Warm
Cool
Return flow
Land breeze
Warm
Cool
Return flow
Sea breeze
Figure 4-14. Land-sea breezes.
Convection currents close to the ground can affect a pilot’s ability to control the balloon. On final approach, for example,
the rising air from terrain devoid of vegetation sometimes produces a ballooning effect that can cause a pilot to overshoot
the intended landing spot. On the other hand, an approach over a large body of water or an area of thick vegetation tends
to create a sinking effect that can cause an unwary pilot to land short of the intended landing spot. This could prove
particularly hazardous to a balloon landing in a small, confined area, as the “undershoot” of the approach could potentially
put the balloon into the trees or power lines.
The Jet Stream
The jet stream refers to relatively strong winds concentrated in a narrow stream in the atmosphere. These winds are
normally horizontal, high altitude winds. The position and orientation of jet streams vary from day to day. General weather
patterns (hot/cold, wet/dry) are related closely to the position, strength, and orientation of the jet stream (or jet streams).
A jet stream at low levels is known as a low level jet stream. Since it is of interest primarily to high level flight, further
discussion is not necessary.
Local and Small-Scale Winds
There are four types of local and small-scale winds to be aware of.
Gradient Winds
Pressure gradients initiate the movement of air and as soon as the air acquires velocity, the Coriolis force deflects it to the
right in the Northern Hemisphere. As the speed of the air along the isobars increases, the Coriolis force becomes equal and
opposite to the pressure gradient force. After a period of time, the air moves directly parallel to the curved isobars if there
is no frictional drag with the surface. The air no longer moves toward lower pressure because the pressure gradient force
is completely neutralized by the Coriolis force and the centrifugal force.
Orographic Winds
The term “orographic” has multiple meanings, when placed in the context of weather phenomena. In a general sense,
according to the American Meteorological Society, wind flows that are caused, affected, or influenced by mountains may
be said to be orographic winds flows. The term has come to mean any winds that are affected by terrain, not just mountains;
this definition is probably the most frequently used, when discussing balloon flight.
As a specific term, “orographic lifting” is defined as an ascending air flow caused by mountains. The mechanisms that
produce the orographic lifting fall into two broad categories:
1. The upward deflection of horizontal large-scale air flow by the terrain acting as an obstacle or barrier.
2. The daytime heating of mountain surfaces to produce an anabatic flow (see below) along the slopes and updrafts in
the vicinity of mountain peaks.
This definition, while strictly referring only to lifting by mountains, is sometimes extended to include the effects of hills or
long sloping terrain. When sufficient moisture is present in the rising air, Orographic fog or clouds may form.
Anabatic Winds
Anabatic winds are those that blow up a steep slope or mountain side. It is sometimes referred to as an upslope flow. These
winds typically occur during the daytime in calm, sunny weather. A hill or mountaintop may be warmed by the sun, which
in turn heats the air just above it. As that air rises through convection, it creates a low pressure region, into which the air at
the bottom of the slope flows, and causes winds.
Katabatic Winds
Katabatic winds are the reverse of anabatic winds; that is, they flow down slope, and most frequently at night. They are
created by the effect of the air near the ground losing heat thru radiational cooling at a faster rate than air at a similar
altitude over the surrounding land mass.
Clouds
Clouds are weather signposts in the sky. They provide the balloon pilot with visible evidence of the atmospheric motion,
water content, and degree of stability. In this sense, clouds are of significant importance to the aeronaut. However, when
they become too numerous or widespread, form at low levels, or show extensive vertical development, they present weather
hazards to ballooning.
Clouds are visible condensed moisture, consisting of droplets of water or crystals of ice. They are supported and transported
by air movements as slow as one-tenth of a mile per hour. Cloud formation is the direct result of saturation producing
processes which take place in the atmosphere. A pilot should be able to identify cloud formations that are associated with
weather hazards. Knowledge of cloud types will also assist the pilot in interpreting weather conditions from weather
reports and existing weather.
Cloud Formation
Clouds are often indicative of future weather. For clouds to form, there must be adequate water vapor and condensation
nuclei (miniscule particles of matter like dust, salt, and smoke), as well as a method by which the air can be cooled. When
the air cools and reaches its saturation point, the invisible water vapor changes into a visible state. Through the processes
of sublimation and condensation, moisture condenses or sublimates onto condensation nuclei. The nuclei are important
because they provide a means for the moisture to change from one state to another.
Cloud type is determined by its height, shape, and behavior. They are classified according to the height of their bases as
low, middle, or high clouds, as well as clouds with vertical development. The International Cloud Classification is designed
to provide a uniform cloud classification system. [ Figure 4-15] Within this system, cloud types are usually divided into
four major groups and further classified in terms of their forms and appearance.
International Cloud Classification
Abbreviations and Weather Map Symbols
Base Altitude
Cloud Type
Abbreviation
Symbol
Bases of high
clouds usually
above 18,000
feet
Cirrus CI
Cirrocumulus C C
Cirrostratus CS
Altocumulus A C
Altostratus AS
* Cumulus CU
* Cumulonimbus C b
Nimbostratus N S
Stratocumulus S C
Stratus St
Bases of middle
clouds range
from 6,500 feet
to 18,000 feet
Bases of low
clouds range
from surface
to 6,500 feet
* Cumulus and cumulonimbus are clouds with vertical
development. Their bases are usually below 6,500 feet, but
may be slightly higher. The tops of the cumulonimbus some
times exceed 60,000 feet.
18,000 FT
6,500 FT
Surface
Figure 4-15. Cloud classification per international agreement.
The four major groups are:
• Low clouds.
• Middle clouds.
• High clouds.
• Clouds with vertical development.
Cloud classification can be further broken down into specific cloud types according to the outward appearance and cloud
composition. Knowing these terms can help identify visible clouds. The following is a list of cloud classifications:
• Cumulus—heaped or piled clouds.
• Stratus—formed in layers.
• Cirrus—ringlets, fibrous clouds, also high-level clouds above 20,000 feet.
• Castellanus—common base with separate vertical development, castle-like.
• Lenticularus—lens shaped, formed over mountains in strong winds.
• Nimbus—rain-bearing clouds.
• Fracto—ragged or broken.
• Alto—meaning high, also middle-level clouds existing at 5,000 to 20,000 feet.
Low clouds are those that form near the Earth’s surface. The low cloud group consists of stratus and stratocumulus clouds.
[Figure 4-16 and Figure 4-17] Clouds in this family create low ceilings, hamper visibility, and can change rapidly. Because
of this, they influence flight planning and can make visual flight rules (VFR) flight impossible. The bases of these clouds
can start near the surface, with the top extending to 6,500 feet or more above the terrain. Low clouds are of great importance
to the balloon pilot, as they can create low ceilings and poor visibility. The heights of the cloud bases may change rapidly.
If low clouds form below 50 feet, they are classified as fog, and may completely blanket landmarks and landing fields.
Figure 4-16. Stratus clouds.
Figure 4-17. Stratocumulus clouds.
Middle clouds form around 6,500 feet above ground level (AGL) and extend up to 20,000 feet AGL. They are composed
of water, ice crystals, and supercooled water droplets. The middle cloud group consists of altocumulus [ Figure 4-18],
altostratus, and nimbostratus [ Figure 4-19] clouds. Altocumulus clouds, which usually form when altostratus clouds are
breaking apart, also may contain light turbulence and icing. Altostratus clouds can produce turbulence and may contain
moderate icing. The altocumulus has many variations in appearance and formation, whereas the altostratus varies mostly
in thickness, from very thin to several thousand feet. Bases of the middle clouds start as low as 6,500 feet and tops can
range as high as 20,000 feet above the terrain. These clouds may be composed of ice crystals or water droplets (which may
be supercooled). Altocumulus rarely produces precipitation, but altostratus usually indicates the proximity of unfavorable
flying weather and precipitation.
Figure 4-18. Altocumulus clouds.
Figure 4-19. Nimbostratus clouds.
High clouds form above 20,000 feet AGL and usually form only in stable air. The high cloud group consists of cirrus,
cirrocumulus, and cirrostratus clouds. The mean base level of these three cloud types starts at 18,000 feet or higher above
terrain. Cirrus clouds [Figure 4-20] may give indications of approaching weather changes. Cirriform clouds are composed
of ice crystals, are generally thin, and the outline of the sun or moon may sometimes be seen through them, producing
a halo or corona effect. High clouds are generally of no interest to the balloon pilot, other than they may indicate future
conditions.
Figure 4-20. Cirrus clouds.
Clouds with extensive vertical development are cumulus clouds that build vertically into towering cumulus or cumulonimbus
clouds, often developing into thunderstorms. The bases of these clouds form in the low to middle cloud region, but can
extend into high altitude cloud levels. Towering cumulus clouds indicate areas of instability in the atmosphere, and the air
around and inside them is turbulent. These clouds generally have their bases below 6,500 feet above the terrain and tops
sometimes extend above 60,000 feet. Clouds with extensive vertical development are caused by lifting action, such as
convective currents, orographic lift, or frontal lift.
Scattered cumulus or isolated cumulonimbus clouds seldom present a flight problem, since these clouds can usually be
circumnavigated without difficulty. However, these clouds may rapidly develop in groups or lines of cumulonimbus. They
may also become embedded and hidden in stratiform clouds, resulting in hazardous flight conditions.
Within the high, middle, and low cloud groups are two main subdivisions. These are:
• Clouds formed when localized vertical currents carry moist air upward to the condensation level. These vertical
development clouds are characterized by their lumpy or billowy appearance, and are designated cumuliform type
clouds, meaning “accumulation” or “heap.” Turbulent flying conditions usually exist in, below, around, and above
cumuliform clouds.
• Clouds formed when complete layers of air are cooled until condensation takes place. These clouds are stratiform
type clouds, meaning “layered out,” since they lie mostly in horizontal layers or sheets. Flight in stratiform cloud
conditions is usually smooth.
In addition to the two main subdivisions discussed above, is the word nimbus, meaning “rain cloud.” These clouds normally
produce heavy precipitation, either liquid or solid. For example, a stratiform cloud producing precipitation is referred to
as nimbostratus, and a heavy, swelling cumulus cloud that has grown into a thunderstorm is referred to as cumulonimbus
Cumulonimbus clouds contain large amounts of moisture and unstable air, and usually produce hazardous weather
phenomena such as lightning, hail, tornadoes, gusty winds, and wind shear. These extensive vertical clouds can be obscured
by other cloud formations and are not always visible from the ground or while in flight. When this happens, these clouds
are said to be embedded, hence the term, embedded thunderstorms.
To pilots, the cumulonimbus cloud is perhaps the most dangerous cloud type. It appears individually or in groups and is
known as either an air mass or orographic thunderstorm. Heating of the air near the Earth’s surface creates an air mass
thunderstorm; the upslope motion of air in the mountainous regions causes orographic thunderstorms. Cumulonimbus
clouds that form in a continuous line are nonfrontal bands of thunderstorms or squall lines.
Knowledge of principal cloud types and the factors that affect them helps the pilot visualize expected weather conditions,
and to recognize potential weather hazards.
Ceilings & Visibilities
Ceilings and visibilities have an important role in the classification of sky conditions, and are critical for the definition of
flight restrictions. It is necessary to define these terms to make those distinctions clear for the balloon pilot.
Ceiling
For aviation purposes, a ceiling is the lowest layer of clouds reported as being broken or overcast, or the vertical visibility
into an obscuration like fog or haze.
Observations are made using the concept of the “celestial dome,” the hemisphere of sky which can be seen from a specific
point on the ground. Cloud coverage is reported as the total cloud cover at and below a specific layer, and is reported in
one-eighth increments (octals). A ceiling is reported as broken when five-eighths to seven-eighths of the sky is covered
with clouds. Overcast means the entire sky is covered with clouds.
Current ceiling information is reported by the aviation routine weather report (METAR) and automated weather stations of
various types. Ceilings are reported in height AGL.
Visibility
Closely related to cloud cover and reported ceilings is visibility information. Visibility refers to the greatest horizontal
distance at which prominent objects can be viewed with the naked eye. Visibilities reported in standard weather reports are
horizontal surface visibilities and are generally considered linear. Predominant visibility is the greatest horizontal distance
over which objects can be seen and identified over at least half of the horizon. In the United States, prevailing visibilities
are reported in statute miles and portions thereof.
Since prevailing visibility is used for reporting purposes, three miles visibility does not mean that a pilot must have one
and one half miles visibility in front of and behind the balloon, but that the predominant visibility in most quadrants must
be three miles.
Current visibility is reported in METAR and other aviation weather reports, as well as automated weather stations. Visibility
information is available during a preflight weather briefing.
Temperature/Dew Point Relationship
The relationship between dew point and temperature defines the concept of relative humidity. The dew point, given in
degrees, is the temperature at which the air can hold no more moisture. When the temperature of the air is reduced to the
dew point, the air is completely saturated and moisture begins to condense out of the air in the form of fog, dew, frost,
clouds, rain, or snow.
As moist, unstable air rises, clouds often form at the altitude where temperature and dew point reach the same value. When
lifted, unsaturated air cools at a rate of 5.4 °F per 1,000 feet and the dew point temperature decreases at a rate of 1 °F per
1,000 feet. This results in a convergence of temperature and dew point at a rate of 4.4 °F. A pilot can determine the height
of the cloud base by applying the convergence rate to the reported temperature and dew point in the following manner:
Temperature (T) = 85 °F.
Dew point (DP) = 71 °F.
Convergence Rate (CR) = 4.4°.
T – DP = Temperature Dew Point Spread (TDS).
TDS ÷ CR = X.
X x 1,000 feet = height of cloud base AGL
Example:
85 °F – 71 °F = 14 °F.
14 °F ÷ 4.4 °F = 3.18.
3.18 x 1,000 = 3,180 feet AGL.
The height of the cloud base is 3,180 feet AGL.
Explanation: With an outside air temperature (OAT) of 85 °F at the surface, and dew point at the surface of 71 °F, the
spread is 14 °F. Divide the temperature dew point spread by the convergence rate of 4.4 °F, and multiply by 1,000 to
determine the approximate height of the cloud base.
This relationship is useful in determining the height of the overlying cloud base when completing preflight preparations.
Fog
Fog is a cloud that begins within 50 feet of the surface. It typically occurs when the temperature of air near the ground is
cooled to the air’s dew point. At this point, water vapor in the air condenses and becomes visible in the form of fog. Fog
is classified according to the manner in which it forms and is dependent upon the current temperature and the amount of
water vapor in the air.
Fog is composed of minute droplets of water or ice crystals suspended in the atmosphere with no visible downward motion.
It is one of the most common and persistent weather hazards encountered by balloonists. Similar to stratus clouds, the base
of fog is at the Earth’s surface while the base of 4-19 a cloud is at least 50 feet above the surface. Fog may be distinguished
from haze by its dampness and gray color. It is hazardous during takeoffs and landings, as well as the in-flight process,
because it restricts surface visibility. Knowledge of fog formation and dissipation processes, as well as types of fog help
the balloon pilot plan a flight more accurately
Fog Formation
Since neither condensation nor sublimation occurs unless the relative humidity is near 100 percent, a high relative
humidity is of prime importance in the formation of fog. The natural conditions which bring about a high relative humidity
(saturation) are also fog-producing processes, such as the evaporation of additional moisture into the air or cooling of the
air to its dew point temperature. A high relative humidity can be estimated, from hourly sequence reports, by determining
the spread (difference in degrees) between the temperature and dew point. Fog rarely occurs when the spread is more than