Hail
Hail associated with any thunderstorm can exist within part of the main rain shaft. Hail can also occur many miles from
the main rain shaft, especially under the thunderstorm anvil. Pea-sized hail does not usually damage a glider, but the large
hail associated with a severe storm can dent metal gliders or damage the gelcoat on composite gliders, both in the air or
on the ground.
Icing
Icing can create a significant problem within a cloud or where visible moisture exists, especially at levels where the outside
temperature is approximately -10 °C. Under these conditions, supercooled water droplets (water existing in a liquid state 0
°C and below) can rapidly freeze upon contact with wings and other glider surfaces. Early precipitation below a cloud base
from a developing storm may be difficult to see. At times, precipitation can even be falling through an updraft feeding the
cloud. Snow, graupel (a snow/frozen water combination), or ice pellets falling from the forming storm above can stick to
the leading edge of the wing and degrade performance. Ice can form on the canopy and interfere with the pilot’s forward
vision.
Low Visibility
Poor visibility due to precipitation or low ceilings as air below a thunderstorm cools creates another concern for the
pilot. Even light or moderate precipitation can reduce visibility dramatically. Often, under a precipitating Cb, confusion
distinguishing between the precipitation and cloud can occur.
Lightning
Lightning strikes are completely unpredictable. Lightning in a thunderstorm may occur within one cloud, jump from one
cloud to another if a nearby storm exists, or jump from cloud to ground. Some strikes emanate from the side of the Cb and
travel horizontally for miles before turning abruptly toward the ground. Inflight damage to gliders has included burnt control
cables and blown-off canopies. In some cases, strikes have caused little more than mild shock and cosmetic damage. At the
other extreme, a composite training glider in Great Britain suffered a strike that caused complete destruction of one wing;
fortunately, both pilots parachuted to safety. In that case, the glider was two or three miles from the thunderstorm. Pilots
and ground personnel should avoid ground launching, especially with a metal cable, with a thunderstorm in the vicinity.
Tornadoes
Severe thunderstorms can sometimes spawn tornadoes a few hundred to a few thousand feet across with winds that can
exceed 200 mph. Tornadoes that do not reach the ground are called funnel clouds. Pilots should consider any tornado watch
or warning information and remain well clear of funnel clouds and tornadoes.
Weather for Slope Soaring
Wind deflects horizontally, vertically, or some combination of the two when it encounters topography. Slope or ridge
soaring relies on updrafts produced by the mechanical lifting of air as it encounters the upwind slope of a hill, ridge, or
mountain.
Individual or isolated hills tend not to produce slope lift because the wind deflects around the hill, rather than over it. A
somewhat broader hill with a windward face of approximately a mile, might produce some slope lift over a small area. The
best ridges for slope soaring span at least a few miles. While ridges only 100 or 200 feet high can produce slope lift, the
pilot might not find sufficient rising air to climb above the top of the ridge.
Slope lift can extend to a maximum of two or three times the ridge height. [ Figure 9-16] Generally, the higher the ridge
extends above the adjacent valley, the higher the glider pilot can climb. The pilot should maintain a safe maneuvering
altitude and sufficient altitude to land if necessary. Pilots should consider flight between 500 to 1,000 feet above the
adjacent valley as a minimum safe height.
Lift zone
Best lift
Figure 9-16. Lift zone for slope soaring.
Depending on the slope, windspeed of 10 to 15 knots blowing nearly perpendicular to the ridge produces usable updrafts.
However, wind directions up to 30° or 40° from perpendicular may still produce slope lift. High ridges may have little
or no wind along the lower slopes, but the upper parts of the ridge may be in winds strong enough to produce slope lift,
creating a vertical wind shear.
The area of best lift varies with height. Below the ridge crest, the best slope lift is found within a few hundred feet of
the ridge, depending on the slope and wind strength. Very steep ridges require extra speed and caution since eddies and
turbulence can form on the upwind side. Above the ridge crest, the best lift usually occurs further upwind from the ridge
as the glider gains altitude. [Figure 9-17]
Eddy
Lift zone
Figure 9-17. Slope lift and eddy with near-vertical slope.
An ideal ridge has a slope on the order of 1 to 4. For each four feet of horizontal travel, the terrain increases vertically by
1 foot. Shallower slopes do not create a vertical wind component strong enough to compensate for the glider’s sink rate.
Very steep, almost vertical slopes create lift, but may also produce turbulent eddies along the lower slope or anywhere
close to the ridge itself. In such cases, only the upper part of the slope may produce updrafts, although steeper slopes allow
a quick escape to the adjacent valley.
Slope lift in stable air can be very smooth, enabling safe soaring close to the terrain. In unstable air, thermals may flow up
the slope. Depending on thermal strength and windspeed, the thermal may rise well above the ridge top, or it may drift into
the lee downdraft and break apart. Downdrafts on the sides of thermals can easily cancel the slope lift, and require extra
speed and caution, especially below the ridge crest near the terrain. The combination of unstable air and strong winds can
make slope soaring unpleasant or even dangerous for an inexperienced glider pilot.
Upwind terrain can block the low-level wind flow of an otherwise promising ridge. Additionally, any waves produced by
an upwind ridge or mountain can enhance or destroy ridge lift downstream, depending on the interference pattern of the
waves. Locally, the downdraft from a thermal just upwind of a ridge can cancel slope lift for a short distance. The pilot
should assume any slope lift could vanish and have plans for that eventuality.
While the flow deflects upward on the windward side of a ridge, it deflects downward on the lee side. [Figure 9- 18] This
downdraft can reach 2,000 fpm or more near a steep ridge in strong winds (panel A). Flat-topped ridges offer little refuge
since sink and turbulence can combine to make flight above the flat challenging or impossible (panel B). Finally, an uneven
upwind slope with ledges or “steps” requires extra caution since small-scale eddies, turbulence, or sink can form (panel
C). If crossing ridges in windy conditions, the pilot should plan for heavy sink on the lee side and stick to a set of personal
minimum altitudes for crossing.
Figure 9-18. Airflow along different ridges.
Three-dimensional effects are important as well. For instance, a ridge with cusps or bowls may produce better lift on the
upwind-facing edge of a bowl if the wind is at an angle from the ridge. However, the pilot may encounter sink on the lee
side of a bowl edge. [Figure 9-19]
Figure 9-19. Three-dimensional effects of oblique winds and bowls.
Moist air rising from contact with a slope that cools sufficiently may form a so-called cap cloud. [Figure 9-20] The cloud
may form above the ridge, and if the air moistens over time, the cloud slowly lowers onto the ridge and down the upwind
slope, limiting the usable height of the slope lift. Under certain conditions, a morning cap cloud may rise as the day warms,
then slowly lower again as the day cools. Since the updraft forms the cloud, a pilot could climb into the cap cloud and lose
outside visual references.
Figure 9-20. Cap cloud.
Mountain Waves
Wind blowing over mountains or ridges can produce waves, the most powerful of which have lifted gliders to above 49,000
feet in the United States. With strong and widespread winds aloft and stable atmosphere, mountain waves can extend
downwind along the entire length of a mountain range. Pilots refer to these waves as mountain waves, lee waves, mountain
lee waves, or standing waves. Pilots have achieved multi-leg flights of over 2,000 kilometers in mountain waves. Note that
for brief periods in some parts of the world, mountain waves reach into the stratosphere and receive a boost from the polar
vortex. Pilots in an experimental pressurized glider reached altitudes above 76,000 feet in 2018 using this phenomenon in
Argentina.
Water flowing in a stream or small river illustrates mountain wave formation. A submerged rock causes ripples (waves) in
the water downstream, which slowly dampen out. In the case of mountain waves, the airflow over the mountain displaces
a parcel of air from its equilibrium level. Since the atmosphere contains variations in the stability profile, wind blowing
over a mountain does not always produce downstream waves. Anyone seeking more information regarding mountain wave
formation than presented in this chapter should consult the Aviation Weather Handbook (FAA-H-8083-28).
Mountain waves differ fundamentally from slope lift. Slope soaring occurs on the upwind side of a ridge or mountain,
while mountain wave soaring occurs on the downwind side. Mountain waves can tilt upward with height, and at times near
the top of the wave, the glider pilot may be almost directly over the mountain or ridge that produced the wave.
Isolated small hills or conical mountains do not form classic lee waves. In some cases, they do form waves
emanating at an angle to the wind flow like water waves created by the wake of a ship. A single peak may require only a
mile or two in the dimension perpendicular to the wind for high-amplitude lee waves to form, though wave lift produced
this way occurs in a relatively small area.
Mechanism for Wave Formation
Stable air can support wave formation when a disturbance causes vertical motion of the air and wind causes horizontal
displacement. As illustrated in Figure 9-21 at dashed line 1, the dry unsaturated parcel (depicted by the red dot) sits at
rest at its equilibrium level. After upward displacement of the parcel (dashed line 2), the lifted parcel, now cooler than
the surrounding air, accelerates downward toward its equilibrium level. It overshoots the level due to momentum and
keeps going down. Dashed line 3 shows that the parcel, now warmer than the surrounding air and at a lower altitude than
at dashed line 1, moves upward again. The process continues with the motion eventually damping out. The number of
oscillations depends on the initial parcel displacement and the stability of the air. In the lower part of the figure, wind has
been added, illustrating the wave pattern the parcel makes as it oscillates vertically. If there were no wind, a vertically
displaced parcel would just oscillate up and down, while damping at one spot over the ground.
Equilibrium
DALR
Parcel motion
Parcel motion
Parcel motion
Parcel motion
Parcel of air with movement
+
+
+
+
––––
Temperature Temperature Temperature Temperature
–T = cooler than surrounding air
+T = warmer than surrounding air
DALR = dry adiabatic lapse rate
Figure 9-21. Parcel displaced vertically and oscillating around its equilibrium level.
The lower part of Figure 9-21 also illustrates two features of any wave. The wavelength is the horizontal distance between
two adjacent wave crests. Typical mountain wavelengths vary considerably, between 2 and 20 miles. The amplitude is half
the vertical distance between the trough and crest of the wave.
Figure 9-22 illustrates a two-dimensional conceptual model of a mountain with wind and temperature profiles. Note the
increase in windspeed (blowing from left to right) with altitude and a stable layer near mountaintop height with less stable
air above and below. As the air flows over the mountain, it descends the lee slope (below its equilibrium level in stable air),
which sets up a series of oscillations downstream. While the wave exhibits smooth flow, a low-level turbulent zone exists
below, with an embedded rotor circulation under each crest. Turbulence, especially within the individual rotors, while
usually moderate to severe, can occasionally become extreme.
Cap cloud
Lower turbulent zone
Roll cloud
ACSL
CCSL
Tropopause
Lee wave region
Height (thousands of feet)
Temperature
Wind
Wind
Stable
Layer
Main downdraft Main updraft
Temperature
Horizontal distance
Temperature
Wind velocity
Standing Lenticular Cirrocumulus Clouds (CCLS)
Altocumulus Standing Lenticular Clouds (ACSL)
Figure 9-22. Mountain lee wave system.
This simple conceptual model has many possible variations. For instance, the topography could have many complex
three-dimensional features, such as large ridges, or spurs at right angles to the main range. Variations can occur when
a North-South range curves to become oriented Northeast-Southwest. In addition, numerous variations of the wind and
stability profiles can occur. In some conditions, the second or third wave crests increases in amplitude, and the pilot can fly
higher if flying that portion of the wave.
Low-level turbulence can range from unpleasant to dangerous. While difficult to predict, the intensity of rotor turbulence
increases with the amplitude of lee waves and can vary based on location and conditions. At times, rotor turbulence is
uniformly rough everywhere below the smooth wave flow. At other times, turbulence becomes severe under wave crests.
On occasion, moderate or severe turbulence exists only within a small-scale rotor under the wave crest. Typically, the worst
turbulence occurs on the leading edge of the primary rotor.
Figure 9-22 above indicates cloud types associated with a mountain wave system. A cap cloud flowing over the mountain
tends to dissipate as the air forced down the mountain slope warms and dries. The first (or primary) wave crest features
a roll or rotor cloud with one or more lenticulars forming above. Wave harmonics further downstream may also create
lenticulars or rotor clouds. If the wave reaches high enough altitudes, lenticulars may also form at cirrus levels.
The entire mountain wave system can form in completely dry conditions without clouds, and the presence of clouds
depends on the amount of moisture at various levels. If only lower-level moisture exists, only a cap cloud and rotor clouds
might occur with no lenticulars above, as in Figure 9-23 (panel A). On other days, only mid-level or upper-level lenticulars
appear with no rotor clouds beneath them. When low- and mid-levels contain enough moisture, a deep rotor cloud may
form, with lenticulars right on top of the rotor cloud, with no clear air between the two cloud types.
Figure 9-23. Cloud cover associated with variations in moisture.
In wet climates, the moist air moving horizontally, can completely close the gap between the cap cloud and primary rotor.
This closure could strand a glider on top of the clouds [Figure 9-23 (Panel B)]. Pilots should consider this possibility when
above rotor clouds in moist conditions.
Wave amplitude depends partly on topography. Relatively low ridges of 1,000 feet or less vertical relief can produce
lee waves, and uniform height of the mountaintops along a range produces better organized waves. The shape of the lee
slope also affects wave production. Very shallow lee slopes can produce waves of sufficient amplitude to support a glider.
Different wind and stability profiles favor different topography profiles, and one mountain height, width, and lee slope
may produce usable waves only in certain weather conditions. Hence, experience with a particular soaring site can assist
pilot prediction of wave conditions.
The weather requirements for wave soaring include sufficient wind and a proper stability profile. Windspeed should be
at least 15 to 20 knots at mountaintop level with increasing winds above. The wind direction should be within about 30°
perpendicular to the ridge or mountain range. Air stability at the DALR near the mountaintop would not likely produce
lee waves even with adequate winds. A well-defined inversion at or near the mountaintop with less stable air above would
increase the likelihood of wave production.
Weak lee waves may form without much increase in windspeed as altitude increases, but an actual decrease in windspeed
with height usually caps the wave at that level. When winds decrease dramatically with height, for instance, from 30
to 10 knots over two or three thousand feet, turbulence is common at the top of the wave. On some occasions, the flow
at mountain level may be sufficient for wave flight, but then begins to decrease with altitude just above the mountain,
leading to a phenomenon called “rotor streaming.” In this case, the air downstream of the mountain breaks up and becomes
turbulent, with no lee waves above.
Lee waves experience diurnal effects, especially in the spring, summer, and fall. Height of the topography also influences
diurnal effects. For smaller topography, as morning leads to afternoon and the air becomes unstable to heights above the
wave-producing topography, lee waves tend to disappear. On occasion, the lee wave still exists but the pilot needs more
height to reach the smooth wave lift. Toward evening as thermals again die down and the air stabilizes, lee waves may
again form. During the cooler season, when the air remains stable all day, lee waves are often present all day, provided the
winds aloft continue. Large-mountain dissipation of lee waves during daytime appears less significant. For instance, during
the 1950s Sierra Wave Project (searchable online), the wave amplitude reached a maximum in mid to late afternoon, with
convective heating at a maximum. Rotor turbulence also increased dramatically at that time.
Topography upwind of the wave-producing range can also affect propagation, as illustrated in Figure 9-24. In the first
case [Figure 9-24 Panel A], referred to as destructive interference, the wavelength of the wave from the first range is out
of phase with the distance between the ranges. Lee waves do not form downwind of the second range, despite favorable
winds and stability aloft. In the second case [Figure 9-24 (Panel B)], referred to as constructive interference, the ranges are
in phase, and the lee wave from the second range has a larger amplitude than it might otherwise.