Mother Nature's Tricks
Have you ever been caught in an un-forecast downpour during a picnic? Why are there down flow winds on the updraft side of a mountain? Occasionally the wind defies all common sense reasoning and visualization.
Sometimes being fooled is fun, but when you are fooled by Mother Nature the fun usually ceases. Suppose you're flying along in an area of anticipated updraft, yet all you can find is downdrafts. What is worse is when you are flying in an area of an updraft and decide to take a short cut to the head of a pass. Upon reaching an area where there aren't many options for escape the updraft turns to a strong downdraft. You're probably in a heap of trouble at this point.
This doesn't happen that often, thank goodness, but occasionally the wind will defy all common sense reasoning. This may be due to one or a combination of the following: subsidence, inversion, terrain modification, valley breeze or mountain breeze.
(This discussion is limited to the northern hemisphere)
A quick review of some basic weather phenomena helps make the point. Circulation refers simply to the movement of air about the earth's surface. The sun heats the Earth's surface unevenly. The most direct rays strike near the equator, heating the equatorial regions more than the polar regions. The equatorial region re-radiates to space less heat than is received from the sun, while the reverse is true at the poles.
Yet the equator does not continue to get hotter and hotter, nor does the polar region get colder. The only explanation is that heat is transferred from one latitude to another by the actual transport of air.
Warm air forced aloft at the equator begins to move north at high elevation. Coriolis force turns it to the right (east). This turning develops a strong band of winds, "prevailing westerlies," at about 30º North latitude.
Similarly, cool air from the poles begins a low-elevation journey toward the equator. It is also deflected to its right by Coriolis force creating a belt of low-level "polar easterlies." The result is to create a temporary impasse that disrupts simple, convective transfer.
The atmosphere seeks stability and in an attempt to reach equilibrium, huge masses of air overturn in the middle latitudes. Cold air masses break through the barriers, plunging southward. The result is a mid-latitude bank of migratory storms with ever-changing weather.
When strong winds blow snow off mountains it is obvious where you will find the downdrafts and turbulence with visualization.
The large air masses are high pressure areas. In the northern hemisphere, high pressure areas circulate in a clockwise direction. The high pressure system depicted on weather maps should be visualized as a mountain of air. The mountain is composed of isobars or lines of equal pressure. Consider the isobars as topographic in nature. If they are far apart, the high pressure area has a shallow topography. When close together, there is a very steep slope to the mountain of air. Where isobars are close together it indicates the air is squeezed into a smaller, more confined area with a steep slope creating a rapid flow of air and strong surface winds.
Between the high pressure areas will be areas of low pressure where the air flows counterclockwise. Visualize the low pressure area as a valley between air masses.
None of the pressure areas are stagnant. The earth's atmosphere is in a constant state of imbalance, but there is always a tendency to regain a state of balance. The tendency to return to a state of balance may go the other way and form a new imbalance in the atmospheric pressure, causing new weather problems.
Three forces act on wind. The pressure gradient force drives the wind. Pressure gradient is the decrease of pressure with distance and is in the direction of greatest decrease, thus, pressure gradient is from higher to lower pressure and perpendicular to the isobars. If pressure gradient was the only force acting on the wind, wind would always blow perpendicular to the isobars.
Rotation of the earth generates a force that deflects from a straight path any mass moving relative to the earth's surface. Coriolis force is zero at the equator and increases with latitude to a maximum at the poles. It is at a right angle to wind direction and is directly proportional to wind speed. Air in motion, due to pressure gradient, blows straight across the isobars from higher to lower pressure. When the air is in motion, Coriolis force begins to act at right angles to the wind, turning it to the right. Coriolis force continues to deflect the wind until is is blowing parallel to the isobars. Coriolis force and pressure gradient force balance, and above surface friction (about 2,000 feet), causes the wind to blow parallel to the isobars.
The winds at the earth's surface do not blow parallel to the isobars. Instead, they cross the isobars at an angle from higher to lower pressure.
Frictional force always acts opposite to wind direction. As friction slows the wind speed, Coriolis force decreases; however, friction has no effect on pressure gradient force. Pressure gradient and Coriolis forces are no longer in balance. Above 2,000 feet AGL the wind blows parallel to isobars. Below that altitude, friction causes the surface wind to blow 45º inward toward a low pressure area and 45º outward from a high pressure area.
Flying toward Independence Pass from Aspen, Colorado, up the Roaring Fork River. Green Mountain is on the right.
Variations in temperature and humidity create a contrast in pressure and density. The pressure differences drive a complex system of air currents in a never-ending attempt to attain equilibrium.
Suppose an air mass (high pressure area) arrives over the plateau area of the upper Arkansas River Valley near Leadville, Colorado. The down flow, sinking are may be a stronger force than the prevailing winds aloft. The pilot departing Aspen and flying up the Roaring Fork River toward Independence Pass will be hard pressed to find an updraft in the face of this down flow. Yet it's always been there before. This pilot may be an accident waiting to happen.
According to Aviation Space Environment Medicine, 232 airplanes crashed within 50 nautical miles of Aspen, CO, between 1964 and 1987. A total of 202 people died and 69 were seriously injured. This points out the need for better training in mountain flying.
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.
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 a 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 (also called a gravity or drainage wind). The slopes cool at a rate faster than they heat up, so the mountain breeze may be stronger then the valley breeze, averaging 10-12 knots. Departing downslope will mean the airplane may be subject to a tailwind.
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. It makes more sense to call it the turn-around point. 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. This will be around 500-feet AGL for most light aircraft. 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.)