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Mountain Weather Factors
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Mountain flying offers the well-prepared pilot some of the most spectacular scenery to be found. However, preparation must include a good knowledge of the weather factors one will find. By there very nature, the mountains influence the weather in way not normally thought of by flatland pilots. High mountain peaks and ridges, like the Rockies found in Colorado and other western states, generally run north and south, perpendicular to the normal westerly flow of winds aloft. As the wind flows up the western slopes of the ridges (orographic lifting), you can find turbulence in proportion to the wind speed. You should take orographic lifting into consideration in your flight planning so as to cross at least 1,000 feet above the ridge line and be at that altitude at least 3 miles before reaching the ridge line. Also, monitor the vertical speed indicator on all climbs to detect decreased or negative climb rates immediately. Winds aloft must be given special attention in mountain flight planning. Most experienced mountain pilots suggest avoid mountain flying when the winds at mountain top level exceed 30 knots. |
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Winds higher than this will cause moderate or greater turbulence and very strong up and down drafts, exceeding the capabilities of most light aircraft. Check the winds aloft forecast for the 9,000 foot and 12,000 foot levels. Mountain weather stations are generally in valleys, and their surface winds are not reliable predictors of the winds across the peaks. Winds aloft can be deceiving. A recently discovered phenomenon known as the Leadville Drainage Wind is believed to be the cause of numerous accidents involving light aircraft since it causes ridge winds to be much different than expected. The drainage wind is caused by a higher barometric pressure at Leadville than at Aspen. This causes the wind across the ridges between Leadville and Aspen to flow east to west instead of the expected west to east. Updrafts and down drafts are not where a pilot might expect them to be. The down drafts will occur on the west side of the ridges instead of the east side. A difference in barometric pressure of 0.10" between Leadville an Aspen should be viewed as a warning sign that the drainage winds are a possibility. Maintaining at least 1,000 feet above the ridges will help reduce the possibility of encountering these winds.
Mountain weather can change very rapidly, surrounding the unwary pilot before she/he knows it and closing off escape routes. In the summer, the convection from daily heating coupled with the orographic lifting of the mountain ridges leads to the development of afternoon showers and thunderstorms that can build very rapidly. As a rule of thumb, planning your flight so as to be out of the mountains by 1 p.m. will help avoid the thunderstorm danger. In the winter, snow showers can develop just as quickly and close off routes. Density altitude is a major consideration when flying in high mountain areas, as well as the Colorado Front Range area. Normally aspirated engines lose about 3 percent of their horsepower for each 1,000 feet above sea level.
At Leadville, Colorado, a 300 horsepower engine will produce only about 210 horsepower! It is critical that you check takeoff and landing performance data for ALL takeoffs and landings in Colorado. To improve aircraft performance in the mountains, reduce the aircraft weight to no more than 90 percent of the maximum certified gross weight. Some of the best information to be had will come from other pilots. Monitor Flight Watch or other advisory frequencies to hear the latest reports of conditions in your area. Also, participate by giving your own pilot reports (and position reports) as you fly. Mountain flying has rewards that are well worth the effort. The scenery is truly inspiring. However, adequate preparation and flight planning are musts to make the journey safe, enjoyable, and low risk. And by all means, FILE A FLIGHT PLAN!
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