Wavelength
Wavelength
Figure 9-24. Destructive and constructive interference.
Wave flight requires planning and appropriate equipment. Additionally, flight in Class A airspace requires Federal Aviation
Administration (FAA) notification. The Soaring Society of America (SSA) offers soaring pilots Lennie Awards for
completing and documenting a wave flight. [Figure 9-25]
While working out of the Bishop, California, airport in the late
1940s, mountain pilot and wave pilot pioneer Robert F.
Symons created a new and unique system of awards for wave
flying, which he called “Iennie” pins. Pilots who soared to great
heights in the Sierra Wave received a one-lennie pin for
attaining an altitude of 25,000 to 35,000 feet, a two-lennie pin
for reaching 35,000 to 40,000 feet, and a three-lennie pin for
exceeding 40,000 feet.
Very early, Symons recognized the excellent soaring
conditions in the Owens Valley and helped organize a soaring
group in 1938. As a professional pilot engaged in cloud
seeding, he learned first hand of the power generated in the
Sierra Wave and became well known for his studies and
lectures on mountain wave phenomena. Although his lists are
incomplete, it is believed that he issued some 35 one-lennie,
16 two-lennie, and 10 three-lennie pins. The awarding of
these pins ceased in 1958, when Symons lost his life in a
glider accident.
In 1962 Carl Burson, Jr., saw one of these pins and upon
learning of its history, became interested in reestablishing
their issuance as a memorial to Bob Symons. In 1963, the
program was reestablished under the official auspices of the
SSA, with each new pin holder also receiving a handsome
wall plaque. The pin itself is 7mm in diameter (the same as the
FAI Gold Badge) and has one, two, or three white lenticular
clouds against a blue background with a silver rim. Each pin
is consecutively numbered.
Figure 9-25. Lennie Awards are given for completing and documenting a wave flight.
Convergence Lift
Convergence lift occurs when two opposing air masses meet, and air moves upward in response to the opposing winds.
Air does not need to meet head on to go up. Wherever air piles up in this fashion, it leads to convergence and rising air.
One type of convergence commonly found near coastal areas results from a sea-breeze. Inland areas heat during the day,
while the air over the adjacent water maintains about the same temperature. Inland heating leads to lower pressure, drawing
in cooler air. Sometimes, as the cooler air moves inland, it behaves like a miniature shallow cold front, and lift forms along
a convergence line. At other times, cooler air acts as a focus for a line of thermals. When unstable inland air exists, a sea-
breeze can cause frontal effects and act as a focus for a line of thunderstorms. Additionally, since the air on the coast side
of the sea-breeze front consists of cool air, passage of the front can end thermal soaring for the day.
Air over water often has a higher dewpoint than drier inland air. As shown in Figure 9-26, a curtain cloud sometimes forms,
which marks the area of strongest lift. Since colder and warmer air mixes in the convergence zone, pilots should expect
turbulence in any thermals associated with a sea breeze front.
Cooler sea air
Warmer inland air
Curtain cloud
Convergence line
Figure 9-26. Sea-breeze front.
Several factors influence the sea-breeze front characteristics (e.g., turbulence, strength, speed of inland penetration, the
degree of inland heating, and the land/sea temperature difference). For instance, a small land/sea temperature difference at
sunrise with overcast cirrus clouds can reduce ground heating and impede or weaken sea-breeze front formation. Another
factor is the synoptic wind flow (general flow of wind for a particular place and time). A weak synoptic onshore flow may
allow quicker inland penetration of the sea-breeze front, while a strong onshore flow may remove conditions that allow
development of a sea-breeze front. A moderate offshore flow generally prevents any inland penetration of a sea-breeze
front.
In a well-defined sea-breeze front marked by a curtain cloud, the pilot can fly straight along the line in steady lift. A poorly
defined, weaker convergence line often produces more lift than sink. The pilot should fly slower in lift and faster in sink.
Convergence can also occur along and around mountains or ridges. In Figure 9-27 Panel A, flow deflected around a
ridgeline meets as a convergence line on the lee side of the ridge. The line may be marked by cumulus or a boundary with
a sharp visibility contrast. The latter occurs if the air coming around one end of the ridge flows past a polluted urban area,
such as in the Lake Elsinore soaring area in southern California. In very complex terrain, with ridges or ranges oriented at
different angles to one another, or with passes between high peaks, small-scale convergence zones can be found in adjacent
valleys, depending on wind strength and direction. Figure 9-27 Panel B illustrates a smaller-scale convergence line flowing
around a single hill or peak and forming a line of lift stretching downwind from the peak.
Convergence line
Convergence line
Figure 9-27. Convergence induced by flow around topography (as viewed from above).
Convergence can sometimes occur along the top of a ridgeline or mountain range. In Figure 9-28, drier synoptic- scale
wind flows up the left side of the mountain, while a moist valley breeze flows up the right side of the slope. The two flows
meet at the mountain top and form lift along the entire range. If clouds are present, the air from the moist side condenses
first, often forming one cloud with a well-defined step, marking the convergence zone. For this scenario, the better lift
conditions occur on the Western dry side rather than the East side where clouds are more likely to form.
Drier winds Moist valley breeze
Figure 9-28. Mountaintop convergence.
As a final example, when daytime heating abates in mountainous terrain, a cool katabatic wind or drainage wind may flow
down mountain slopes. The flow down the slope converges with air in the adjacent valley to form an area of weak lift.
The convergence, often too weak to create lift to support glider flight, may act as a trigger for the last thermal of the day.
[Figure 9-29]
Slow Lift
Figure 9-29. Convergence induced by flow around topography.
Obtaining Weather Information
For visual flight rules (VFR) flights, federal regulations require pilots to review weather reports and forecasts if they plan
to depart the airport vicinity. Even for a local flight, glider pilots should know the current and forecast weather to avoid
flight in hazardous conditions.
For additional details regarding available weather services and products, glider pilots should refer to the current version
of the Pilot’s Handbook of Aeronautical Knowledge, FAA-H-8083-25, Chapter 12, Aviation Weather Services and the
Aviation Weather Handbook, FAA-H-8083-28. These sources include a wealth of information. A pilot familiar with
relevant and available weather reports and forecasts can analyze weather hazards and mitigate any associated risks. Pilots
who understand weather can experience safe flight in a variety of conditions.
Preflight Weather Briefing
The FAA delivers flight services to pilots in the CONUS, Alaska, Hawaii, and Puerto Rico. Services are provided by phone
at 1-800-WX-BRIEF, on the internet through the Flight Service Pilot Web Portal, and in person (Alaska only) at 17 Flight
Service Stations (FSS). Services include, but are not limited to: preflight weather briefings, flight planning, and in-flight
advisory services.
Weather briefers do not actually predict the weather; they simply translate and interpret weather reports and forecasts
within the vicinity of the airport, route of flight, or the destination airport if the flight is a cross-country. A pilot may request
one of four types of briefings: standard, abbreviated, soaring [Figure 9-30], or outlook.
Soaring Forecast
National Weather Service Denver/Boulder, Colorado
645 AM MDT Wednesday August 25, 2010
This forecast is for Wednesday August 25, 2010:
If the trigger temperature of 77.3 F/25.2 C is reached...then
Thermal Soaring Index....................... Excellent
Maximum rate of lift........................ 911 ft/min (4.6 m/s)
Maximum height of thermals.................. 16119 ft MSL (10834 ft AGL)
Forecast maximum temperature................... 89.0 F/32.1 C
Time of trigger temperature.................... 1100 MDT
Time of overdevelopment........................ None
Middle/high clouds during soaring window....... None
Surface winds during soaring window............ 20 mph or less
Height of the -3 thermal index................. 10937 ft MSL (5652 ft AGL)
Thermal soaring outlook for Thursday 08/26..... Excellent
Wave Soaring Index............................. Poor
Wave Soaring Index trend (to 1800 MDT)......... No change
Height of stable layer (12-18K ft MSL)......... None
Weak PV A/NV A (through 1800 MDT)................ Neither
Potential height of wave....................... 14392 ft MSL (9107 ft AGL)
Wave soaring outlook for Thursday 08/26........ Poor
Remarks...
Sunrise/Sunset.................... 06:20:55 / 19:42:44 MDT
Total possible sunshine........... 13 hr 21 min 49 sec (801 min 49 sec)
Altitude of sun at 13:01:25 MDT... 60.82 degrees
Upper air data from rawinsonde observation taken on 08/25/2010 at 0600 MDT
Freezing level.................. 15581 ft MSL (10296 ft AGL)
Additional freezing level....... 54494 ft MSL (49209 ft AGL)
Convective condensation level... 13902 ft MSL (8617 ft AGL)
Lifted condensation level....... 14927 ft MSL (9641 ft AGL)
Lifted index.................... -3.4
K index......................... +9.7
* * * * * * Numerical weather prediction model forecast data valid * * * * * *
08/25/2010 at 0900 MDT | 08/25/2010 at 1200 MDT
|
K index... +4.0 | K index... -0.7
This product is issued twice per day, once by approximately 0630 MST/0730
MDT (1330 UTC) and again by approximately 1830 MST/1930 MDT (0130
UTC). It is notcontinuously monitored nor updated after its initial issuance.
The information contained herein is based on rawinsonde observation and/or
numerical weather prediction model data taken near the old Stapleton
Airport site in Denver, Colorado at
North Latitude: 39 deg 46 min 5.016 sec
West Longitude: 104 deg 52 min 9.984 sec
Elevation: 5285 feet (1611 meters)
and may not be representative of other areas along the Front Range of the
Colorado Rocky Mountains. Note that some elevations in numerical weather
prediction models differ from actual station elevations, which can lead to
data which appear to be below ground. Erroneous data such as these should
not be used.
The content and format of this report as well as the issuance times are subject
to change without prior notice. Comments and suggestions are welcome and
should be directed to one of the addresses or phone numbers shown at the
bottom of this page. To expedite a response to comments, be sure to mention
your interest in the soaring forecast.
DEFINITIONS:
Convective Condensation Level - The height to which an air parcel possessing
the average saturation mixing ratio in the lowest 4000 feet of the airmass,
if heated sufficiently from below, will rise dry adiabatically until it
just becomes saturated. It estimates the base of cumulus clouds that are
produced by surface heating only.
Convection Temperature (ConvectionT) - The surface temperature required to
make the airmass dry adiabatic up to the given level. It can be considered a
"trigger temperature" for that level.
Freezing Level - The height where the temperature is zero degrees Celsius.
Height of Stable Layer - The height (between 12,000 and 18,000 feet above
mean sea level) where the smallest lapse rate exists. The location and
existence of this feature is important in the generation of mountain
waves.
K Index - A measure of stability which combines the temperature difference
between approximately 5,000 and 18,000 feet above the surface, the amount
of moisture at approximately 5,000 feet above the surface, and a measure
of the dryness at approximately 10,000 feet above the surface. Larger
positive numbers indicate more instability and a greater likelihood of
thunderstorm development. One interpretation of K index values regarding
soaring in the western United States is given in WMO Technical Note 158
and is reproduced in the following table:
below -10 no or weak thermals
-10 to 5 dry thermals or 1/8 cumulus with moderate thermals
5 to 15 good soaring conditions
15 to 20 good soaring conditions with occasional showers
20 to 30 excellent soaring conditions, but increasing
probability of showers and thunderstorms
above 30 more than 60 percent probability of thunderstorms
Lapse Rate - The change with height of the temperature. Negative values
indicate inversions.
Lifted Condensation Level - The height to which an air parcel possessing the
average dewpoint in the lowest 4000 feet of the airmass and the forecast
maximum temperature must be lifted dry adiabatically to attain saturation.
Lifted Index - The difference between the environmental temperature at a level
approximately 18,000 feet above the surface and the temperature of an air
parcel lifted dry adiabatically from the surface to its lifted condensation
level and then pseudoadiabatically thereafter to this same level. The
parcel`s initial temperature is the forecast maximum temperature and its
dewpoint is the average dewpoint in the lowest 4000 feet of the airmass.
Negative values are indicative of instability with positive values showing
stable conditions.
Lift Rate - An experimental estimate of the strength of thermals. It is
computed the same way as the maximum rate of lift but uses the actual
level rather than the maximum height of thermals in the calculation.
Also, none of the empirical adjustments based on cloudiness and K-index
are applied to these calculations.
Maximum Height of Thermals - The height where the dry adiabat through the
forecast maximum temperature intersects the environmental temperature.
Figure 9-30. Soaring forecast.
Weather-Related Information
Pilots can also find weather-related information on the Internet, including sites directed toward aviation. Pilots should
verify the timeliness and source of the weather information provided by any Internet sites to ensure the information is up
to date and accurate. For example, a source of accurate information includes the NWS website.
Interpreting Weather Charts, Reports, & Forecasts
Knowing how to interpret and understand weather information requires knowledge and practice. Weather charts and reports
record observed atmospheric conditions at certain locations at specific times. The NWS collects data from automated
sources or from trained observers using electronic instruments, computers, and personal observations as well as radiosonde
observations or model generated soundings to produce the weather products pilots use to determine if a flight can be
conducted safely. This same information can be used by soaring pilots to determine if sufficient lift for a planned flight
may exist, where to find it, and how long the lift should last.
Chapter Summary
Pilots use several different types of lift to make extended glider flights. The lift comes from thermals, mountain ridges,
wave formation, or convergence zones. The stability of the atmosphere affects several of these sources. Each source of lift
has its own set of hazards and associated risks. For example, weather leading to thermals can also lead to thunderstorms.
Pilots looking for weather conditions that favor extended flight should also know how to identify conditions and hazards
that prompt them not to fly. Pilots should not fly near thunderstorms, in low visibility, or in high winds that affect the safety
of takeoffs and landings. Pilots can get general and specific information from a variety of sources and should make certain
that any source of weather information used before flight is accurate. The FAA has information and documents regarding
weather products and services on its website.