BalloonLife,
May 1999
Wind
APh.D.influiddynamicsisnot
needed to understand the behaviorof wind.
Togetabetterideahow air behavesin
motion,standonabridgeoverasmall
stream. As youwatchthe water pass un-
derneath,observethechangesthattake
place inits flow as the water encounters
obstacles.Thesedisturbancescreateed-
dies and changes in speed. The same thing
happenswith air.
All weather phenomena, is caused by
onefactor—thesun.Asthesun’srays
reach the earth’s surface, drastic changes
occur accordingtothe absorption of heat
at different points on the surface. This rate
of absorption is dependent onthe earth’s
topographicalmakeup.Theearth’ssur-
face will either absorb or reflect much of
thesun’sradiantenergy.Whenthese
eventsoccur,differentpressurepatterns
areformedbydifferentheating, andthe
phenomena of wind isborn.
Inordertohavewind there must be
convection—themovementoflighter,
warmer air over cooler, denser air.Con-
vection occurs when the earth’s surface is
heatedandtheairaboveitbecomes
warmer.Thiswarmerairexpandsand
becomeslighterorlessdense.Thisair
rises,spreads, andcools;eventuallyde-
scendstocomplete the convective circu-
lation. This convective current—rising of
the warmer air and sinking of cooler air—
willcontinueaslongasthereisuneven
heating of the earth’ssurface.
Fromthis,highandlowpressure
areas are formed. Nature, in a never-end-
ingattempttoreachequilibrium,sends
pressurefromahighpressureareatoa
low pressure area in theform of a horizon-
talconvectivecurrentwecallwind.For
you technical buffs the horizontal move-
ment of air or windis known as “advec-
tion.”
Thus,therearetwotypesofwind:
those causedby pressurechangesor ad-
vection, andthose causedby thermal ac-
tivityorconvection.Thespeedofthe
windorairflowisdeterminedbythe
pressure differences. These create a force
known as “pressure gradient force.” The
stronger the force, the stronger the wind.
A moving mass of air tends to travel
ina straight line until acteduponbyan
outsideforce.Forthepurposesofour
discussion,theoutsideforceisanob-
stacle whichimpedesthe flow of air.As
the air flows around these obstacles—like
ourstreammovingaroundrocks—ed-
dies, whirls, androtorsare created. Dis-
turbance of the air flow,whether caused
by mechanical means like objects or ther-
malactivity,createstheturbulenceof
gustsandlulls.
AirflowAroundObstacles
A few more definitions. The surface
boundarylayer isthe area closest tothe
ground.Dependinguponwhousesthe
term, this layer mightbe anywhere from
300 to1,500feetabovetheground. The
rougher the terrain, the higher the surface
boundarylayer.Forexample,overflat
terrain,thelayerwouldbeat300feet,
while over mountainous terrain, the layer
wouldbe ofagreater heightbecause of
the topography’sinfluence.
Above the surface boundarylayer is
thefrictionlayer.Thislayeriswhere
laminarairflow,orsmoothair,andthe
surface boundary layer meet.
Becauseofobstacles,airdoesnot
flowsmoothlyallthetimeitgenerates
turbulent pockets onthe downwindside.
Three primary factors that determine
theextentofturbulencebeyondanob-
stacle are the obstacle’s dimensions, per-
meability,andthespeedoftheair.In
additiontoairspeed,theangleatwhich
thewindstrikestheobstacleisalsoa
factorusedtocalculatethedegreeof
turbulence.
The dimensions of the obstacle have
obvious importance. Alarge obstacle will
producemoreturbulencethanasmall
one, if allother conditionsarethesame.
Height,width,depth,andsurfaceofan
obstacle also influence the flow of air. Air
moving over large obstacles will acceler-
ate and form large vertical eddies that spin
endover end.
Manystudiesinfluiddynamicsare
basedonthestandardthatthe objecten-
counteredissolid. While this applies for
objectssuchashills,mountains,build-
ings, and other solidbarriers, these prin-
ciplesdonottranslate topermeable ob-
jects such as trees, hedges or the surfaceof
alake.Permeabilityoftheseobstacles
influence the degree of air turbulence and
thedistancedownwindfromtheobject
that it occurs. This information is factored
intocriteriaforlaunchandlandingsite
selectiontodetermine the neededclear-
ances from any obstacles in order to avoid
turbulence.
Airspeedhasa directimpact onthe
amount of turbulence found downwind of
an obstacle. If abreeze is 3 mph one would
experiencelittleturbulencewhengoing
arounda solidobjectsuchasa house or
moving over a hill. However, if there is a
15 mph breeze, a great deal of turbulence
canbeexpectedontheleesideofthe
buildingora strongrotor canbeantici-
pated onthe lee side of a hill. Even small
changes in wind speed can increase turbu-
lence.Thestrongertheflowofairthe
greater margin ofclearance isnecessary
tosafelycleartheobstacleandnotbe
caughtin an eddyor rotor.
by TomHamilton
BalloonLife,May1999
and the air outside, or by the flow of air
over the top of the balloon.
In your trainingyou were toldabout
the effects of air flowing over the top of
the envelope. While the balloon is static
on the ground, the shape of the top forces
the flow of air to compress over the top,
creating alow pressure area.This low
pressure area creates lift the same way an
airplane wing does.
As the balloon takes off and acceler-
ates to the speed of the air mass thisflow
of air over the top diminishes. Thus any
liftcreatedby the eventisno longer avail-
able.If you were at equilibrium before
take off and have notchanged the density
of theair insideyour balloon, you willnot
have sufficient lift to stay in the air be-
cause you have lost part of your lift.
Let’s take a lookat whythishappens.
While you are static on the ground there
are two lift forces at work. The first isthe
heating of theair insidethe envelope. The
second is the flow of air over the top. The
liftfrom each of these elementscombines
tocreatethe liftnecessary tobe at equilib-
rium. With a littlemore heatyou are able
to rise.
What youhave beentaught isthatthe
air flow over the top is called false lift
because it was not created by applying
heat to the envelope. This is correct to a
point. The lift is real, and as long as the
speedoftheairflowandtheballoon
remain sufficiently different, the lift will
continue to be generated. This is where
the problemarises. It’s like going to a
Chinese restaurant for dinner, the food is
great and fills you up, but an hour later
you’re hungryagain. The same thinghap-
penshere, only sooner. As youaccelerate,
you lose the air flow lift and must add
more heat to maintain the samerate of
ascent. (The same holds true at equilib-
riumorin a constant rate ofdescent.)
There is nothing false about this lift. It is
real. Itjust doesn’t stay with you and you
need to replaceit with something else,
like heat.
Thesamething cantakeplacein
flight with a low-level jet stream. If you
are flyingalongandthe top of the balloon
is in a faster moving layer of air, you are
at equilibrium with two components of
lift—heat and air flow. You would like to
descend so you allow the balloon to cool
orventtobegina descent. Asthe topofthe
balloon moves out of the faster moving
air, you lose one of your lift components.
This, combined with the descent you ini-
tiated, accelerates your descent. You are
now moving much faster than planned.
You apply heat, but if the balloon is in
close proximitytothe ground, there is not
sufficienttime torecover. Pow!You plant
one. Backinto theair yougo, a–swingin’.
You’retotallyembarrassedandwon-
der what iswrongwithyourpilotingskills
today. If youare notaware of the presence
of low-level wind shear, you will prob-
ably pass it off as sloppy flying. And this
doesn’thave tohappenclosetotheground,
it can easily happen in a shear at 3,000 feet,
only you recover before hitting the dirt.
Thesecondwayinwhichawind
shear can increase your rate of descent is
by diminishing the capacity of your enve-
lope. Our topic ofdiscussion here is about
getting pulled down, but again this situa-
tion can happen just as easily in the oppo-
site direction.
Flying along, you choose a landing
spot and begin to descend. Below you is a
low level wind shear with the air near the
surface moving much faster than the air
mass you are in. As you enter the lower air
mass it is like getting hit witha sucker
punch. The shear knocks in the side of the
envelope, decreasesthe capacityof the
envelope and pushes theairout the mouth.
The greater the differencebetweenthe
winds of the two air masses, the greater
the effect.
Nowyou no longer havethe lift capa-
bilityyou had afew secondsago. You
begin heating, but again, there isn’t suffi-
cient time before you smack the ground
and recoil into the air, a–swingin’.
This can also happen when traveling
from a faster moving air mass to a slower
movingairmass.Thistimeinsteadof
getting hit in the back, the envelope is hit
in the front by the wall of slower moving
air. The effect is the same. Again, this can
happen at any altitude.
Twootherelementsthatwillalso
increasedescent rates when going through
a wind shear are dynamic low pressure
anddownwardlift. Bothhave minimal
effect on the balloon but do exist.
Dynamic low pressure results when a
fast-moving layer of air passes over the
mouth of the balloon. The air movement
can cause the air to be sucked out of the
envelope. It is like the diminished capac-
ity discussed above. As the airmoves over
the mouthof the balloon, it creates the
dynamiclowpressurewhichpullsthe
static air inside the envelope out.
Downward lift is the sameas the false
lift discussed earlier, only now it is hap-
pening to another part of the balloon, and
the lift generated has a downward compo-
nent.
If we were to take a balloon, place it
in a wind tunnel and blow smoke by it, we
would notice that the air is compressed
andforcedallaroundtheballoon.We
alreadyknowwhathappenswhenthis
occurs at the top of the balloon—lift with
an upward component. The same events
are taking place over the entire surface of
When the top part of the
envelop e is in a faster
moving layer of a ir the
venturi effect over the
top of the balloo n gen-
erates lift.
BalloonLife,May1999
the balloon. What happens if this wind is
only over the bottom portion of the bal-
loon, below the equator? Lift is being
created, butthis time there isa downward
component to the lift. The lower half of
the balloon enters the faster moving air
massand thesurface ofthe ballooncauses
the air flow to generate lift. This lift is
tangent to the surface of the balloon and,
because it is below the equator where the
tangent line points in a downward direc-
tion, the lift has a downward component.
This liftpulls the balloon down. Because
there is less skintension onthe lower half
ofthe balloon, theresultsare notasdrastic
as when air flows over the top.
Shears
Windshear isa change inwindspeed
and/ordirection overashort distance,
whichresults in atearing orshearing
action. Thisshear creates eddies between
the wind currents and is described as the
shearzone.Sincewindshearmaybe
associated with either a change in wind
directionor speed, or both, itmay occurat
any altitude and can be vertical as wellas
horizontal. Shears are of most concern to
the balloon pilot when these occur close
tothe surface. They are of particular con-
cernduringthe takeoffandlandingphases
offlight.However,dependingon therela-
tive strength oftheshears thesewind
speed changes can be hazardous at any
altitude.
Fatalballoonaccidentshaveoccurred
because ofwind shears.Manyinjuries
and inadvertent obstruction contacts oc-
curredduringtakeoffandlandingcanalso
beattributedto thisshear effect. It is
imperative that the balloon pilot acquire
the knowledge, skill, and understanding
of how and when these shears occur and
how to fly them.
Although we frequently think of the
possibilityof shearswhen there are major
shiftsinwindvelocityanddirectioninthe
winds aloft forecast, we should also be
thinking ofwind shear potential under
numerous other situations. The list, near
by, lists many of these conditions which
have thepotentialtoproduce windshears.
Thereareseveralphenomenathat
produce low-level wind shear. These in-
cludefronts; lowleveljetsassociated
with temperatureinversions; funneling
winds; mountain waves; and microburst
downdraftsfrom thunderstorms,raincol-
Situations with Wind
Shear Potential
Radiation Inversions
Nocturnal Jet
Fronts
High Altitude Jet
Convection/Thermals
Sea and Land Breezes
Mountain Waves
Canyon Funneling
Obstruction Blocking
Heavy Aircraft Vortices
Virga
Thunderstorms and Gust
Fronts
Microbursts
Afront between two dry, stableairmasses
can be devoid of clouds, but abrupt wind
changes in the frontal zone can cause
severe wind shear conditions. (Remem-
ber there is always a wind shift at frontal
passage.) Fast-moving fronts with tem-
perature differentials of at least 10ºF have
the most severe shears, but balloonists
should beconcerned with any frontal passage.
Low level wind shears occur on the
surface after the cold front passes. Be-
cause cold fronts have a greater slope and
usually move faster than warm fronts,
low-level, cold front wind shear duration
BalloonLife,May1999
and land/seabreezes.Variations of10
knotsor more inthesustainedwindspeed,
orstrongsurface windsblowingpastbuild-
ings,terrainobstructionsor treescan pro-
duce localized shears. This type of shear
can be particularly hazardous to balloon
flight. Due to temperature inversions as
well as obstruction phenomenon, pilots
commonly experience a wind shear just
above tree-top level.
Landandsea breezesoccurnear large
lakes, bays, or ocean shores. As you re-
member, air flow to or from the water is
causedby differentialheatingand cooling
of land and water surfaces. These breezes
are mini frontsas deep as 2,000 feet AGL
and can penetrate the land areaaround
them as far inland 15 to 20 miles. This
condition is most prevalent by late after-
noon.Landbreezesareless deepand
intense, and are formed at night. How-
ever,landbreezescaninfluenceearly
morning balloon flights over large lakes
and over other largebodies of water—
especiallythose low splash and dash, and
low-level flights near the shoreline.
loon,furthercausing asignificantde-
crease in lift.
Due to internal pressures on the en-
velope, a lightly loaded balloon will dis-
tort more than will a heavily loaded bal-
loonwhen itencounters a shear or gust. It
is the strength of the internal pressure in
the envelopewhich offers theprimary
resistancetoenvelopedistortionfrom
those windforces.Excessive envelope
temperaturewill weaken thefabric,so
watch your loadandtemperaturechartsin
your aircraft flight manual.Thereis a
definite trade off in safety here.
In addition to internal heat loss, the
dead air layer atthe surface of the balloon
actsasaninsulating layer.Whenthis
layer is blown away by a gust or sheer,
rapid heat loss occurs. The net result is
further cooling and significant decrease
in lift.
When descending into a shear layer,
the samephenomenon occurs with the
distortion and cooling, but the distortion
originateson thelowerportion ofthe
envelope.Negativeliftpressuresalso
come intoplay and false heavyaddstothe
pilot’sproblem of controllingthedescent.
Becauseof these thin shear layers, and
particularly with temperature inversions,
theflightprofilelookslike a cork bobbing
onthe ocean or likea spacecraftbouncing
off the atmosphere. The craft’sangle and
rates of penetrationupon entry are critical.
A serious hazard for balloonflightis
enteringa veryheavy shear or gust. These
can displace the vertical axis of the bal-
loonby asmuchas30 to40 degrees. Ifthe
pilot is operating the burner at that exact
momentand does not or can not compen-
sate the angle of the flame relative to the
angle of the envelope, severe structural
damage can occur with probable disas-
trousoutcomes. Newersystemswithburn-
ers which gimbal areless prone to this
problem only if the pilot is observant of
the envelopeandflameposition atall
times and uses the burner gimbal appro-
priately.
There is a common misconception
among many balloon pilots that the best
way toflyshears or inversionsis topunch
through these withhigh rates of climb or
descent. It should be obvious toyou from
the previous discussion that thebetter
wayto handletheseshearlevelsis to
make shallow entry angles withmoderate
ratesof climbor descent. The strongerthe
shear, the easier and slower your entry
should be.
When in turbulence and shears, be
extremely alert. Be ready to shut off the
burner (blastvalve)immediatelyandkeep
awatchfuleye up atyour envelope aswell
as outand around you. Know the limitsof
your burner gimbal. When ataltitudeit is
rarelyurgenttomaintainequilibrium.
Therefore, don’t operate the burner when
heavyenvelope distortion occurs.You
mayseveralhundredfeetof altitudewhile
your envelope reshapes, but then you can
arrest your descent in the normal manner
without serious damage to the structure.
Vertical shears are occasionally en-
counteredwithstrongcold air downdrafts
or from thermals and mountain waves.
These tend to flatten the vertical size of
the envelope, again reducing the volume
and lift. The amount of distortion is usu-
ally more visible to an observer on the
ground or from another balloon than to
theoccupantofthedistortedballoon.Other
than worryingaboutthelength ofthe
flame,the pilot does not have as much
concern for potential structural damage.
Tops of balloons have been pushed down
several feet by these pressures,but the
balloon usually regains its shape quickly
asthe pilot brings thetemperatureup
aboveequilibrium.Beawarethatyou
may still be descending at arapid rate
even though you are heating. Be careful
not to go over redline in this situation. It
won’thelpthatmuchandyoucould
weaken the fabric to the point of failure.
These you may just have to ride out.
Envelopes withhigh numbers of op-
erating hoursand/orahistoryofhigh
operating temperatures should be flown
moreconservativelyin turbulenceand
shear.Youmaywanttohavea lighterload
eventhough thedistortion potential is
greater, the pressure on the fabric is less.
When fastersurface windsexist,plan
ahead. Use gradual descent rates and try
to get stabilized at equilibrium at as low
an altitude as possible well before reach-
ing the landing point.
One finalthought:be a studentof the
various wind conditionswhich can cause
shears. When shears are likely to affect
your flight, use common sense and good
judgement in making go-no/go decision.
If you do get caught by ashear,don’t
panic, use your skills.
