Introduction
Glider pilots face a multitude of decisions, starting with the decision to take to the air. Pilots must determine if weather
conditions are safe and if current conditions support a soaring flight. Gliders, being powered by gravity, are always sinking
through the air. Therefore, glider pilots must seek air that rises faster than the sink rate of the glider to enable prolonged
flight. Glider pilots refer to rising air as lift, not to be confused with the lift created by the wing. This chapter focuses
on weather that commonly affects glider flights. However, all pilots should understand weather theory, weather hazards
to aviation, and the technical details of aviation weather products. The Aviation Weather Handbook (FAA-H-8083-28)
provides comprehensive information.
The Atmosphere
Without the atmosphere, wind, clouds, precipitation, and protection from solar radiation would not exist. The height of the
atmosphere represents a small distance when compared to the 3,438 nautical mile radius of the earth. There is no specific
upper limit to the atmosphere—it simply thins to a point where it fades away into space. The atmosphere below about 27
NM (164,000 feet) contains 99.9 percent of atmospheric mass. At that altitude, atmospheric density drops to approximately
one-thousandth of its value at sea level. [Figure 9-1]
Altitude (feet)
53,000
18,000
Sea level
164,000 99.9% of
mass below
this level
90% of
mass below
this level
50% of
mass below
this level
Figure 9-1. Atmospheric mass by altitude.
Chapter 9: Glider Flight & Weather
Composition
Two gases, nitrogen (N~2) and oxygen (O~2), comprise 99 percent of the volume of the total atmosphere on average.
The remaining volume contains various trace gases and small amounts of water, ice, dust, and other particles. While the
proportion of nitrogen to oxygen remains the same to approximately 260,000 feet, the amount of water vapor (H~2O) in
the air can vary. For example, water vapor content above tropical areas and oceans accounts for up to 4 percent of the
atmosphere by volume and displaces some nitrogen and oxygen gas. Conversely, the water vapor in the atmosphere over
deserts and at high altitudes consists of much less than 1 percent of the total volume. [Figure 9-2]
260,000 ft
1% other gases 78%
21%Nitrogen
Oxygen
Atmosphere
Figure 9-2. The composition of the atmosphere.
Although water vapor exists in the atmosphere in small amounts as compared to nitrogen and oxygen, it has a significant
impact on weather. The additional physical states of water vapor as a liquid and solid contribute to the formation of clouds,
precipitation, fog, and ice.
Atmospheric Measurements
Temperature, density, and pressure measurements provide information about the atmosphere. These variables change over
time, and combined with vertical and horizontal differences, the measurements and trends comprise data for weather
reports and forecasts.
Temperature
People often describe air temperature in terms of whether the air feels hot or cold. In aviation, quantitative measurements
use the Celsius (°C) scale or the Fahrenheit (°F) scale. Temperature of the atmosphere depends on the average kinetic
energy of molecules. Fast-moving molecules have high kinetic energy and higher temperatures. Conversely, slow-moving
molecules have lower kinetic energy and lower temperatures.
Density
The density of any substance gives its mass per unit of volume. Low air density means a smaller number of air molecules
(or less massive molecules on average) in a specified volume, while high air density means a greater number of air
molecules (or more massive molecules on average) in a specified volume.
Pressure
Molecules in a volume of air not only possess a certain temperature and density, but they also collide with other gas
molecules and push on nearby objects. The collisions result in measurable pressure or a force per unit of area. Since
the force created by moving gas molecules acts equally in all directions, localized measurements of gas pressure using
calibrated equipment should equal each other. Common units of measure for pressure include pounds per square inch (lb/
in*2), inches of mercury ("Hg), and the equivalent units of millibars (mb) or hectopascals (hPA).
Ideal Gas Law
How do temperature, pressure, and density relate to each other? Dry air behaves almost like an ideal gas, meaning it obeys
the ideal gas law P/DT = R, where P is pressure, D is density, T is temperature, and R is a constant. In general, the density,
pressure, and temperature of an air parcel change predictably in accordance with the variables in the formula.
Standard Atmosphere
To provide a common reference used for temperature and pressure, scientists established the International Standard
Atmosphere (ISA). This standard atmosphere uses a representative vertical distribution of temperature and pressure
variables for pressure altimeter calibrations. The standard conditions are also a starting point for most aircraft performance
data. At sea level, the standard atmosphere consists of a barometric pressure of 29.92 "Hg, 1,013.2 mb, or 14.7 lb/in*2,
and a temperature of 15 °C or 59 °F.
Since temperature normally decreases with altitude at a predictable rate, a standard lapse rate calculation gives the standard
temperature at various altitudes. Below 36,000 feet, the standard temperature lapse rate is 2 °C (3.5 °F) per 1,000 feet of
altitude change. Pressure does not decrease linearly with altitude, but a 1 "Hg decrease for each 1,000 feet of increased
altitude approximates the rate of pressure change below 10,000 feet. Pilots can use the standard lapse rates for flight
planning purposes with the understanding that variations from standard conditions exist in the atmosphere. [Figure 9-3]
50,000
40,000
30,000
20,000
10,000
Sea level
−55 −35 15 0 +15
Altitude (feet)
Temperature (°C)
Figure 9-3. Standard atmosphere temperatures.
Layers of the Atmosphere
Scientists divide earth’s atmosphere into five layers: troposphere, stratosphere, mesosphere, thermosphere, and exosphere.
[Figure 9-4] The rate of change in temperature as altitude increases defines these layers. The lowest layer, called the
troposphere, exhibits an average decrease in temperature from the earth’s surface to about 36,000 feet above mean sea
level (MSL). The troposphere extends to a higher altitude over the tropics and a lower altitude over the polar regions. It
also varies seasonally, being higher during the summer and lower during the winter.
Troposphere
Stratosphere
Mesosphere
Thermosphere
Exosphere
Ozone layer
Tropopause
Figure 9-4. Layers of the atmosphere.
Almost all of earth’s weather occurs in the troposphere as most of the water vapor and clouds are found in this layer. The
lower part of the troposphere interacts with the land and sea surface, providing thermals, mountain waves, and sea-breeze
fronts. Although temperatures decrease as altitudes increase in the troposphere, local areas where temperature increases
with altitude (inversions) commonly occur.
The top of the troposphere or tropopause has a pressure of about ten percent of MSL pressure (0.1 atmosphere) and density
drops to about 25 percent of its sea-level value. Temperature reaches its minimum value at the tropopause, approximately
–55 °C (–67 °F). For pilots, this is an important part of the atmosphere because it is associated with a variety of weather
phenomena, such as thunderstorm tops, clear air turbulence, and jet streams. The vertical limit of the tropopause varies
with the height of the troposphere.
The tropopause separates the troposphere from the stratosphere. With increasing height in the stratosphere, the temperature
tends to change very slowly at first. However, as altitude increases the temperature increases to approximately 0 °C (32
°F) reaching its maximum value at about 160,000 feet MSL. Unlike the troposphere in which the air moves freely both
vertically and horizontally, the air within the stratosphere generally moves horizontally.
Gliders have reached into the lower stratosphere using mountain waves. At high altitudes, supplemental oxygen requirements
become mandatory. Layers above the stratosphere have some interesting features that are normally not of importance to
glider pilots. However, interested pilots may refer to any general text on weather or meteorology.
Scale of Weather Events
When preparing forecasts, meteorologists consider atmospheric circulation on different scales. To aid the forecasting
of short- and long-term weather, various weather events have been organized into three broad categories or scales of
circulation. The size and lifespan of the phenomena in each scale are roughly proportional, so that larger scales coincide
with longer lifetimes. The term “microscale” refers to features with spatial dimensions of 0.1 to 1 NM, which last for
seconds to minutes. An example is an individual thermal. The term “mesoscale” refers to the horizontal dimensions of 1 to
1,000 NM, which last minutes to weeks. Examples include mountain waves, sea breeze fronts, thunderstorms, and fronts.
Research scientists break down the mesoscale into further subdivisions to better classify various phenomena. The term
“macroscale” refers to the horizontal dimensions greater than 1,000 NM, which last weeks to months. These include the
long waves in the general global circulation and the jet streams embedded within those waves. [Figure 9-5]
10,000
1,000
0.1
General circulation,
monsoon circulation
Jet stream
Occluded front,
hurricane
Land/sea breeze,
lee wave
Thunderstorm,
downburst
Tornado, dust devil,
thermal, turbulence
second minute hour day week month year
Space scale (NM)
Time scale (lifetime)
Standard Atmosphere
A A
E F
Microscale
Macroscale
Mesoscale
Figure 9-5. Scale of circulation—horizontal dimensions and life spans of associated weather events.
Smaller scale features are embedded in larger scale features. For instance, a microscale thermal may be just one of many
thermals in a mesoscale convergence line, like a sea breeze front.
The sea breeze front may occur only under certain synoptic (i.e., simultaneous) conditions controlled by the macroscale
circulations. The scales interact, with feedback from smaller to larger scales and vice versa, in ways not fully understood
by atmospheric scientists. Generally, the behavior and evolution of macroscale features are more predictable, with forecast
accuracy decreasing as scale diminishes. For instance, forecasts of up to a few days for major events, such as a trough with
an associated cold front, have become increasingly accurate. However, no one can forecast the exact time and location
of an individual thermal an hour ahead of time. Since most of the features of interest to soaring pilots lie in the smaller
mesoscale and microscale ranges, prediction of gliding weather presents a significant challenge.
Pilots interpreting forecasts should begin with the macroscale, which identifies large-scale patterns that may produce good
gliding conditions. This varies from site to site and depends on whether the goal is thermal, ridge, or wave soaring. Then,
mesoscale features should be considered. This may include items such as the cloudiness and temperature structure of the
air mass behind a cold front, as well as the amount of rain produced by the front. Developing an understanding of lift types
and environments in which they form, can help a pilot predict local weather conditions that affect glider flights.
Thermals
Thermals are the most common form of rising air used to sustain glider flight. The paragraphs in this section explore topics
related to thermals, including thermal structure, atmospheric stability, and air masses conducive to flight in thermals.
Thermal Shape & Structure
Convection describes a form of heat transfer involving the movement or flow of mass in a fluid (gas or liquid). Rising
convection currents of air or “thermals” are one means by which the atmosphere transfers heat energy vertically. Thermals
do not necessarily develop on a warm sunny day. Subtle differences in the atmosphere make the difference between a
warm, sunny day with plenty of thermals and a warm, sunny day that produces no thermals. Glider pilots who understand
the conditions that create thermals can use their own forecasting skills to predict thermal activity.
Two conceptual models exist for the structure of thermals: the bubble model and the column or plume model. These models
simplify a complex and often turbulent phenomenon, so pilots should expect many exceptions and variations while flying
in thermals. Many books, articles, and Internet resources provide further reading on this subject.
The bubble model describes an individual thermal resembling a vortex ring, with rising air in the middle and descending air
on the sides. The air in the middle of the vortex ring rises faster than the entire thermal bubble. The model fits occasional
reports from glider pilots. At times, two glider pilots in the same thermal will find different amounts of lift. For example,
one glider may be at the top of the bubble climbing only slowly, while a lower glider climbs rapidly in the stronger part
of the bubble below. [ Figure 9-6] Often, a glider flying below another glider circling in a thermal can contact the same
thermal and climb, even if the gliders are displaced vertically by 1,000 feet or more. This suggests the column or plume
model of thermals is more common. [Figure 9-7]
Figure 9-6. The bubble or vortex ring model of a thermal.
Figure 9-7. The column or plume model of a thermal.
The applicability of the models may depend on the supply of warm air near the surface. Within a small, heated area, one
single bubble may rise and take all the warmed surface air with it. On the other hand, if a large area becomes warm and one
spot acts as the initial trigger, surrounding warm air can flow into the relative void left by the initial thermal. The in-rushing
warm air follows the same path, creating a thermal column or plume. Since all the warmed air near the surface does not
usually have the exact same temperature, a column could exist with a few or several imbedded bubbles. Individual bubbles
within a thermal plume may merge, while at other times, two adjacent and distinct bubbles may exist side by side.