MOUNTAIN WAVE [top]
When airflow over mountainous terrain meets certain criteria, a phenomenon known as a standing wave or mountain wave may exist. The three requirements for a standing wave are:
1. Wind flow that is within 30 degrees of perpendicular to the ridge, with velocities of 20 knots or more at mountain top levels.
2. A wind profile which shows an increase in wind velocity with altitude and a strong steady flow at higher levels up to the tropopause.
3. An inversion or stable layer somewhere below 15,000 feet.
Intensities of any given wave are determined by mountain height, degree of slope and the strength of the wind flow.
An inversion tends to hold the flow down at lower altitudes while the high upper flow accelerates the lower flow. When this air flow hits a ridge, the flow is squeezed and a venturi-type acceleration may occur. A wave may form that can extend downwind from the ridge for many miles, often several times the height of the ridge. With a 50 knot wind, the turbulence can extend downrange hundreds of miles. Satellite photos have shown the wave extending over 700 miles downwind from the mountains.
Associated with a standing wave in air containing sufficient moisture are the "sign post" clouds indicated the wave.
1. Lenticular clouds, the smooth, lens shaped clouds that are found at altitudes from just above the ridges to more than 40,000 feet.
2. Rotor clouds which occur downwind and parallel to a ridge, may appear as a horizontal tornado or something as benign as a fair-weather cumulus. They can produce updrafts and downdrafts in excess of 5,000 to 8,000 feet per minute.
3. Cap clouds appear as a stationary, white cap covering the top of a mountain. As with the lenticular, the wind may be flowing through this cloud at high velocity. As the air moves up the windward side of the ridge, moisture is condensed out, then causes dissipation of the cloud on the lee side as it moves down and the heat of compression reabsorbs the moisture.
The existence of a standing wave indicates probable turbulence. The type of turbulence will vary from barely perceptible to extreme. Above the ridges you may encounter very smooth up and downdrafts.
Meteorologists are able to accurately forecast standing waves. You can also predict a standing wave by requesting information on air temperature and the winds aloft from the surface to 40.000 feet. If it shows a wind velocity at the level of the peaks of 30 knots or more, with higher winds above and the wind direction relatively constant, then there is a good possibility of a standing wave.
In planning a mountain flight, a pilot who suspects or is informed by the weather briefer that a standing wave is forecast, should know what to expect and plan accordingly. Check PIREPS and find out what other pilots are reporting. Consult with local pilots for the best routes to fly and routes to avoid.
NOTE: Standing waves may last from two hours to two days. When a high wind, 20 knots or greater, is flowing over a mountain range, whether the flow is in the form of a standing wave or just wind flowing over a mountain, a potentially dangerous situation exists. The standing wave situation is more predictable at altitudes above the summit. When there is a mountain wave or a strong wind flow, it is too dangerous to fly light aircraft near and below the ridges.
Pilots who understand the limitations
of observations and forecasts usually
Recent studies of aviation forecasts indicate the following:
Up to 12 hours—and even beyond—a forecast of good weather, ceiling 3,000 or more, and visibility three miles or greater is much more likely to be correct than a forecast of conditions below 1,000 feet or less than one mile.
If poor weather is forecast to occur within three to four hours, the probability of occurrence is better than 80 percent.
Forecasts of poor flying conditions during the first few hours of the forecast period are most reliable when there is a distinct weather system, such as a front, a trough, precipitation, etc. There is a general tendency to forecast too little bad weather in such circumstances.
The weather associated with fast-moving cold fronts and squall lines is the most difficult to forecast accurately.
Errors occur when attempts are made to forecast a specific time that bad weather will occur. Errors are made less frequently, of course, when forecasting that bad weather will occur during some period of time.
Surface visibility is more difficult to forecast than ceiling height.
Visibility in snow is the most difficult of all visibility forecasts. Skill In these forecasts leaves much to be desired.
Available evidence shows that forecasters can predict the following at least 75 percent of the time:
The passage of fast-moving cold fronts or squall lines within plus or minus two hours, as much as 10 hours in advance.
The passage of warm fronts or slow-moving cold fronts within plus or minus five hours, up to 12 hours in advance.
The rapid lowering of ceilings below 1,000 fee in pre-warm front conditions within plus or minus 200 feet and within plus or minus four hours.
The time rain or snow will begin, within plus or minus five hours.
Forecasters cannot predict the following with an accuracy that satisfies Present aviation operational requirements:
The time freezing rain will begin
The location and occurrence of severe or extreme turbulence.
The location and occurrence of heavy icing.
Ceilings of 100 feet or zero before they exist.
The onset of a thunderstorm that has not yet formed.
Turbulence, in the mountains, is not always easy to forecast. When flying in the mountains, you may find turbulence in many unexpected places.
Turbulence can be divided into three general categories:
1) Convective- thermals occurring on a hot summer afternoon.
2) Mechanical- the wind flowing around and over mountains, buildings, trees, and other obstructions.
3) Sheer- two air masses moving in different directions producing shear at their interface, such as that occurring with cold fronts and downbursts.
When you encounter turbulence, always slow down. If the turbulence is light, be sure to slow the airspeed to that below the yellow range on the airspeed indicator. If moderate turbulence is encountered, slow to the aircraft's maneuvering speed.
During the summer months, it is always best to plan your flights into and out of the mountains in the early morning hours or the late evening hours. As the sun generates the most heat during the middle of the day, that is when the most turbulence will be encountered. During the winter months, when there are no weather systems in the area, very little turbulence will be encountered.
OBTAINING MOUNTAIN WEATHER INFORMATION [top]
AWOS/ASOS TELEPHONE NUMBERS
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OTHER METHODS OF OBTAINING WEATHER INFORMATION [top]
ALPINE COUNTY, CA (530) 694-2231 ALPINE COUNTY SHERIFFS DEPARTMENT
BRIDGEPORT, CA (760)932-7153 OR 932-5252
CHESTER, CA (530) 258-3616
DUNSMUIR, CA (530) 926-2519
EAGLE LAKE, CA (530) 251-8288
FALL RIVER MILLS, CA (530) 336-5549 OR 336-5465
QUINCY, CA (530) 283-6210
SOUTH LAKE TAHOE, CA (530) 542-6180
SUSANVILLE, CA (530) 257-2030