independent of surrounding cells in the same storm. Each thunderstorm progresses through a life cycle from 1 to 3 hours,
depending upon the number of cells contained and their stage of development. In the initial stage (cumulus), the cloud
consists of a single cell. As the development progresses, however, new cells may form as older cells dissipate.
Stages in Thunderstorm Development
The life cycle of each thunderstorm cell consists of three distinct stages:
• Cumulus.
• Mature.
• Dissipating.
Cumulus Stage
Although most cumulus clouds do not become thunderstorms, the initial stage of a thunderstorm is always a cumulus
cloud. [Figure 4-24] The chief distinguishing feature of the cumulus or building stage is an updraft that prevails throughout
the entire cell. This updraft may vary from a few feet per second to as much as 6,000 fpm (65 knots) in mature cells. As an
updraft continues through the vertical extent of the cell, water droplets grow in size and raindrops are formed.
Figure 4-24. Congestive cumulus cloud
Mature Stage
The formation of a downdraft and the beginning of surface rain and additional updrafts and downdrafts initiates the
mature stage. [ Figure 4-25] By this time, the average cell has attained a height of 25,000 feet. As the drops begin to
fall, the surrounding air begins a downward motion because of frictional drag. This descending air will be colder that its
surroundings and its rate of downward motion is accelerated, forming the downdraft. The downdraft reaches maximum
speed a short time after rain begins to fall in the cloud. Downdrafts occur at all levels within the storm, and their speed
ranges from a few feet per minute to about 2,500 fpm (25 knots). Significant downdrafts never extend from the top of
the cell because moisture is not sufficient in the upper levels for raindrops to form. At these high levels, only ice crystals,
snowflakes, and supercooled water are present. Therefore, their rate of fall is insufficient to cause appreciable downdrafts.
The mature cell generally extends far above 25,000 feet—in some cases up to 70,000 feet. In the middle levels, around
14,000 feet, strong updrafts and downdrafts are adjacent to each other. A shear action exists between these drafts and
produces strong and frequent gusts.
Figure 4-25. Mature thunderstorm.
Dissipating or Anvil Stage
Throughout the life span of the mature cell, more and more air aloft is entrained by the falling raindrops. Consequently,
the downdraft spreads out to take the place of the weakening updrafts. As this process progresses, the entire lower portion
of the cell becomes an area of downdraft. Since updrafts are necessary to produce condensation and release latent heat
energy, the entire structure begins to dissipate. [ Figure 4-26] The strong winds aloft carry the upper section of the cloud
into the familiar anvil form (cumulonimbus cloud). However, the appearance of the anvil does not always indicate that the
thunderstorm is dissipating.
Figure 4-26. Dissipating thunderstorm.
A significant thunderstorm hazard is the rapid change in wind direction and wind speed immediately prior to storm passage
at the surface. These strong winds are the result of the horizontal spreading of the storm’s downdraft current as they
approach the surface of the earth. This initial wind surge, as observed at the surface, is known as a first gust. The speed
of this first gust may exceed 75 knots and vary 180° in direction from the previously prevailing surface winds. Firstgust
speeds average about 15 knots over prevailing velocities, and average an approximately 40° change in the direction of the
wind. First gusts usually precede the heavy precipitation, and strong gusts may continue for approximately 5 to 10 minutes
with each thunderstorm cell. First gusts are not limited to the area ahead of the storm’s movement. They may be found in
all sectors, including the area back of the storm’s movement.
surface variations generally occur. These variations usually occur in a particular sequence characterized by:
• An abrupt fall in pressure as the storm approaches.
• An abrupt rise in pressure associated with rain showers as the storm moves on and the rain ceases.
All thunderstorms are similar in physical makeup, but for purposes of identification they are divided into two general
groups: frontal and air mass. This division gives the balloon pilot an indication of the method by which the storms are
formed and the distribution of the clouds over the area. The specific nomenclature of these thunderstorms depends upon
the manner in which the lifting action occurs.
Frontal Thunderstorms
Thunderstorms may occur within the cloud system of any front: warm, cold, stationary, or occluded. Frontal thunderstorms
are caused by the lifting of warm, moist, conditionally unstable air over a frontal surface. Thunderstorms may also occur
many miles ahead of a rapidly moving cold front, and are called prefrontal or squall line thunderstorms.
Warm front thunderstorms are caused when warm, moist, conditionally unstable air is forced aloft over a colder, denser
shelf of retreating air. Because the frontal surface is shallow, the air is lifted gradually. The lifting condensation level, the
level where air becomes saturated when lifted, is normally reached well before the level of free convection (where lifted
air is warmer than the environmental air and rises on its own), thus producing stratus clouds. The level of free convection
is normally reached in isolated areas along the frontal surface. This is the area where the greatest amount of water vapor is
present in the warm air being lifted. Therefore, warm front thunderstorms are generally isolated to scattered in coverage.
When the level of free convection is reached, warm front thunderstorms may form. These thunderstorms may prove
particularly hazardous, as they are frequently obscured by the surrounding stratiform clouds.
Cold front thunderstorms are caused by the forward motion of a wedge of cold air under a mass of warm, moist, conditionally
unstable air (cold front), increasing the possibility for thunderstorms to develop. The slope of a typical cold frontal surface
is relatively steep, so the lifting condensation level and the level of free convection are usually near the same altitude.
Cold front thunderstorms are typically positioned along the frontal surface in what appears to be a continuous line. These
storms are easily recognized, because they are partly visible from the front and rear of the storm line. However, if the slope
of the frontal surface is shallow, the lifting action is not sufficient to produce thunderstorms in lines (line squalls). With a
shallow front, the thunderstorms form behind the surface front and are widely scattered. Such storms may be concealed by
the surrounding cloud layers.
Lines of thunderstorms frequently develop ahead of rapidly moving cold fronts. These are known as prefrontal squall lines,
and frequently form parallel to the cold front. Prefrontal squall line thunderstorms are usually more violent than cold front
thunderstorms. They are most active between noon and midnight. The cold front cloud system usually weakens during
the period of the greatest prefrontal squall line activity because the warm air displaced by the frontal surface has lost its
moisture and energy in the prefrontal thunderstorms. In the United States, tornadoes are frequently associated with strong
prefrontal squall lines. Pre-frontal squall line thunderstorms are indicated on a surface weather map by an alternate dash-
dot-dash line (display).
The distribution of stationary front thunderstorms is controlled by the slope of the frontal surface. Steeply sloped stationary
fronts tend to have lines of storms, whereas shallow stationary fronts tend to have widely scattered storms.
Occluded front thunderstorms are associated with the two types of occluded fronts (warm front and cold front occlusions)
and are usually cold front thunderstorms that have been moved into the area of warm frontal weather by the occlusion
process. They are found closer to the low-pressure center and are normally strongest for a distance of 50 to 100 miles north
of the peak of the warm sector.
Air Mass Thunderstorms
The two types of air mass thunderstorms are locally convective and orographic. Both types form within air masses, and are
randomly distributed throughout the air mass.
Convective thunderstorms are often caused by solar heating of the land, which provides heat to the air, thereby resulting
in thermal convection. Relatively cool air flowing over a warmer water surface may also produce sufficient convection to
cause thunderstorms. The land-type convective thunderstorms normally form during the afternoon hours, after the Earth
has gained maximum heating from the sun. If cool, moist, conditionally unstable air is passing over this land area, heating
from below will cause convective currents, thereby resulting in towering cumulus or thunderstorm activity. Dissipation
usually occurs during the early evening hours, as the land begins to lose its heat to the atmosphere. Although convective
thunderstorms form as individual cells, they may become so numerous over a particular geographical area that continued
flight cannot be maintained.
Thunderstorms over the ocean are most common during the night and early morning. They frequently occur offshore when
a land breeze is blowing toward the water. The cool land breeze is heated by the warmer water surface, which results in
sufficient convection to produce thunderstorms. After sunrise, heating of the land surface reverses the airflow (sea breeze).
The thunderstorms then dissipate over the water, but they may re-form over the warmer land surface. As an example, the air
mass weather that exists in Florida combines both types of convective thunderstorms. Circulation around a semipermanent
high pressure system off the southeastern United States (Bermuda high) carries moist ocean air over the warm land surface
of the Florida Peninsula. At night, thunderstorms off the Florida Coast are caused by the warm water of the Gulf Stream
heating the surface air, while the upper air is cooling by radiation to space. This heating from below produces thermal
convection over the water. When the sun rises, the heat balance necessary to maintain storm formation over the water
is destroyed. By day, the storms appear to move inward over the land areas, but actually dissipate off the coast and re-
form over the hot landmass. The heated land surface sets up an unstable lapse rate over the Peninsula and causes storm
development to continue until nocturnal cooling occurs. Usually, convective type storms are randomly distributed and
easily recognized.
Orographic thunderstorms will form on the windward side of a mountain if conditionally unstable air is lifted above the
level of free convection. The storm activity is usually scattered along the individual peaks of the mountains. Occasionally,
however, this activity may form a long unbroken line of storms similar to a squall line. The storms persist as long as
the circulation causes upslope motion. From the windward side of the mountains, identification of orographic storms
may sometimes be difficult because the storm clouds are obscured by other clouds (usually stratiform). Almost without
exception, orographic thunderstorms enshroud mountain peaks or hills.
Minimum Factors
The minimum factors essential to the formation of a thunderstorm are conditionally unstable air with relatively high
moisture content and some type of lifting action. Lifting of warm air will not necessarily cause free convection. The
air may be lifted to a point where the moisture condenses and clouds form. These cloud layers, however, will be stable
if the level of free convection has not been reached by the lifting. Conversely, it is possible for dry heated air to rise
convectively without the formation of clouds. In this condition, turbulence might be experienced in perfectly clear weather.
Cumulonimbus cloud formations require a combination of conditionally unstable air, some type of lifting actions, and high
moisture content. Once a cloud has formed, the latent heat of condensation released by the change of state from vapor to
liquid tends to make the air more unstable.
Some type of external lifting action is necessary to bring the warm surface air to the point where it will continue to rise
freely (the level of free convection). For example, an air mass may be lifted by thermal convection, terrain, fronts, or
convergence.
Thunderstorms
By no means is the information contained here a complete discussion of all the weather information and factors affecting
balloon flight. There are many resources available, both through government and private agencies, which may be of value
to the pilot in planning a flight. A pilot should take the time to explore the internet, read weather books, and gain a complete
understanding of the myriad of weather information and products that are available.
The second section of this chapter will expose the reader to some of the weather reporting products available, both through
FSS briefings and Internet searches. With the knowledge gained from the first half of this chapter, the pilot will be able to
make a good interpretation of the reports, and determine how present and future conditions will affect the decision to fly.
How to Obtain Weather Information
An integral part of flight preparation for any pilot is checking the weather conditions expected to occur during the flight.
FAA regulations place the responsibility for flight planning on the pilot. To effectively plan a flight, a pilot needs to
understand what weather information is available, how to obtain it, and how it can be applied to a flight.
While weather forecasts are not 100 percent accurate, meteorologists, through careful scientific study and computer
modeling, have the ability to predict the weather patterns, trends, and characteristics with increasing accuracy. Through a
complex system of weather services, government agencies, and independent weather observers, pilots and other aviation
professionals receive the benefit of this vast knowledge base in the form of up-to-date weather reports and forecasts. These
reports and forecasts enable pilots to make informed decisions regarding weather and flight safety.
Sources for Weather Information
There are many sources available for today’s pilot when gathering information about weather prior to a flight. A review
of pertinent weather reports and information is required by FAA regulations and the following sources provide excellent
weather information. For the balloon pilot, experience and study helps them determine the preferred sources for weather
information.
FAA Flight Service Stations (FSS)
The FAA FSS is the primary source for preflight weather information. FSS can be contacted by calling 1-800- WXBRIEF. It
also logs pilot contacts to provide background information in the event of an accident or incident, as well as substantiating
workload statistics. It is one of the two sources of an official weather briefing.
The FAA has contracted flight briefings to private contractor. To receive information on a flight briefing, you can either
call 1-800-wxbrief (992-7433) or go to their website and create a profile.
To get a flight briefing, you will need to input a variety of information related to your flight, including cruising speed and
cruising altitude (referred to as ‘level’). For the speed, you will need to follow the ICAO format, by typing N0 followed
by your anticipated speed (in knots). For the (flight) level, the pilot has multiple options. For the majority of flights, pilots
could just type ‘VFR’ in the space provided. If a pilot is planning on an extended flight at a particular level below 18,000
feet MSL, they can enter that particular level by AXXX where the first placeholder is tens of thousands of feet, the second
place holder is in thousands of feet, and the third placeholder is in hundreds of feet. For example, if a pilot was planning
a flight at 8500 feet MSL, the correct entry would be A085.
Internet Sources
A wealth of internet sources exist for the balloon pilot seeking information about current weather conditions.
• The National Weather Service has weather offices across the country that supply the latest weather information. At
this site, you can click on the map anywhere in the country which will take you to the local office webpage. On the
local office page, one can click of the local map to get a localized forecast along with the closest current observation.
On the local forecast page under the “forecasts” sub-menu, there is an option tilted “forecast discussion”. There you
can find a discussion where the greatest weather concerns over the next seven days are discussed. There, you can
also find an aviation discussion. This discussion provides a window into the head of the forecaster that drafted the
TAFs, and may highlight weather concerns in greater detail than the TAFs allow.
Figure 4-27. This map can be used to gather hourly AWOS and ASOS type weather information from various sites in each state.
• National Center for Atmospheric Research and the University Corporation for Atmospheric Research —a
collaborative effort of research centers, universities, and weather offices around the United States, this site provides
numerous real-time and forecast weather products and graphics.
• Aviation Weather Center (AWC)— makes text, digital and graphical forecasts, analyses, and observations of aviation
related weather variables available to the aviation community.
• Helicopter Emergency Medical Services (HEMS) Weather Display —the ADDS development team created an
experimental tool designed to show weather conditions for short-distance and low-altitude flights common for the
helicopter emergency medical services (HEMS) community at the request of the FAA. This interactive site allows
the user to determine ceiling and visibility and winds (at 500 foot increments) for an area as small as 5 km*2. While
not specifically targeted nor designed for the balloon pilot’s use, the information obtained from this site is helpful to
the pilot planning a flight at some distance from a normal weather reporting facility.
Interpreting Weather Charts & Reports
A weather chart is any chart or map that presents data and analysis that describe the state of the atmosphere over an
extended region at a given time. Weather charts provide a picture of the overall movement of major weather systems and
fronts and are used in flight planning.
Three useful weather charts for balloon pilots are: surface analysis, weather depiction, and radar summary charts. These
three charts present current weather information and provide “big picture” information for weather systems across the
United States. The composite moisture stability chart, constant pressure analysis charts, and significant weather prognostic
charts provide additional information for flight planning.
Knowledge of all these weather charts, reports, and forecasts may not be necessary for the pilot planning a local flight, but
an understanding of large scale weather patterns and systems bring a greater understanding of how those systems affect
local weather. It is important to gain an understanding of the primary charts used, and develop interpolation skills to be able
to perform safe, adequate flight planning.
Surface Analysis Chart
The surface analysis chart is computer-generated, covers the contiguous 48 states and adjacent areas, and is transmitted every
3 hours with an analysis of the current surface weather. It shows the areas of high and low pressure, fronts, temperatures,
dew points, wind directions and speeds, local weather, and visual obstructions. [Figure 4-28]
Figure 4-28. Example of a section of a surface analysis chart with station model legend (inset).
Surface weather observations for reporting points across the United States are also depicted on this chart. Each of these
reporting points is illustrated by a station model.
• Type of observation—a round model indicates an official weather observer made the observation. A square model
indicates the observation is from an automated station. Stations located offshore give data from ships, buoys, or
offshore platforms.
• Sky cover—the station model depicts total sky cover and will be shown as clear, scattered, broken, overcast, or
obscured/partially obscured.
• Clouds—cloud types are represented by specific symbols. Low cloud symbols are placed beneath the station model,
while middle and high cloud symbols are placed directly above the station model. Typically, only one type of cloud
will be depicted with the station model.
• Sea level pressure—sea level pressure given in three digits to the nearest tenth of a mb. For 1,000 mb or greater,
prefix a ten to the three digits. For less than 1,000 mb, prefix a nine to the three digits.
• Pressure change/tendency—pressure change in tenths of mb over the past 3 hours. This is depicted directly below
the sea level pressure.
• Precipitation—a record of the precipitation that has fallen over the last 6 hours to the nearest hundredth of an inch.
• Dew point—dew point is given in degrees Fahrenheit.
• Present weather—over 100 different weather symbols are used to describe the current weather.
• Temperature—temperature is given in degrees Fahrenheit.
• Wind—true direction of wind is given by the wind pointer line, indicating the direction from which the wind is
coming. A short barb is equal to five knots of wind, a long barb is equal to ten knots of wind, and a pennant is equal
to 50 knots.
Weather Depiction Chart
A weather depiction chart details surface conditions as derived from METAR and other surface observations. It is prepared
and transmitted by computer every 3 hours beginning at 0100 Zulu time (0100Z), and is valid at the time of the plotted data.
Designed to be used for flight planning, it gives an overall picture of the weather across the United States. [Figure 4-29]
Figure 4-29. Weather depiction chart. On aviationweather.gov, the user can display data they are interested in, and zoom in on regions
of the country if interested. Surface observations are color-coded based on flight category. Other user defined overlays include:
radar, satellite, sigmets, airmets and METARS.