This seminar addresses some general meteorological phenomena that you might encounter when flying in any mountainous area. Specifically we will investigate phenomena that are unique to mountainous areas where there are deep river canyons and associated local winds and weather patterns.
Before any flight it is always a good idea to get a normal FSS weather briefing to give you an idea of position and movement of major fronts, precipitation, convective activity, winds aloft, and aviation weather reports. Television and Internet weather services are also a good source of information, but keep in mind that weather in mountainous areas can change rapidly, and forecasts may not be valid past a few hours. In the summer months major weather concerns will be morning fog in valleys and canyons, wind, and evening thunderstorm activity. Wind and weather can change rapidly and vary greatly from one location to another that is only a few miles away. In some areas you may be able to obtain a backcountry radio report from remote Forest Service facilities or private ranches, which are a good source of weather information. Additionally, it will be important to talk to other pilots about fog, surface wind conditions, and convective activity unique to a certain area.
When wind blows in the mountains it creates wind shear, lift, sink, and turbulence. It is prudent to maintain a constant awareness of wind direction and strength. You can get an idea of local wind is by looking at trees, smoke, and mountain lakes. Cloud shadows can give an indication of the prevailing wind at the level of the cloud layer. Always be alert because wind speed and direction can change constantly and are not consistent, even from one ridge or canyon to the next.
When winds aloft blow more than 25 knots perpendicular to a mountain range such as the
Cascades or Sierra Nevada, they may produce the classic mountain wave effect. Wind may be fairly calm on the windward side, but turbulent on the leeward side with rotors and downdrafts. The actual waves may extend for miles downwind of the mountain range, and wavelengths can vary, but generally are about eight to ten miles. The shorter the wavelength the greater the amplitude of the wave. Cap clouds, rotor clouds, and standing lenticular clouds may form if the air has enough moisture. When you suspect any mountain wave activity you should use extreme caution. Keep in

mind that forecast high velocity winds aloft, cirrus clouds, and lenticular clouds all indicate increased wind flow over the mountains.
Wind velocity increases as wind crosses any ridge, and you could encounter large downdrafts on the lee side of ridges, sometimes without much turbulence. Use extreme caution when crossing ridges, and always do so at the recommended 45
 angle so that you can turn away if you encounter downdrafts. In narrow mountain passes you may encounter an increase in wind speed called a
"venturi effect", even when the prevailing wind is only light to moderate. Venturi effects are discussed later in this section.
Knowing the winds on the ground is critical when operating in and out of mountain and canyon airstrips. In the summer, many of the strips in mountainous areas, and especially in canyons, are unsafe to fly into or out of by late morning because of wind. Landing with a good headwind at a oneway strip means you would be departing with a tailwind. This subject will be covered in more detail in the section on landing operations. Generally, if there is wind you should consider returning to land later when there is no wind, or plan to stay at a one-way strip until the wind decreases.
In the northwestern USA, where there is a lot of mountainous terrain, atmospheric circulation is generally from the west or southwest, which has an effect on weather and wind patterns with which you will be concerned. In the Oregon and Washington Cascades, summer brings southwesterly to westerly flow aloft and mountain waves are not uncommon to the east of the range and up to flight level altitudes. Additionally, strong westerly winds may prevail in central Washington during the summer months.
During the summer in Montana and Idaho, mountain areas usually see wind blowing from the west or southwest. In the fall, winter, and spring, winds and circulation patterns are more unpredictable. If winds are from the north or east it usually means a system is coming in from Canada and the low is to the northeast east. In central Idaho north or east winds generally bring a lot of unstable weather, precipitation, IFR weather, and unpredictable turbulence and winds. Some of the strongest winds that create the most turbulence are encountered in Idaho, western Wyoming, and western Montana when prevailing winds aloft are from the south.
In mountainous areas atmospheric pressure patterns may vary greatly. This means that you can travel only a short distance and the altimeter setting might be different. Keep this in mind as you operate in and out of airstrips that are more than a few miles apart, or that are situated in the bottom of canyons or on top of ridges or in high mountain basins. If two canyons are in the same drainage system, pressure usually equalizes quickly between them and the altimeter setting is nearly uniform.
However, local winds and pressure patterns can be very different in river drainage systems only a few

miles apart. If you fly from one canyon across a ridge into a different canyon that is not connected, there may be a pressure differential between the two canyons. In that case, you can expect wind and turbulence at the divide between the two drainage systems. The different pressure in the new canyon may cause your altimeter to be off by several hundred feet until you can get a new altimeter setting.
All pilots sharte the same problem, so be aware that other airplanes may be reporting altitudes with a different altimeter settimg from yours. Set your altimeter to the field elevation each time you land, and verify altimeter settings with other pilots when passing. Remember if flying from high to low pressure, or hot to cold, your altimeter will indicate higher than your true altitude, so "look out below".
One of the most important aspects of flying in mountain and canyon areas is knowing where to find lift and how to use it to your advantage. When flying ridges or canyons, you want to fly the
"updraft" side (where the lift is) which will help climb performance and terrain clearance. On the downdraft side, you may find a rough ride and downdrafts that exceed the climb capability of your aircraft. Most mountain pilots expect wind to flow down into a canyon and then up the other side, creating an updraft on the downwind side of the canyon. However, that is not always true since actual lift is the result of the interaction between orographic lift (created by wind and mountains) and solar lift, or heat. Additionally, the updraft side may change from one side of a canyon to the other as they canyon twists and turns and different sides face the sun or wind. This concept is explored in the following sections on orographic and thermal effects.
Orographic lift is mechanical lift created by wind flowing over terrain. As air flows over mountains it is lifted, which causes it to cool. If the lifted air has a lot of moisture it can form clouds
(such as a cap cloud), orographic precipitation, or upslope fog. As the heavy, cool air descends down a mountain slope or canyon it is heated and dried, which can cause local winds, such as the
Santa Ana winds in the Sierra Nevada or Chinook winds in the Colorado Rocky Mountains.
In the western United States, and particularly the northwest, prevailing winds are generally from the west and southwest. Thus you can usually find upslope winds on west facing slopes and downslope winds on east facing slopes. This is especially true with well defined major north-south mountain ranges that are oriented perpendicular to the prevailing westerly winds such as the Oregon and Washington Cascades, the Sierra Nevada, or the Rocky Mountains.
In areas where more randomly oriented mountains and canyons prevail, determining local wind directions and locations of orographic lift are not straightforward. For example, the mountains of central Idaho and western Montana are formed by a large mass of randomly oriented ridges cut by

deep, sinuous rivers. Thus, when the prevailing winds flow over them it is more like waves crashing over a jumble of rocks. This means there can be many different local winds that will produce patterns of random, unpredictable areas of lift and sink. This phenomenon is called a "haystack" effect and means that midlevel, or ridge top winds are often hard to predict and will vary widely in speed and direction.
Every process of weather on the planet is related to uneven heating of the earth's surface, and mountain and canyon terrain is no exception. Early in the morning you may find that the sun heats the east facing slopes, and there may be some local lift, but it generally does not last long. Sunlit slopes heat up more quickly than the surrounding terrain, causing thermal or anabatic lift. Thermal lift is most common on south and west facing slopes as the day progresses. Thermal lift can be quite significant, and mountain pilots seek it out. Thermal lift usually extends about 50 to 200 feet out from the face of the mountain ridge or canyon wall as well as directly above a ridgeline. This means you need to be close to mountain faces or canyon walls to use the lift. Bare rocks and slopes will heat up faster and generate more lift than areas that are heavily forested, so look for the "brighter" slopes. In fact, there may be enough difference between bare slopes and forested areas to create turbulence at the boundary between them due to wind shear produced by differential lift. Likewise, expect to find differences in lift between areas that are in sun and in shadow, and turbulence at the boundary between those areas. Using thermal, or anabatic, lift to gain altitude is called "contouring" a ridge.
When contouring a ridge always turn away from the ridge (into the wind) to make your circle back toward the lift.
In summer months when the sun is shining on southwest and west facing slopes and the wind is out of the southwest or west, orographic and thermal lift can combine and greatly augment the lifting force. But what if the wind is blowing from another direction? If thermal and orographic lift work against one another, the results can be much different, depending on wind velocity. Thermal lift will create updrafts on a sun-facing slope, but if that slope is on the lee side of a ridge or canyon, orographic effects produce downdrafts. The updrafts meet the downdrafts and create turbulence. As wind velocity increases, this turbulence reaches deeper into the canyon and can become severe to extreme. In very deep, rocky canyons that are exposed to the sun for a long time during summer days, thermal lift may cause updrafts powerful enough to create a "cushion" of air in the canyon bottoms that deflects the orographic downdrafts. This may result in turbulence at any level in the canyon depending on the relative strength of the opposing thermal and orographic effects. Such

turbulence is often greatest at ridge top level. Solar lift on the upwind side of the canyon may be stronger than orographic lift on the downwind side of the canyon, depending on temperature and g eographic location, thus making it better to fly on the upwind, or “wrong” side of the canyon.
Predicting which of these phenomena will occur is difficult, so it is best to stay out of canyons when the wind is blowing strongly. You will find local pilots to be a great resource, since they know where and when to expect turbulence and lift. It may be safe to fly with winds of 15-20 knots in one area and not in another. It is best to check with pilots who are familiar with an area before mixing mountains, canyons, and wind.
1. With a light wind, solar flit is stronger than orographic effects, and the upwind, sunny side of the canyon is the best place to fly
2. With a stronger wind, rising air from solar lift creates a cushion of air in the canyon bottom. There is lift low in the canyon, but turbulence near the ridge tops.
3. With a very strong wind, solar lift interacts chaotically with orographic effects creating turbulence and unpredictable winds throughout the canyon.
Several factors such as varying slope, color, latitude, and elevation contribute to uneven surface heating. Many canyons are hundreds of miles long and the elevation between their upper and lower ends may differ by thousands of feet. Changes that occur on a daily cycle in these deep, long canyons are the result of the interplay between heat from the sun, rising and falling air, and the shape and elevation of the terrain. These daily changes are called diurnal effects, and are greatest in the summer months when the sun is highest and the days are longest. When the morning sun strikes canyon walls, it starts a thermal heating process: air from the lower part of the canyon heats up and begins to rise. The rising air simply follows the canyon upstream. Thus, canyon breezes normally blow up-canyon (or upstream) beginning midmorning. These winds can become quite strong by mid afternoon. During evening hours the air cools off more quickly at the upper (higher altitude) end of a canyon. This cooling air becomes dense and sinks downstream. Thus, canyon winds normally flow down-canyon (downstream) during evening and ighttime hours.

This cycle of winds blowing upstream in the mornings and downstream in the evenings is a dynamic process that repeats daily, but there can be many exceptions locally due the shape of the canyon itself, as well as the influence from winds entering the canyon from tributary canyons.
A phenomenon that occurs where canyons join is called a convergence effect. Canyons may converge at various angles and in varying directions. Diurnal winds may be stronger and develop early in the day in large canyons, but may develop later, or not at all, in smaller tributary canyons.
This means the wind might be blowing upstream in the main canyon but blowing downstream in one of the side canyons that feeds into the main canyon. Flying past this confluence, you should expect turbulence, which could be severe, depending on the relative strength of the opposing winds. When planning a flight into mountain and canyon areas, you should study your aeronautical chart and pay special attention to areas where canyons converge, so you can anticipate turbulence and plan your route and altitude accordingly. Because of converging rivers or streams, canyons are typically wider at confluences, and sandbars or benches tend to form there. Thus, airstrips are often located near river confluences. If you are operating at an airstrip near a confluence, be alert for varying winds, wind shear, and turbulence.
Many canyon airstrips have more than one windsock to help you determine the effects of converging winds. It is not unusual for these windsocks to point in opposite directions, even though they are only a few thousand feet apart. If the converging canyons are not deep, winds can converge over the top of the ridge that separates them; if an airstrip is at the top of such a ridge, beware of wind shear and turbulence.
The venturi effect is an increase in wind speed through a constriction or bottleneck, such as a narrow mountain pass or a narrow spot in a canyon. If a large open valley or intermountain basin narrows to form the entrance to a canyon, the wind will accelerate as it passes into the canyon, causing a decrease in pressure, which creates a downdraft. The downdraft can be insidious, as there may not be a lot of turbulence associated with it. Alternatively, when a wind is blowing up or down a canyon that opens out into a wide valley, the wind will diverge and typically flow outward away from the constricted area, which can cause wind shear in horizontal and vertical directions. When studying your charts during preflight planning, note any such constrictions and anticipate a venturi effect. You might also expect a venturi effect in a narrow canyon that makes a sharp bend, causing the wind to change direction rapidly.

Turbulence can make any flight uncomfortable, and could damage your aircraft. Throughout this two-part discussion, we noted situations and locations in which a pilot can expect turbulence when flying in mountains and canyons. On this page is a summary of where you should expect turbulence, which should help you in your quest to avoid it. You should be constantly vigilant for phenomena such as convergence and venture effects, and learn to anticipate areas of turbulence. It is best to seek out updrafts, avoid downdrafts, and try to stay out of turbulence.
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The foregoing discussion should impress on you that although there are some general rules pertaining to wind, lift, and turbulence, they may apply only to a local ridge or mountain; overall you should expect conditions in mountainous areas and in canyons to be somewhat unpredictable. Often lift is where you find it, so you should actively seek out updrafts and always be wary. Although we have looked at some general rules pertaining to wind, lift, and turbulence when flying in canyons, it is best to expect the unexpected. If the winds aloft are strong, you should reassess your flight plan; you may need to change your route of flight, your altitude, or postpone your flight. Knowledge of these phenomena can help make your mountain and canyon flying safe and enjoyable. A good rule of thumb::