Mother Nature's Tricks - 2

Inversion

Often there is a layer of air within the troposphere that is characterized by an increase of temperature with altitude. It is called an inversion and is usually confined to a shallow layer.

Widespread sinking air (subsidence) is heated by compression and may become warmer than the air below it causing the inversion. The most frequent type of inversion over land is that produced immediately above the ground on a clear, still night. The ground loses heat rapidly through terrestrial radiation, cooling the layer of air next to it. Frontal inversions are also found in association with movement of colder air under warm air or the movement of warm air over cold air.

In a valley, expect the prevailing westerly winds to flow down the east-facing side of the mountain on the downwind side, pass through the valley and flow up on the west-facing upwind side of the next mountain. An inversion may place a cap over the area preventing the wind from flowing down the mountain. But when the wind strikes the terrain on the downwind side of the valley, it may tuck and move down the mountain side. With enough velocity, it may continue across the valley and up the other side.

Terrain Modification

Figure 1

Figure 2

Figure 3

Figure 1 – Anabatic Lift

Uneven terrain features may cause the air flow to be deflected downslope on what is considered the updraft side of the mountain. In the absence of wind, the sun's heating of the surface will produce convection currents known as anabatic lift.

Figure 2 – Valley Breeze

During the day, the sun warms the valley walls and its adjacent air. The heated air being less dense will—lacking strong prevailing winds—rise gently upslope and is known as a valley breeze. The east facing mountain will receive the benefit of the sun's rays first and may cause a downslope wind on the west-facing slope as air rushes down to fill the evacuated air.
The valley breeze begins early in the morning and depending on the elevation of the mountain and the heat of the sun, may reach a peak speed of around 10 knots by noon. The significance of this is that when landing on an airstrip in a drainage, there will be a tailwind to contend with. The average wind speed is 6-8 knots.

Figure 3 - Mountain Breeze

During the late afternoon and evening the valley walls cool quickly, cooling a layer of air next to the slope. This more dense air moves downslope into the valley causing the mountain breeze (gravity or drainage wind). The slopes cool at a rate faster than they heat up, so the mountain breeze may be stronger than the valley breeze, averaging 10-12 knots. Departing downslope will mean the airplane may be subject to the tailwind.

Coping

We tend to think in constants when contemplating the weather and associate whatever is happening as affecting a large area. Often a phenomena is isolated or may crop up in various isolated areas. Despite what is happening or where it is happening, it is important to visualize what is going on.

Air is fluid, similar to water—although less dense. Ask "What would water do in this situation?" More often than not the picture becomes clear, you will know where there are areas of lift, sink and turbulence.

So what will happen to the pilot heading up the Roaring Fork River toward Independence Pass? As long as he remains in a position where he can turn to lowering terrain and does not fly beyond the point of no return, Mother Nature will not have a chance to perform a "got-cha."

The "point of no return" is defined as a point on the ground of rising terrain where the terrain out climbs the aircraft. The turn-around point is determined as the position where, if the throttle is reduced to idle, the aircraft can be turned around during a glide without impacting the terrain.

Never fly beyond this point of no return. Turn around and maneuver for additional altitude prior to continuing.

(By the way, it is not proper technique to reduce the throttle for the turn around, this merely denotes the point where the turn around must be initiated.)