A light wind is generally favorable for fog formation. It causes a gentle mixing action, which spreads surface cooling
through a deeper layer of air and increases the thickness of the fog.
Although most regions of the Earth have sufficient condensation nuclei to permit fog formation, the amount of smoke
particles and sulphur compounds in the vicinity of industrial areas is pronounced. In these regions, persistent fog may occur
with above average temperature-dew point spreads.
Fog tends to dissipate when the relative humidity decreases. During this decrease, the water droplets evaporate or ice
crystals undergo sublimation, and the moisture is no longer visible. Either strong winds or heating processes may cause
the decrease in relative humidity.
Some Fog Types & Characteristics
There are four main types of fog you should be aware of.
Radiation Fog
Radiation fog forms after the Earth has radiated back to the atmosphere the heat gained during daylight hours. By early
morning, the temperature at the surface may drop more than 11 °C. Since the dew point temperature (moisture content) of
the air normally changes only a few degrees during the night, the temperature-dew point spread will decrease as the air is
cooled by contact with the cold surface. If the radiational cooling is sufficient, and other conditions are favorable, radiation
fog will form. Radiation fog is most likely when the:
• Sky is clear (maximum radiational cooling).
• Moisture content is high (narrow temperature-dew point spread).
• Wind is light (less than 7 knots).
Advection Fog
The movement of warm moist air over a colder surface creates advection fog which is common along coastal regions where
the temperature of the land surface and the water surface contrasts. The southeastern area of the United States provides
ideal conditions for advection fog formation during the winter months. If air flows (advection) from the Gulf of Mexico or
the Atlantic Ocean over the colder continent, this warm air is cooled by contact with the cold ground. If the temperature of
the air is lowered to the dew point temperature, fog will form. Advection fog, forming under these conditions, may extend
over larger areas of the nation east of the Rockies. It may persist throughout the day or night until replaced by a drier air
mass
If advection fog forms over water, it is often referred to as sea fog. Cold ocean currents, such as those off the coast of
California, may cool and saturate moist air coming from the warmer areas of the open sea. Sea fog is often dense offshore,
as well as onshore.
As advection fog moves inland during the winter, the colder land surface often causes sufficient contact cooling to keep the
air saturated. The fog may then persist during the day or with a wind speed of 10 to 15 knots.
Valley Fog
During the evening hours, cold dense air will drain from areas of higher elevation into low areas of valleys. As the cool
air accumulates in the valleys, the air temperature may decrease to the dew point temperature, causing a dense formation
of valley fog. While higher elevations may often remain clear throughout the night, the ceiling and visibility become
restricted in the valley.
Fog formed by the addition of moisture to the air is called evaporation fog. The major types of evaporation fog are frontal
fog and steam fog
Frontal fog is normally associated with slow-moving winter frontal systems. Frontal fog forms when liquid precipitation,
falling from the maritime tropical air above the frontal surface, evaporates in the polar air below the frontal surface.
Evaporation from the falling drops may add sufficient water vapor to the cold air to raise the dew point temperature to the
temperature of the air. The cold air will then be saturated, and frontal fog will form. Frontal fog is common with active
warm fronts during all seasons. It occurs ahead of the surface front in an area approximately 100 miles wide. It is, therefore,
frequently mixed with intermittent rain or drizzle. When fog forms ahead of the warm front, it is called prefrontal fog. A
similar fog formation may occur in the polar air along a stationary front. Occasionally, a slow-moving winter cold front
with light wind may generate fog. This fog forms in the polar air behind the surface front and is known as postfrontal fog.
Steam fog forms when cold stable air flows over a non-frozen water surface that is several degrees warmer than the air. The
intense evaporation of moisture into the cold air saturates the air and produces fog. Conditions favorable for steam fog are
common over lakes and rivers in the fall and over the ocean in the winter when an offshore wind is blowing.
Atmospheric Stability & Instability
A stable atmosphere resists upward or downward movement, and small vertical disturbances dampen out and disappear.
An unstable atmosphere allows an upward or downward disturbance to grow into a vertical or convective current allowing
small vertical air movements to become larger, resulting in turbulent airflow and convective activity. Instability can lead to
significant turbulence, extensive vertical clouds, and severe weather.
Rising air expands and cools due to the decrease in air pressure as altitude increases. The opposite is true of descending air;
as atmospheric pressure increases, the temperature of descending air increases as it is compressed. This adiabatic process
(heating or cooling) 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 water vapor is lighter than air, moisture decreases air density, causing it to rise. Conversely, as moisture decreases,
air becomes denser and tends to sink. Since moist air cools at a slower rate, it is generally less stable than dry air since
the moist air must rise higher before its temperature cools to that of the surrounding air. The dry adiabatic lapse rate
(unsaturated air) is 3 °C (5.4 °F) per 1,000 feet. The moist adiabatic lapse rate varies from 1.1 °C to 2.8 °C (2 °F to 5 °F)
per 1,000 feet.
The combination of moisture and temperature determine the stability of the air and the resulting weather. Cool, dry air is
very stable and resists vertical movement, which leads to good and generally clear weather. The greatest instability occurs
when the air is moist and warm, as it is in the tropical regions in the summer. Typically, thunderstorms appear on a daily
basis in these regions due to the instability of the surrounding air.
The normal flow of air tends to be horizontal. If this flow is disturbed, a stable atmosphere will resist any upward or
downward displacement. It will tend to return quickly to normal horizontal flow. An unstable atmosphere, on the other
hand, will allow these upward and downward disturbances to grow, resulting in rough (turbulent) air. An example is the
towering thunderstorm which grows as a result of large and intensive vertical movement or air. It climaxes in lightning,
thunder, and heavy precipitation, sometimes including hail.
Atmospheric resistance to vertical motion, called stability, depends upon the vertical distribution of the air’s weight at a
particular time. The weight varies with air temperature and moisture content. In comparing two parcels of air, warmer air
is lighter than colder air, and moist air is lighter than dry air. If air is relatively warmer or moister that its surroundings,
it is forced to rise and would be unstable. If the air is colder or dryer than its surroundings, it will sink until it reaches its
equilibrium, and would be stable. The atmosphere can be at equilibrium only when light air is above heavier air.
Temperature has a significant effect on the stability or instability of the air mass. Air heated near the Earth’s surface on
a hot summer day will rise. The speed and vertical extent of its travel depends on the temperature distribution of the
atmosphere. Vertical air currents, resulting from the rise of air, can vary from the severe downdraft and compensating
downdraft associated with thunderstorms to the closely spaced upward and downward bumps that are felt on warm days
when flying at low levels. Since the temperature of air is an indication of its density, a comparison of temperatures from
one level to another can approximate the degree of the atmosphere’s stability, or how much it will tend to resist vertical
motion.
Types of Stability
The five types of atmospheric stability are:
• Absolute stability.
• Absolute instability.
• Conditional instability.
• Neutral instability.
• Convective instability.
Absolute stability occurs when the actual lapse rate in a layer of air is less than the moist adiabatic lapse rate; that air is
absolutely stable regardless of the amount of moisture it contains. A parcel of absolutely stable air which is lifted becomes
cooler than the surrounding air and sinks back to its original position as soon as the lifting force is removed.
Similarly, if forced to descend, it becomes warmer than the surrounding air; like a cork in water, it rises to its original
position upon removal of the outside force.
Absolute instability exists when the actual lapse rate in a layer of air is greater than the dry adiabatic lapse rate; that air is
absolutely unstable regardless of the amount of moisture it contains. A parcel of air lifted even slightly will immediately
be warmer than its surroundings, and, as with a hot air balloon, will be forced to rise.
Conditional instability exists when the temperature lapse rate of the air involved lies between the moist and dry adiabatic
rates of cooling. Before the displaced air actually becomes unstable, it must be lifted to a point where it is warmer than the
surrounding air. When this point has been reached, the relatively warmer air continues to rise freely until, at some higher
altitude, its temperature has cooled to the temperature of the surrounding air. In the instability process, numerous variables
tend to modify the air. One of the most important of these variables is the process called entrainment. In this process, air
adjacent to the cumulus or mature thunderstorm is drawn into the cloud primarily by strong updrafts within the cloud. The
entrained air modifies the temperature of the air within the cloud as the two become mixed.
Neutrally stable air is air with the same temperature, and there is no parcel to rise or descend. For example, the surface area
in contact with that air is of the same temperature.
The term convective instability refers to a condition in which the air becomes unstable after lifting. From a physical
standpoint, it closely resembles a conditionally unstable air mass, but has the mechanical lifting of thermal activity
impacting on the overall characteristics of the air.
Effects of Stable & Unstable Air
The degree of stability of the atmosphere helps to determine the type of clouds formed, if any. For example, if very
stable air is forced to ascend a mountain slope, clouds will be layer-like with little vertical development and little or no
turbulence. Unstable air, if forced to ascend the slope, would cause considerable vertical development and turbulence in
the cumulus-type clouds.
If air is subsiding (sinking), the heat of compression frequently causes an inversion of temperature which increases the
stability of the subsiding air. When this occurs, as in winter high pressure systems, a surface inversion formed by radiational
cooling is sometimes already present. The subsidence-produced inversion, in this case, will intensify the surface inversion,
placing a strong “lid” above smoke and haze. Poor visibility in the lower levels of the atmosphere results, especially near
industrial areas. Such conditions frequently persist for days, notably in the Great Basin region of the western United States.
Weather Hazards
There are many weather hazards that you need to consider.
Turbulence
Turbulence is the irregular motion of the atmosphere as indicated by gusts and lulls in the wind. Since turbulence is
associated with many different weather situations, knowledge of its causes and its behavior will help a balloon pilot avoid
or minimize its effects.
Turbulence can be divided into four categories according to the specific causes:
• Thermal—caused by localized convective currents due to surface heating or unstable lapse rates and cold air moving
over warmer ground or water.
• Mechanical—resulting from wind flowing over irregular terrain or obstructions.
• Frontal—resulting from the local lifting of warm air by cold air masses, or the abrupt wind shift (shear) associated
with most cold fronts.
• Wind shear—marked gradient in wind speed and/or direction due to general vibrations in the temperature and
pressure fields aloft.
Two or more of the above causative factors often work together. In addition, turbulence is produced by man-made
phenomena.
Thermal
A thermal is simply the updraft in a small-scale convective current. Convective currents (vertical or horizontal air
movements) develop in air, which is heated by contact with a warm surface. This heating from below occurs when either
cold air is advected (moved horizontally) over a warmer surface or the ground is strongly heated by solar radiation.
The strength of convective currents depends in part on the extent to which the Earth’s surface has been heated, which
depends upon the nature of the surface. Barren surfaces, such as sandy or rocky wasteland and plowed fields, are heated
more rapidly than surfaces covered in vegetation. Thus, barren surfaces generally cause stronger convection currents. In
comparison, water surfaces are heated more slowly.
When air is very dry, convective currents may be present although convective-type clouds (cumulus) are absent. The
general upper limits of the convective currents are often marked by the tops of cumulus clouds, which form in them when
the air is moist, or by haze lines. However, turbulence may extend beyond this boundary. Varying types of surfaces, and the
resultant thermal conditions, can affect a balloon to a considerable extent.
The balloon pilot caught in a thermal will recognize the condition by an increase in altitude without application of heat
from the balloon’s heater. This ascent can be rapid and may exceed the maximum rate of climb limitations in the balloon’s
flight manual. Since the air mass is also rising with the balloon, there is no significant pressure against the top of the
balloon. Thus, the top cap will not be pushed open (commonly referred to as “floating the top”).
Depending on their size, some thermals may have a rotative motion similar to a small low pressure system. This motion
draws the balloon in and forces it to fly in an uncontrolled circle. For balloons caught in a thermal, remember the adage
“altitude is your friend.” First, the pilot should insure there is sufficient altitude to clear potential obstacles. Second,
maintain the temperature in the balloon appropriate for level flight. Many pilots attempt to descend immediately, but this
may put the balloon, as well as the passengers, at risk of an uncontrolled descent with possible injury. Most thermals have
a short lift span. In almost all cases, the thermal will “spit” the balloon out the top after a short time, and the pilot may
descend and land as necessary.
Mechanical
When the air near the surface of the Earth flows over obstructions, such as irregular terrain, (bluffs, hills, mountains)
and buildings, the normal horizontal wind flow is disturbed. As a result, it is transformed into eddies or other irregular
air movements. Figure 4-21 shows how the buildings or other obstructions near a launch site or landing field can cause
turbulence.
Figure 4-21. Surface obstructions cause eddies and other irregular air movements.
The strength and magnitude of mechanical turbulence depends on:
• The speed of the wind.
• The nature of the obstruction.
• The stability of the air.
• The angle at which the wind moves over the obstacle.
Stability seems to be the most important factor in determining the strength and vertical extent of the mechanical turbulence.
Frontal
Frontal turbulence is cause by the lifting of warm air by a frontal surface, leading to instability and/or the mixing or shear
between the warm and cold air masses. The vertical currents in the warm air are strongest when the warm air is moist and
unstable. The most severe cases of frontal turbulence are generally associated with fast moving cold fronts. In these cases,
mixing between the two air masses, as well as the differences in wind speed and/or direction add to the intensity of the
turbulence.
Wind Shear
Wind shear is a relatively steep gradient in wind velocity along a given line of direction (either vertical or horizontal) and
produces churning motions (eddies) which result in turbulence. The greater the change of wind speed and/or direction in
the given direction, the greater the shear and associated turbulence.
Clear-air turbulence (CAT), or sudden severe turbulence that occurs in cloudless regions, is associated with wind shear,
particularly between the core of a jet stream and the surrounding air. CAT is not limited to the vicinity of the jet stream and
may occur in isolated regions of the atmosphere. For example, the turbulence in a mountain wave can also be classified as
CAT because the identifying clouds in the wave do not necessarily have to occur for the turbulence to be present.
Sometimes during a climb or descent, a balloon encounters a narrow zone of wind shear with its accompanying turbulence
at the top of a temperature inversion. These inversions occur anywhere from just above the surface to the tropopause.
Strong inversions near the ground are an extreme form of wind shear that adversely affect balloon takeoffs and landings.
For example, a pocket of calm, cold air forms in a valley as a result of nighttime cooling, but the warmer air moving over
it has not been affected appreciably. Due to the difference between the two bodies of air, a narrow layer of very turbulent
air may form. A balloon climbing or descending through this zone will usually encounter considerable turbulence, as well
as changes in lift.
Low Level Wind Shear
Wind shear is a sudden, drastic change in windspeed and/or direction over a very small area. Wind shear can subject a
balloon to violent updrafts and downdrafts, as well as abrupt changes to the horizontal movement of the balloon. While
wind shear can occur at any altitude, low level wind shear is especially hazardous due to the proximity of a balloon to the
ground. Directional wind changes of 180° and speed changes of 50 knots or more are associated with low level wind shear.
Low level wind shear is commonly associated with passing frontal systems, thunderstorms, and temperature inversions
with strong upper level winds (greater than 25 knots).
Wind shear is hazardous to a balloon for several reasons. The rapid changes in wind direction and velocity changes the
wind’s relation to the balloon disrupting the normal flight attitude and performance of the balloon.
Obstructions & Wind
As mentioned earlier, obstructions on the ground affect the flow of wind and can be an unseen danger, causing yet another
atmospheric hazard for pilots. For example, ground topography and large buildings can break up the flow of the wind
and create wind gusts that change rapidly in direction and speed. Obstructions range from manmade structures, such as
hangars, to large natural obstructions, such as mountains, bluffs, or canyons. A safe pilot is vigilant when flying in or out
of launch or landing sites that have large buildings or natural obstructions located near them.
The intensity of the turbulence associated with ground obstructions depends on the size of the obstacle and the primary
velocity of the wind. This can affect the takeoff and landing performance of a balloon, and can present a very serious
hazard. During the landing phase of flight, a balloon may “drop in” due to the turbulent air and be too low to clear obstacles
during the approach. Disrupted airflow often extends horizontally as much as ten times the height of the object, if the
winds are in the eight to ten knot range. Balloon pilots should be aware of this, and make adjustments accordingly when
attempting to launch next to an obstruction, or when landing just past one.
This same condition is even more noticeable when flying in mountainous regions. While the wind flows smoothly up the
windward side of the mountain and the upward currents help to carry an aircraft over the peak of the mountain, the wind on
the leeward side does not act in a similar manner. As the air flows down the leeward side of the mountain, the air follows
the contour of the terrain and is increasingly turbulent. This tends to push an aircraft into the side of a mountain. The
stronger the wind, the greater the downward pressure and turbulence become.
Due to the effect terrain has on the wind in valleys or canyons, downdrafts may be severe. Thus, a prudent balloonist is
well advised to seek out another balloon pilot with mountain flying experience and get a mountain “checkout” before
conducting a flight in or near mountainous terrain.
Mountain Wave
A mountain wave is the wavelike effect, characterized by updrafts and downdrafts, that occurs above and behind a mountain
range when rapidly flowing air encounters the mountain range’s steep front. The characteristics of a typical mountain wave
are represented in Figure 4-22 which illustrates how air flows with relative smoothness in its lifting component as the
wave current moves along the windward side of the mountain. Wind speed gradually increases, reaching a maximum near
the summit. On passing the crest, the flow breaks into a much more complicated pattern, with downdrafts predominating.
Rotor turbulence
Breaking wave
WIND
Figure 4-22. Characteristics of a mountain wave
An indication of the possible intensities in the mountain wave is reflected in verified records of sustained downdrafts
and updrafts in excess of 3,000 feet per minute (fpm). Turbulence in varying degrees can be expected, with particularly
severe turbulence in the lower levels. Proceeding downwind 5 to 10 miles from the summit, the airflow begins to ascend
as part of a definite wave pattern. Additional waves, generally less intense than the primary wave, may form downwind.
This event is much like the series of ripples that form downstream from a rock submerged in a swiftly flowing river. The
distance between successive waves (wavelength) usually ranges from 2 to 10 miles, depending on existing wind speed and
atmospheric stability, although waves up to 20 miles apart have been reported.
Characteristic cloud forms peculiar to wave action provide the best means of identification. Lenticular clouds, formed by
mountain waves, are smooth in contour. [Figure 4-23] These clouds may occur alone or in layers at heights above 20,000
feet MSL, and be quite ragged when the airflow at that level is turbulent. The roll cloud forms at a lower level, generally
near the height of the mountain ridge. The cap cloud must always be avoided in flight because of turbulence, concealed
mountain peaks, and strong downdrafts on the lee slope. The lenticulars, like the roll and cap clouds, are stationary. They
are constantly forming on the windward side and dissipating on the lee side of the mountain wave.
Figure 4-23. Multiple lenticular clouds over Mount Shasta, California.
Rotors
Rotors or eddies can also be found embedded in mountain waves. Formation of rotors can also occur as a result of down
slope winds. Their formation usually occurs where wind speeds change in a wave or where friction slows the wind near the
ground. These rotors are often experienced as gusts or wind shear. Clouds may also form within a rotor.
Research on mountain waves and rotors or eddies continues as these phenomena are quite complex, but there is no doubt
that pilots need to be aware of these phenomena and take appropriate precautions. Although mountain wave activity
is normally forecast, many local factors may affect the formation of rotors and eddies. When planning a flight, a pilot
should take note of the winds and terrain to assess the likelihood of waves and rotors. There may be telltale signs in flight,
including disturbances on water or wheat fields and the formation of clouds, provided there is sufficient humidity to allow
cloud formation.
Thunderstorms
A thunderstorm is a local storm produced by a cumulonimbus cloud and accompanied by lightning and thunder.
Thunderstorms and cumulonimbus clouds contain many of the most severe atmospheric hazards for the balloon pilot.
They are almost always accompanied by strong gusty winds, severe turbulence, heavy rain showers, and lightning. These
hazards may extend well away from the central core of the thunderstorm mass, sometimes as much as 30 miles or more.
Ceilings and visibility in the precipitation areas under the thunderstorms are normally poor. Because of the heavy
precipitation, the ceiling reported is at best an estimate of where the pilot may break out into visual contact with the surface.
The weather observer determines the vertical visibility into the precipitation, which may be significantly different from the
slant-range of the pilot.
Potentially hazardous turbulence, as well as many other hazards associated with thunderstorms, make a thunderstorm a
dangerous weather formation. The safe balloon pilot avoids any conditions that may expose them to potential thunderstorm
activity. They exercise discretion and good judgment when potential thunderstorm conditions exist.
Structure of Thunderstorms
It is important to understand the structure of thunderstorms.
Convective Cells
The fundamental structural element of the thunderstorm is the unit of convective circulation known as a convective cell.
A mature thunderstorm contains one or more of these cells in different stages of development, each varying in diameter
from one to five miles. By radar analysis and measurement of drafts, it has been determined that each cell is generally