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Mother Nature's tricks
Why are there down flow winds on the updraft side of a mountain?
 By Sparky Imeson—April
 
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.
          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.

CIRCULATION
To make my point it might be helpful to review some basic weather phenomena. The term 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 the earth near the equator, heating the equatorial regions more than the Polar Regions. In addition, the equatorial region reradiates 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 30-degrees North latitude. Similarly, cool air from the poles begins a low-elevation journey toward the equator. It too, is deflected to its right by Coriolis force creating a belt of low‑level “polar easterlies.” The result of Coriolis force is to create a temporary impasse that disrupts the simple, convective transfer.
          The atmosphere seeks stability and in an attempt to reach equilibrium huge masses of air overturns in the middle latitudes while 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.)

AIR MASS
The large air masses are high pressure areas. In the northern hemisphere all high pressure areas circulate in a clockwise direction. When you look at a surface weather map and see the high pressure area depicted you should visualize this as a mountain of air. The mountain is composed of isobars or lines of equal pressure. These can be used to visualize the shape of the mountain by considering the isobars as topographic in nature. If the isobars are far apart, it means the high pressure area has a shallow topography. When the isobars are close together, it means there is a very steep slope to the mountain of air. When air is squeezed into a smaller, more confined area, the air flow will increase. Therefore, where the isobars are very close together you will find a steep slope with a rapid flow of air and the surface winds will be strong.
          Between high pressure areas you will find an area of low pressure where the air flows counterclockwise. Visualize the low pressure area as a valley of air between air masses.
          None of the pressure areas are stagnant. They are in a constant state of change where their pressure is increasing or decreasing. The earth's atmosphere is in a constant state of imbalance because of this, 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.

WIND
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 it is blowing parallel to the isobars. Coriolis force and pressure gradient force exactly balance, and above surface friction, causes the wind to blow parallel to the isobars.








(Flying toward Independence Pass from Aspen, Colorado, up the Roaring Fork River. Note the airport in center of photo.) 







The winds at the earth's surface do not blow parallel to 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 the isobars. Below 2,000 feet AGL, friction causes the surface wind to blow 45° inward toward a low pressure area and 45° outward from a high pressure area. 

SUBSIDENCE
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 upper Arkansas River Valley plateau area near Leadville, Colorado. The down flow, sinking air 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 find updrafts en route, but close to Independence Pass he 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.

INVERSION
Often there is a layer of air within the troposphere that is characterized by an increase of temperature with altitude rather than a decrease. It is called an inversion. It 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 an 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. Inversions (frontal) 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, we expect the prevailing westerly winds to flow down the side of the mountain (east‑facing) on the downwind side, pass through the valley and flow up on the upwind side (west‑facing) 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. 






(Proper positioning along the edge of the canyon allows an easy escape should a downdraft be encountered.)

 

 





TERRAIN MODIFICATION
Uneven terrain features may cause the air flow to be deflected downslope on what is considered the updraft side of a mountain.

VALLEY AND MOUNTAIN BREEZES
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.
          During the late afternoon and night 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 wind 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.

SECRET TO COPING
We tend to think in constants when contemplating the weather and associate whatever is happening as affecting a large area. Often a phenomenon 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 a fluid just like water, although less dense, that can be visualized. Ask yourself “What would water do in this situation?” More often than not you will be able to picture what is happening; to 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 provide a “got‑cha.” 

Sparky Imeson has received the FAA Northwest Region's Flight Instructor of the Year award in 1974, 1979 and 1995, for providing effective and creative flight and ground instruction. Of his 19,200+ flight hours, the majority have been in small airplanes in the mountains.  He has written 19 books, most on mountain flying. To contact Sparky go to his website – www.mountainflying.com

reprinted with permission from Sparky Imeson
posted 7/16/06


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