often associated with limited vertical movement and stratiform clouds, not the instability and turbulence that arises from this specific scenario. High pressure systems typically bring clear skies and calm weather, which contrasts with the instability expected from a cold air mass interacting with warmer conditions. Lastly, low wind speeds and fog are not the defining characteristics of this situation; rather, the dynamic lifting of air provides a much different atmospheric condition. Thus, the correct choice highlights the inherent instability, cloud formation, and turbulence that are prominent when cold air encounters a warmer surface.