Introduction
Weather is the state of the atmosphere at a given time and place, with respect to temperature, moisture content, stability,
visibility, and cloudiness. These factors interact to form the following five major meteorological elements:
• Atmospheric pressure (high or low).
• Wind (calm or storm).
• Clouds (clearness or cloudiness).
• Precipitation (rain, sleet, snow).
A solid understanding of weather theory provides the tools necessary to understand the reports and forecasts obtained from
a Flight Service Station (FSS) weather specialist and other aviation weather services. No other means of aviation relies
more heavily on knowledge and understanding of weather for its safety than ballooning. It is important to note, however,
that there is no substitute for experience.
There are many excellent texts and online sources available for learning more about weather that are referenced at the
appropriate point in this chapter. Much of the following information can be found in the Aviation Weather Handbook
FAA-H-8083-28, which may be found on the FAA’s website.
Other online sources for weather information are also helpful. The National Oceanic and Atmospheric Administration
(NOAA) offers a weather tutorial. Developed to meet the needs of educators, weather professionals, and others interested
in learning more about weather, it provides a number of concise explanations of weather theory. Additionally, there is a
site developed as part of the Weather World 2010 project by the Department of Atmospheric Sciences at the University of
Illinois at Urbana-Champaign.
This chapter is designed to give balloon pilots a basic knowledge of weather principles, acquaint them with the weather
information available for flight planning, and help them develop sound decision-making skills as they prepare for and
execute a safe flight.
The Atmosphere
The atmosphere is the envelope of air that surrounds the Earth. Approximately one-half of the air, by weight, is within the
lower 18,000 feet. The remainder of the air is spread over a vertical distance in excess of 1,000 miles. No definite outer
atmospheric boundary exists, but the air particles become less numerous with increasing altitude until they gradually
overcome Earth’s gravity and escape into space. In addition to the rotation of the air with the rotation of the Earth, another
type of air movement occurs within the atmosphere. This movement of air around the surface of the Earth is called
atmospheric circulation.
Composition
The atmosphere is a blanket of air composed of a mixture of gases that surrounds the Earth and reaches over 560 kilometers
(km), 348 miles, from the surface. This blanket of gases provides protection from ultraviolet rays, as well as supporting
human, animal, and plant life. Nitrogen accounts for 78 percent of the gases comprising the atmosphere, while oxygen
makes up 21 percent. [ Figure 4-1] Argon, carbon dioxide, and traces of other gases make up the remaining 1 percent.
Within this envelope of gases, there are several recognizable layers of the atmosphere as defined by altitude.
Chapter 4: Weather Theory & Reports
1%21%
78%
Trace gases
Oxygen
Nitrogen
Figure 4-1. Composition of the atmosphere.
The first layer, closest layer to the surface, known as the troposphere, extends from sea level up to 20,000 feet (8 km) over
the northern and southern poles and up to 48,000 feet (14.5 km) over the equatorial regions. The vast majority of weather,
clouds, storms, and temperature variances occur within this first layer of the atmosphere.
The Standard Atmosphere
To provide a common reference when discussing weather, the International Standard Atmosphere (ISA) has been
established. To arrive at the standard atmosphere, conditions throughout the atmosphere with respect to latitudes, seasons,
and altitudes were averaged. The standard reference point is 59 °F or 15 °C, and 29.92 inches of mercury ("Hg) or 1013.2
millibars (mb). Pressure does not decrease linearly with altitude, but for the first 10,000 feet, 1 "Hg for each 1,000 feet
approximates the rate of pressure change. There is also a standard temperature lapse rate of 3.5 °F or 2 °C per 1,000 feet
of altitude, up to 36,000 feet.
At sea level, the atmosphere exerts pressure on the Earth at a force of 14.7 pounds per square inch (psi). This means a
column of air one inch square, extending from the surface up to the upper atmospheric limit, weighs about 14.7 pounds.
A person standing at sea level also experiences the pressure of the atmosphere, but the pressure is a force of pressure over
the entire surface of the skin. The actual pressure at a given place and time will differ with altitude, temperature, and density
of the air. These conditions also affect balloon performance, especially with regard to useful load and burner performance.
Measurement of Atmospheric Pressure
Measurement of Atmospheric Pressure Constant pressure charts and hurricane pressure reports are written using millibars
(mb). Since weather stations are located around the globe, all local barometric pressure readings are converted to a sea
level pressure to provide a standard for records and reports. To achieve this, each station converts its barometric pressure
by adding approximately 1 "Hg for every 1,000 feet of elevation gain. For example, a station at 5,000 feet above sea
level, with a reading of 24.92 "Hg, reports a sea level pressure reading of 29.92 "Hg. Using common sea level pressure
readings helps ensure aircraft altimeters are set correctly, based on the current pressure readings. In order to compensate
for pressure variations due to different station elevations, all observations are mathematically corrected to mean sea level
(MSL). Altimeter settings are obtained by mathematically reducing station pressure to MSL. This enables the pilot to read
MSL altitudes on the altimeter.
When charting atmospheric pressures over various areas of the Earth, the meteorologist is primarily interested in the
pressure difference per unit of horizontal distance—the pressure gradient.
The MSL pressure is plotted in mb at each reporting station on a surface weather map. The isobars outline pressure areas
in somewhat the same manner that contour lines outline terrain features on contour maps. The standard procedure on
surface weather maps in North America is to draw isobars at four mb intervals, with intermediate, two mb spacing when
appropriate. Although the isobar patterns are never the same on any two weather maps, they do show patterns of similarity.
By tracking barometric pressure trends across a large area, weather forecasters can more accurately predict movement
of pressure systems and the associated weather. For example, tracking a pattern of rising pressure at a single weather
station generally indicates the approach of fair weather. Conversely, decreasing or rapidly falling pressure usually indicates
approaching bad weather and possible severe storms.
Temperature
Temperature is a measurement of the amount of heat and expresses a degree of molecular activity. Since different substances
have different molecular structures, equal amounts of heat applied to equal volumes of two different substances will result
in unequal heating. Every substance has its own unique specific heat. For example, a land surface becomes hotter than
a water surface when equal amounts of heat are added to each. The degree of “hotness” or “coldness” of a substance is
known as its temperature, and is measured with a thermometer.
The Earth’s surface is heated during the day by the sun. This incoming solar radiation is called insolation, while heat
radiated from the Earth by outgoing radiation is called terrestrial radiation. The cooling that occurs at night is terrestrial
radiation.
Temperature Scales
Two temperature scales are important to the balloon pilot: Fahrenheit (F) and Celsius (C). On the Fahrenheit scale, the
freezing point is 32° and the boiling point is 212°, a difference of 180°. On the Celsius scale, the freezing point is 0°
and the boiling point is 100°. For many years, the Celsius scale was the choice for technicians and those countries and
organizations utilizing the metric system. In recent years, the United States has transitioned to almost exclusive use of the
Celsius scale in weather reports, primarily because of the International Civil Aviation Organization (ICAO) convention
agreements. [Figure 4-2]
Water boils
Water freezes
Dry ice (solid CO2)
Liquid air
Absolute zero
Fahrenheit
Celsius
212 °F
98.6 °F
32 °F
–40 °F
–108 °F
–312 °F
–459 °F
100 °C
50 °C
0 °C
–40 °C
–78 °C
–200 °C
–273 °C
Figure 4-2. Comparison of Fahrenheit and Celsius temperature scales.
A quick and easy way to convert Fahrenheit to Celsius is to subtract 30, and divide the number by two. To convert
Celsius to Fahrenheit, double the number, and add 30. These formulas give a good approximation for most calculations in
ballooning. Conversion charts are also available on the Internet.
Temperature Variations
The amount of solar radiation (insolation) received by any region varies with the time of day, with seasons, and with
latitude. These differences in insolation and changes in temperature of various air masses create temperature variations.
Temperatures also vary with differences in topographical surface and with altitude. These temperature variations create
forces that drive the atmosphere in its motion. Simply stated, heat and, therefore, temperature differences cause weather.
Diurnal variation is the change in temperature from day to night brought about by the daily rotation of the Earth. The
Earth receives heat during the day by insolation, but continually loses heat by terrestrial radiation. Warming and cooling
depend on an imbalance of insolation and terrestrial radiation. During the day, insolation exceeds terrestrial radiation
and the surface becomes warmer. At night, insolation ceases, but terrestrial radiation continues and cools the surface.
Cooling continues after sunrise until insolation again exceeds terrestrial radiation. Minimum temperature usually occurs
after sunrise, sometimes as much as one hour after. The continued cooling after sunrise is one reason that fog sometimes
forms shortly after the sun is above the horizon.
Temperature Variations with Topography
Temperature variations are also induced by water and terrain. Water absorbs and radiates heat energy with less temperature
change than does land. Large, deep water bodies tend to minimize temperature changes, while large land masses induce
major temperature changes. Wet soil, such as that found in swamps and marshes, is almost as effective as water in
suppressing temperature changes. Thick vegetation tends to control temperature changes since it contains some water and
also insulates against heat transfer between the ground and the atmosphere. Arid, barren surfaces generate the greatest
temperature changes.
These topographical influences are both diurnal and seasonal. For example, the difference between a daily maximum and
minimum may be 10° or less over water, near a shore line, or over a swamp or marsh, while a difference of 50° or more is
common over rocky or sandy deserts.
Abrupt temperature differences develop along lake and ocean shores. These variations generate pressure differences and
local air flows or winds, which may become a consideration in the balloon pilot’s study of the air mass
Prevailing wind is also a factor in temperature control. In an area where prevailing winds are from large water bodies,
temperature changes are rather small. Most islands enjoy fairly constant temperatures. On the other hand, temperature
changes are more pronounced where prevailing wind is from dry, barren regions.
Temperature Variation with Altitude
Temperature normally decreases with increasing altitude throughout the troposphere. This decrease of temperature with
altitude is defined as lapse rate. The standard lapse rate seldom exists. In fact, temperature sometimes increases with
height. An increase in temperature with altitude is defined as an inversion.
An inversion often develops near the ground on clear, cool nights when wind is light. The ground radiates and cools much
faster than the overlying air. Air in contact with the ground becomes cold, while the temperature a few hundred feet above
changes very little. Thus, temperature increases with height. Inversions may also occur at any altitude when conditions
are favorable. For example, a current of warm air aloft overrunning cold air near the surface produces an inversion aloft.
Low level inversions, which are usually of most interest to the balloon pilot, generally dissipate through the daylight hours
as the air mixes with insolation.
Heat Transfer
The heat source for this planet is the sun; energy from the sun is transferred through space and the Earth’s atmosphere to
the Earth’s surface. As this energy warms the Earth’s surface and atmosphere, some of it is or becomes heat energy. Heat
is transferred into the atmosphere in three ways: radiation, conduction, and convection.
Radiation
Radiation is the transfer of heat by electromagnetic waves. No medium of transfer is required between the radiator and the
body being irradiated (receiving the radiation). Heat waves, a form of this electromagnetic energy, may be reflected. In
meteorology, the principal reflectors are the Earth’s surface, water vapor in the air, and particulate matter in the atmosphere.
Conduction
Conduction is the transfer of heat energy from one substance to another or within a substance. As with electricity, some
materials are good conductors while others are poor conductors. Poor conductors are considered to be insulators. Air is
one of the poorest conductors of heat in comparison to silver, one of the best conductors. Silver will pass 20,000 times
more heat than an equal mass of air across a similar temperature difference during a fixed period of time. Conduction in
the atmosphere is considered to be a significant method of heat exchange only at the Earth’s surface, where the lowest few
centimeters of the atmosphere are actually in contact with the ground or water.
Convection
Convection is the transfer of heat energy in a fluid. This type of heating is most commonly seen in the kitchen when a liquid
boils. This type of heat transfer occurs in the atmosphere when the ground is heated by the sun. The warm ground heats
the air above it by radiation and conduction, causing the warm air to rise. Meanwhile, the dense cooler air aloft moves in
to take the warm ground air’s place to be heated.
Heat can be transferred by convection in either a vertical or a horizontal direction. In meteorology, “advection” is the term
used for the horizontal transport of heat by wind. It is important to differentiate between the vertical and horizontal paths
of convection. In the atmosphere, the amount of heat transferred horizontally over the surface of the Earth by advection is
about 1,000 times greater than that transferred by convection.
The Adiabatic Process
The adiabatic process is the change of the temperature of air without transferring heat. In an adiabatic process, compression
results in warming, and expansion results in cooling. The adiabatic process takes place in all upward and downward
moving air. When air rises into an area of lower pressure, it expands to a larger volume. As the molecules of air expand, the
temperature of the air lowers. As a result, when a parcel of air rises, pressure decreases, volume increases, and temperature
decreases. When air descends, the opposite is true.
Since air is composed of a mixture of gases subject to heating when compressed and cooling when expanded, air will rise,
seeking a level where the pressure of the body of air is equal to the pressure of the air that surrounds it. Whatever the cause
of the lifting, the air rises, and the pressure decreases, allowing the “parcel of air” to expand. This continues until it reaches
an altitude similar in pressure and density to its own. As it expands, it cools through the adiabatic process and no heat is
added or withdrawn from the system in which it operates. As air rises, it is cooled because it is expanding by moving to
an altitude where pressure and density is less. This is adiabatic cooling. When the process is reversed and air is forced
downward, it is compressed, causing it to heat by a process called adiabatic heating
Air Masses
An air mass is a large body of air (usually 1,700 kilometers or more across) whose physical properties (temperature
and humidity) are horizontally uniform. The weather is a direct result of the continuous alternation of the influences of
warm and cold air masses. Warm air masses predominate in the summer, and cold air masses predominate in the winter.
However, both cold and warm air, alternately, may prevail almost anywhere in the temperature zone at any season. The
basic characteristics of any air mass are temperature and humidity. These properties are relatively uniform throughout the
air mass, and it is by measurement of these properties that the various types of air masses are determined.
Characteristics
Air masses acquire the characteristics of the surrounding area, or source region. The characteristics of an air mass consist
of the basic properties of moisture and temperature, which include:
• Stability.
• Cloud Types.
• Sky coverage.
• Visibility.
• Precipitation.
• Icing.
• Turbulence.
The terrain surface underlying the air mass is the primary factor in determining air mass characteristics.
A source region is typically an area in which the air remains relatively stagnant for a period of days or longer. During
this time of stagnation, the air mass takes on the temperature and moisture characteristics of the source region. Areas of
stagnation can be found in polar regions, tropical oceans, and dry deserts. Air masses are classified by region of origination:
• Polar or tropical.
• Maritime or continental.
A continental polar air mass forms over a polar landmass and is characterized by cool and dry conditions. Maritime tropical
air masses form over warm tropical waters like the Caribbean Sea and bring warm, moist air. As the air mass moves from
its source region and passes over land or water, the terrain it passes over modifies its qualities, and thus modifies the nature
of the air mass.
An air mass passing over a warmer surface will be warmed from below, and convective currents form, causing the air to
rise. This creates an unstable air mass with good surface visibility. Moist, unstable air causes cumulus clouds, localized
showers, and turbulence to form. Conversely, an air mass passing over a colder surface does not form convective currents,
but instead creates a stable air mass with poor surface visibility. The poor surface visibility is due to the fact that moisture,
smoke, dust, and other particles cannot rise out of the air mass and are instead trapped near the surface by an inversion. A
stable air mass can produce low stratus clouds and fog.
Pressure Systems
The differences that occur with heating and cooling the atmosphere in the lower levels also cause density variations.
These variations cause small horizontal pressure differences that are only about one ten-thousandth of the magnitude
of the normal change of pressure with altitude, but they significantly impact atmospheric circulation and most weather
phenomena. [Figure 4-3]
Ridge line
Trough line
Figure 4-3. High and low pressure systems.
A low or cyclone is a pressure system in which the barometric pressure decreases towards the center and the wind flow
around the system is counterclockwise in the Northern Hemisphere. Unfavorable flying conditions in the form of low
clouds, restricted visibility by precipitation and fog, strong and gusty winds, and turbulence are common in low pressure
systems. Thermal low pressure systems caused by intense surface heating and resulting low air density over barren
continental areas are relatively dry with few clouds and practically no precipitation. Thermal lows are stationary and
predominate over continental areas in the summer. General airflow in a low pressure system, since the atmosphere is
attempting to achieve equilibrium, is in (towards the center of the low pressure system), and up. This tendency can affect
the overall dynamic of the low pressure system.
A high is a pressure system in which the barometric pressure increases toward the center and the wind flow around the
system is clockwise in the Northern Hemisphere. Flying conditions are generally more favorable in highs than in lows
because of fewer clouds, light or calm winds, and less concentrated turbulent areas. But, in some situations, visibility may
be reduced due to early morning fog, smog, or haze at flight levels. High pressure systems predominate over cold surfaces
where the air is dense. General airflow in a high pressure system, in reverse of the low pressure dynamic, is out (away from
the center of the pressure system) and down. Again, these airflow tendencies can affect the dynamic of the high pressure
system, much like the low.
In the Northern Hemisphere, a general cycle of highs and lows moves through the temperate zones from west to east. The
movement of the pressure systems is more rapid in the winter season when the low pressure systems are most intense and
the high pressure systems extend farthest to the south. [Figure 4-4]
Hill of air Hill of air
Depression or valley of air
2,000 FT
1,500 FT
1,000 FT
500 FT
Figure 4-4. A cross-section of the pressure systems depicted in Figure 4-3.
A trough is an elongated area of low pressure, with the lowest pressure along the trough line. The weather in a trough is
commonly violent. Also, troughs can be slow moving.
A ridge is an elongated area of high pressure with highest pressure along the ridge line. The weather in a ridge is generally
favorable for flying.
Fronts
Fronts are the boundaries between two air masses and are classified as to which type of air mass (cold or warm) is replacing
the other. For example, a cold front demarcates the leading edge of a cold air mass displacing a warmer air mass. A warm
front is the leading edge of a warmer air mass replacing a colder air mass. Fronts are also transition zones (boundaries)
between air masses that have different densities. The density of air is controlled primarily by the temperature of the air.
Therefore, fronts in temperate zones usually form between tropical and polar air masses, but they may also form between
arctic and polar air masses. A typical surface weather map shows air mass boundary zones at ground level. Designs on
the boundary lines indicate the type of front and its direction of movement. On weather maps in local weather stations,
fronts may also be indicated by colored lines. A working knowledge of fronts and their accompanying weather hazards is
important to pilots.
Types of Fronts
The four major frontal types are:
• Cold.
• Warm.
• Stationary.
• Occluded.
A front type is determined from the movement of the air masses involved.
Cold Front
A cold front is the leading edge of an advancing mass of colder air. A cold front occurs when a mass of cold, dense,
and stable air advances and replaces a body of warmer air. Cold fronts move more rapidly than warm fronts, generally