FAA Ground School Practice Test

What is often a sign of stable air when flying over mountainous regions?

1. Severe downdrafts

2. Visible lenticular clouds

3. Sheet-like stratus clouds

4. Upper-level turbulence

The correct answer is: Visible lenticular clouds

Visible lenticular clouds are indeed often a sign of stable air, particularly in mountainous regions. These clouds form as moist air flows over a mountain range and cools, causing the moisture to condense and create a lens-shaped cloud that can appear stationary. The presence of lenticular clouds typically indicates a stable atmosphere because they tend to form in smooth, undisturbed air areas where the temperature lapse rate is less favorable for convection. Other cloud formations or weather phenomena mentioned, such as severe downdrafts, sheet-like stratus clouds, and upper-level turbulence, are usually associated with instability in the atmosphere. Severe downdrafts tend to occur in unstable air where strong vertical motions and convective activity are present. Sheet-like stratus clouds are associated with layered, stable air but do not specifically indicate the unique stability characteristics of a mountainous region. Upper-level turbulence is generally linked to dynamic atmospheric conditions, which are often indicative of instability rather than stability. Thus, the presence of lenticular clouds stands out as a clear marker of stable air when flying over mountains.