Whether behaving as a bubble or column, the air in the middle of the thermal rises faster than the air near the edges of the
thermal. A horizontal slice through an idealized thermal provides a bull’s-eye pattern; however, cross sections of real-world
thermals exhibit dissymmetry. [Figure 9-8]
Figure 9-8. Cross-section through a thermal. Darker green is stronger lift; red is sink.
A typical thermal cross-section has a diameter of 500–1,000 feet, though the size can vary considerably. Typically, due
to mixing with the surrounding air, thermals expand as they rise. Thus, the thermal column may resemble a cone, with
the narrowest part near the ground. Thermal plumes also tilt in a steady wind and can distort in the presence of vertical
shear. In strong vertical shear, thermals can become very turbulent or become completely broken apart. Figure 9-9 shows
a schematic of a thermal lifecycle in windshear.
WIND
Figure 9-9. Lifecycle of a typical thermal with cumulus cloud.
A stable atmosphere hinders vertical motion, while an unstable atmosphere promotes vertical motion. A certain amount
of atmospheric instability supports development of thermals. However, moist air and strong atmospheric instability may
lead to thunderstorm formation. Thus, an understanding of atmospheric stability promotes recognition of favorable flight
conditions as well as recognition of weather hazards and associated risk.
When discussing atmospheric stability, a layer of air in the atmosphere represents the dynamic system and a parcel of
air represents the displaced element. In a stable dynamic system, a displaced element returns to its original position. In
an unstable dynamic system, a displaced element continues to move farther away from its original position. In a neutral
dynamic system, a displaced element neither returns to nor moves farther away from its original position.
A parcel of dry or unsaturated air moving upward in the atmosphere expands and cools as it rises due to decreasing
pressure. By contrast, a descending parcel of dry or unsaturated air compresses and warms due to increasing pressure.
When no transfer of heat between the displaced parcel and the surrounding ambient air occurs, the process is called
adiabatic. During this adiabatic process, a rising unsaturated parcel cools at a lapse rate of 3 °C (5.4 °F) per 1,000 feet. This
dry adiabatic lapse rate (DALR) approximates what happens in nature although some mixing of air occurs as thermals rise.
Figure 9-10 below demonstrates one means to predict whether a layer of the atmosphere will function like a stable, unstable,
or neutral dynamic system. Panels A and B represent two scenarios with the same temperature of 20 °C at the surface, but
with different air layer temperatures at 3,000 feet above ground level (AGL). Using the DALR for both scenarios, a parcel
of 20 °C air that lifts from the surface cools to 11 °C by the time it reaches 3,000 feet AGL. In scenario A, the lifted parcel is
still warmer than the surrounding air and will continue to rise by convection—a condition of instability that could produce
a good thermal. In scenario B, the lifted parcel at 3,000 feet AGL has cooled to a lower temperature than the surrounding
air and will descend. In this case, the layer exhibits system stability. See the Aviation Weather Handbook (FAA-H-8083-28)
for more information on atmospheric stability.
9 °C 13 °C3,000 ft 3,000 ft
11 °C 11 °C
20 °C
20 °C
Figure 9-10. Unstable (A) and stable (B) air.
Changing the values in Figure 9-10 illustrates factors that would affect atmospheric stability. A stable layer can turn
unstable in one of two ways. In scenario B, if the surface temperature warms by more than 2 °C (to greater than 22 °C), or
if the air at 3,000 feet cools by more than 2 °C (to less than 11 °C), the atmospheric layer to 3,000 feet becomes unstable.
Warming of lower layers or cooling of higher layers of the atmosphere with no other changes reduces stability and leads to
a better environment for thermals. If the layer aloft and at the surface warm or cool by the same amount, then the stability
of the layer remains unchanged. The layer exhibits greater stability if the air temperature aloft remains constant, but the
surface air cools.
An inversion occurs when the troposphere warms as altitude increases. Inversions can occur at different altitudes and vary
in strength. In strong inversions, the temperature can rise as much as 10 °C in a few hundred feet of altitude gain. Along
with trapping haze or pollution below, an inversion also effectively caps any thermal activity.
Although moisture in the form of water vapor makes up a small percentage of the atmosphere, it can affect the temperature
lapse rate of a rising parcel of air. A rising parcel of air cools at the DALR until it reaches its dewpoint, at which time the
water vapor in the parcel begins to condense. The condensation process releases heat, referred to as latent heat, within the
rising parcel of air. Therefore, a rising parcel of saturated air cools at a rate lower than the DALR. This saturated adiabatic
lapse rate (SALR), varies substantially with altitude. At lower altitudes, it approximates of 1.2 °C per 1,000 feet, whereas
at middle altitudes it increases to 2.2 °C per 1,000 feet. Above approximately 30,000 feet, little water vapor exists to
condense, and the SALR approaches the DALR.
Air Masses Conducive to Thermal Soaring
Generally, the best air masses for thermals are those with cool air aloft, with conditions dry enough to allow the sun’s
heating radiation to warm the surface and to limit extensive formation of cumulus clouds. This cool air aloft can originate
after passage of a Pacific cold front in the Western United States or from polar continental regions such as interior Canada
in the Eastern United States. In both cases, high pressure building into the region often includes an inversion aloft, which
keeps cumulus from growing into rain showers or thunderstorms. However, as the high pressure builds after the second or
third day, the inversion often lowers to the point that thermals suitable for gliding no longer form. This can lead to warm
and sunny, but very stable conditions. Fronts that arrive too close together can also cause poor postfrontal soaring, as high
clouds from the next front keep the surface from warming enough. Very shallow cold fronts from the northeast direction
(with cold air only one- or two- thousand feet deep) often have a stabilizing effect along the plains directly east of the
Rocky Mountains. This is due to cool low-level air undercutting warmer air aloft flowing from the west.
In the desert southwest, the Great Basin, and intermountain west, good summertime thermals often result from intense
heating from below, even in the absence of cooling aloft. This dry air mass with continental origins produces cumulus bases
10,000 feet AGL or higher. At times, this air spreads into eastern New Mexico and western Texas. Later in the summer,
however, some of these regions come under the influence of the North American Monsoon, which can lead to widespread
and daily late morning or early afternoon thundershowers. [Figure 9-11]
L H
COLORADO
NEW MEXICO
TEXAS
MEXICO
Figure 9-11. Typical North American monsoon flow.
Cloud Streets
Cumulus clouds often appear randomly distributed across the sky, especially over relatively flat terrain. Under the right
conditions, however, cumulus clouds can align in a long band, called a cloud street. An individual cloud street can extend
50 miles or more while an entire field of cloud streets can extend hundreds of miles. The spacing between streets is
typically three times the height of the clouds. Cloud streets align parallel to the wind direction and indicate the pattern of
rising and descending air. Glider pilots can often fly many miles with little or no circling, sometimes achieving glide ratios
far exceeding the still-air value by flying near and parallel to the clouds while avoiding the space between the clouds. Thus,
cloud streets mark an ideal location for flying a downwind cross-country flight.
A cross-section of an idealized cloud street formation illustrates a distinct circulation, with updrafts under the clouds and
downdrafts in between. [Figure 9-12] Due to the circulation, sink between streets may be stronger than typically found the
same distance away from random cumulus clouds.
3H
Inversion
Figure 9-12. Circulation across a cloud street.
Cloud streets usually occur over land with cold air outbreaks, for instance, following a cold front. Brisk surface winds and
a wind direction remaining nearly constant up to the cloud base are favorable cloud street conditions. Windspeed should
increase by 10 to 20 knots between the surface and cloud base, with a maximum somewhere in the middle of or near the
top of the convective layer. Thermals should be capped by a notable inversion or stable layer.
Thermal streets, with a circulation like Figure 9-12, may exist without cumulus clouds. Without clouds as markers, use of
such streets becomes difficult. A glider pilot flying upwind or downwind in consistent sink should alter course crosswind
to avoid inadvertently flying along a line of sink between thermal streets that may exist.
Cloud Streets
Figure 9-13 shows a wavelike form for the inversion capping the cumulus clouds. If winds above the inversion run
perpendicular to the cloud streets and increase at 10 knots per 5,000 feet or more, cloud street waves can form in the stable
air above. Though usually relatively weak, thermal waves can produce lift of 100 to 500 fpm and allow smooth flight along
streets above the cloud base.
Lower wind
Lower wind
Cloud street
Cloud street
Upper wind
Upper wind
Figure 9-13. Cloud street thermal wave.
So-called cumulus waves may exist where the cumulus clouds do not organize in streets. Cumulus waves require a capping
inversion or stable layer and increasing wind above cumulus clouds. However, directional shear is not necessary. Cumulus
waves may also be short lived, and difficult to work for any length of time. An exception occurs when the cumulus
cloud anchors to some feature, such as a ridge line or short mountain range. As a final note, thermal waves can also form
without clouds. Without clouds, the possible influence of a ridge or mountain in creating the wave lift becomes difficult
to determine.
Thunderstorms
Forecasters sometimes use the term “deep convection” to refer to convection that rises to high levels, which usually
means thunderstorms, and they use the term “convective activity” to refer to thunderstorms. The tremendous amount
of energy associated with cumulonimbus clouds stems from the release of latent heat as condensation occurs within the
growing cloud. While an unstable atmosphere can provide great conditions for thermal formation, an atmosphere that is
moist and unstable can create cumulonimbus (Cb) or thunderclouds. Cb clouds are the recognized standard marker of
thunderstorms. When Cb builds sufficiently, it changes from rainstorm to thunderstorm status. Not all precipitating, large
cumulus formations are accompanied by lightning and thunder, but the presence of these clouds indicates that hazardous
conditions exist or may intensify.
Thunderstorms can occur any time of year, though they are more common during the spring and summer seasons. They can
occur anywhere in the continental United States but are not common along the immediate West Coast, where an average of
only about one per year occurs over a given location. During the summer months, the desert southwest locations, extending
northeastward into the Rocky Mountains and adjacent Great Plains, experience an average of 30 to 40 thunderstorms
annually. Additionally, in the southeastern United States, especially Florida, between 30 and 50 thunderstorms occur in an
average year per location. [Figure 9-14] Thunderstorms in the cool seasons usually occur in conjunction with some forcing
mechanism, such as a fast- moving cold front or a strong upper-level trough.
Thunderstorms
Summer (June–August)
40 50
50 40
Figure 9-14. Thunderstorm frequency in the summertime.
The lifecycle of an airmass or ordinary thunderstorm consists of three main stages: cumulus, mature, and dissipating. The
term “ordinary” describes the type of thunderstorm consisting of a single Cb, since individual thunderstorms can develop
in a uniform large-scale air mass. The entire lifecycle of an ordinary thunderstorm takes on the order of an hour, though a
remnant cloud from the dissipated Cb can last substantially longer.
The cumulus stage of a thunderstorm is characterized by a cumulus cloud growing to a towering cumulus (Tcu). As air
rises within the cloud during this stage, the intensity of the updraft increases, and the cloud base broadens to a few miles
in diameter. [Figure 9-15] As the cloud increases in size, the strong updraft in the middle of the cloud does not entrain
or carry along dryer air surrounding the cloud, and general downward motion of air around the Tcu may suppress other
smaller cumulus in the vicinity. Toward the end of the cumulus stage, downdrafts and precipitation begin to form within
the cloud. On some days, small cumulus can be around for hours, before Tcu form, while on other days, unstable air allows
any cumulus cloud that forms to rapidly transform into a Tcu.
Figure 9-15. A cumulus cloud becoming a towering cumulus.
As the development of a thunderstorm continues, it reaches the mature stage. By this time, downdrafts known as downbursts
or microbursts reach the ground and spread out creating strong and sometimes damaging surface winds. Glider pilots
should avoid flight toward these downdrafts and their associated windshear, as the glider might lose the capability to reach
a selected landing area.
Note: hazardous windshear creates a significant risk for any ground launch or aerotow, and these operations should not
occur if the pilot suspects a windshear encounter associated with Cb may occur. Depending on the size of windshear, the
towplane could be in a tailwind while the glider is in a headwind.
Pilots need to watch dissipating thunderstorms closely for new dark, firm bases that indicate formation of a new cell. In
addition, outflow from a Cb may cause the air it encounters to rise. The relatively cool air in the outflow can provide nearby
air with a boost, leading to formation of a nearby Cb, not connected to the original Cb.
The risk from thunderstorms involves several hazards, including turbulence, strong updrafts and downdrafts, strong
shifting surface winds, hail, icing, poor visibility or low ceilings, lightning, and even tornadoes. Once a cloud has grown
into Cb, these hazards may develop, with or without obvious signs. Since thermal soaring weather can rapidly deteriorate
into thunderstorm weather, understanding the risk associated with these hazards should prompt a glider pilot to remain on
the ground or avoid them in the air. The following paragraphs provide more information about these hazards.
Turbulence
A pilot should never intentionally fly into a Cb since severe or extreme turbulence leading to structural failure can occur
anywhere within the thunderstorm. Violent updrafts can be followed a second or two later by violent downdrafts, with
occasional side gusts. Severe turbulence commonly occurs close to the storm, and moderate turbulence may exist within
several miles of a thunderstorm. Below the base of the Cb, moderate to severe turbulence can occur along the boundary
between the cool outflow and warm air feeding the Cb. Pilots should expect turbulence near the surface from a gust front as
cool outflow spreads from the storm. Unpredictable smaller scale turbulent gusts can occur anywhere near a thunderstorm
and avoiding the gust front does not guarantee avoidance of severe turbulence.
Updrafts and Downdrafts
Large and strong updrafts and downdrafts accompany thunderstorms in the mature stage. Updrafts feeding the Cb from
under the base can exceed 1,000 fpm. Near the cloud base, a pilot may have difficulty determining the distance to the edge
of the cloud, and strong updrafts could suck a glider into the storm cloud. During the late cumulus and early mature stage,
updrafts feeding the cloud can cover many square miles. As the storm enters its mature stage, downbursts or microbursts
can occur even without very heavy precipitation present. Downbursts can also cover many square miles with descending
air of 2,000 fpm or more. A pilot flying under a forming downburst, which may not be visible, could encounter sink of
3,000 fpm or greater in extreme cases. Such a downburst encountered at pattern altitude can cut the normal time available
to the pilot for executing an approach. While a normal pattern from 800 feet AGL to the ground could span 3 minutes,
contact with the ground occurs in 19 seconds in 2,500 fpm sink.
Outflow Winds
When a downburst or microburst hits the ground, the downdraft spreads out, leading to strong surface winds, known as
thunderstorm outflow. Typically, the winds strike quickly and give little warning of their approach. While flying, pilots
should keep a sharp lookout between any storm and the intended landing spot for signs of a wind shift. Blowing dust, smoke,
or wind streaks on a lake caused by wind from the storm may indicate a rapidly approaching gust front. Thunderstorm
outflow winds usually travel at speeds of 20 to 40 knots for a period of 5 to 10 minutes before diminishing. However,
winds can easily exceed 60 knots, and in some cases, with a slow- moving thunderstorm, strong winds can last substantially
longer. Although damaging outflow winds usually do not extend more than 5 or 10 miles from the Cb, winds of 20 or 30
knots can extend 50 miles or more from large thunderstorms.