Winds — DGCA CPL Aviation Meteorology

Why that is the answer

Mountain waves require a specific vertical wind and stability profile. For a well-developed wave to form: the air must be stable (often a layered structure — stable near the surface, sometimes with a less stable layer aloft, capped by a stable layer at height, i.e., 'three-layer' theory), the wind must blow roughly perpendicular to the ridge line, and crucially the wind speed must INCREASE with height above the ridge with little change in direction, reaching or exceeding about 15-25 kt near ridge-top and continuing to strengthen aloft without significant veering/backing. A wind that decreases or reverses with height (option a) would disrupt the wave, and large direction changes would break up the coherent wave pattern needed for the classic lee-wave/rotor structure to develop and persist. Steady, strengthening flow with height allows the wave energy to propagate upward and remain organized, producing the characteristic lenticular clouds and rotor turbulence. The answer is c: increase with little change in direction.

1. Mountain wave formation needs: stable air, wind near-perpendicular to ridge, sufficient wind speed at ridge top
2. Above ridge level, wind speed must continue to INCREASE with height
3. Direction must remain fairly constant (little veer/backing) for the wave to stay organized
4. A decreasing or reversing wind (option a) would disrupt/destroy the wave pattern
5. Correct requirement: wind increases with height, little change in direction — answer c

Why that is the answer

Jet streams sit at the tropopause on the boundary between cold polar air (poleward) and warmer tropical air (equatorward), with the temperature gradient at right angles to the flow, cold air to the left of the flow direction and warm air to the right in the Northern Hemisphere. Below the jet core, the thermal wind relationship means the wind backs (in the NH) as you descend through the frontal zone associated with the jet. If you fly at right angles across the jet axis in Europe (Northern Hemisphere) and note the outside air temperature decreasing, you are moving toward the cold (poleward) side of the jet, i.e., toward the polar air mass. Below the core, with cold air being approached, the wind direction experienced by the aircraft shifts such that the wind comes from the left of track. This is a direct consequence of the geostrophic/thermal wind balance: the jet flows with cold air on its left (looking downstream) in the NH, and crossing toward cold air below the core produces a wind-from-the-left sensation relative to the crossing track. Options a and b (headwind/tailwind changes) do not follow from a temperature change on a track flown at right angles to the jet, and option d (wind from the right) is the mirror-image case of approaching warmer air, not colder. Therefore option c is correct.

1. Jet stream lies at the boundary of cold (poleward) and warm (equatorward) air masses
2. Flying at right angles to the jet, decreasing OAT means heading toward the cold/polar side
3. Thermal wind balance below the core means approaching cold air gives a wind shift from the left
4. Increasing OAT would instead indicate wind from the right
5. Correct answer: c

Why that is the answer

Wingtip vortices sink slowly and drift with any crosswind component, so a following aircraft should stay at or above the flight path of the aircraft ahead and plan to touch down beyond the point where the heavier aircraft touched down, keeping clear of the descending vortex core that trails and sinks behind the preceding aircraft's flight path. Landing below that flight path, or touching down before the heavier aircraft's touchdown point, both increase the risk of flying directly into the sinking vortex pair, which can produce an uncommanded roll or upset that may exceed the following aircraft's roll control authority, particularly for smaller aircraft following much heavier ones.