What combination of conditions is most favorable for mountain wave formation?

What combination of conditions is most favorable for mountain wave formation?

• A. Unstable air at ridge top altitude and wind above 40 knots
• B. Stable air at mountaintop altitude and a wind of 15-25 knots blowing across the ridge
• C. Moist air with no wind
• D. Very cold air with strong downslopes

Answer: (B)

Mountain waves form most readily when the air is stable and the wind crosses the ridge. A stable atmosphere allows the flow to oscillate vertically as it moves over the mountain, creating a series of lee waves that can extend downwind. The wind needs to be strong enough to energize the wave pattern but not so strong that severe rotor turbulence dominates; about 15 to 25 knots is typically ideal for well-defined waves. If the air is unstable at mountaintop level, convection disrupts the orderly wave pattern. Moist air with no wind lacks the cross-ridge component needed to kick off the waves. Very cold air with strong downslope flow isn’t providing that crosswind interaction and instead leads to other phenomena like katabatic flows, not clean mountain waves.

Mountain waves form most readily when the air is stable and the wind crosses the ridge. A stable atmosphere allows the flow to oscillate vertically as it moves over the mountain, creating a series of lee waves that can extend downwind. The wind needs to be strong enough to energize the wave pattern but not so strong that severe rotor turbulence dominates; about 15 to 25 knots is typically ideal for well-defined waves. If the air is unstable at mountaintop level, convection disrupts the orderly wave pattern. Moist air with no wind lacks the cross-ridge component needed to kick off the waves. Very cold air with strong downslope flow isn’t providing that crosswind interaction and instead leads to other phenomena like katabatic flows, not clean mountain waves.