Balloon Life,July 2000

20

Ever notice thedifferencein the performance of a balloon on cool
days versus its performance on warm days? True, the hotter the
ambienttemperature,themoreheatyouwillneedtogenerate
inside the balloon, but what is really happening?
If you fly in varying terrain—such as a valley at sea level or
intheRockyMountains—younoticeadifferenceinballoon
performance.The changeinaltituderesultsina change inthe
densityoftheair.Thus,athigheraltitudes,thelessdenseair
requires more heat. The balloon will also take longer to respond.
Whatcauses your balloontoresponddifferentlyis that the
relative altitudeof your favorite launch siteis going up and down.
Sure,yousetthesamemeansealevel(MSL) altitudeonthe
altimeter before every flight, and you knowof no unusual ground
movement.;yet,therelative altitudeof thefieldrisesandde-
scendslike an oceantide.
Changesinairdensityarecausedbyvariationsinatmo-
sphericpressure andtemperature. Changesinthe watervapor
contentalsoaffectthedensityoftheair,buttheamountis
negligible (The maximum altitude change is no more than +250
feet at a dew point temperature of 100ºFahrenheit). The perfor-
mance of a balloon is greatly affected by the varying densities of
the atmosphere.
Densityaltitudeisdefinedasthepressurecorrectedfor
temperature. Pressure altitude is the distance measured from the
29.92-inch pressure level (the standard datum plane). The theo-
retical performance of your aerostat, found inthe flight manual
for your balloon, is based on standard atmospheric conditions. In
actual practice, these conditionsare usuallynot encountered.
Alaunch field may have a density altitudethat varies several
thousand feet from the mean sea level elevation of the field. Ifthe
density altitude is higher than standard for the field, this field has
a high (+) densityaltitude. For example, if at a launch field with
a 5,000 foot (MSL) and a density altitude of 10,000 feet, balloons
would be flyingin air of the same densitynormally foundin the
standard atmosphere at 10,000 feet. The efficiency of the balloon
maybeseriouslyaffectedinhighdensityaltitudes,especially
whenthe balloon is critically loaded.
If thedensityaltitude islower(-) thannormal for a given
altitude,theefficiencyoftheballoonisincreased.Apilot
operating from a field at 5,000 feet with a density altitudeof 1,000
will bein the same density of air that normally exists at 1,000 feet.
The densityaltitude usually varies throughout the day with
movementof pressuresystems,diurnalheating, andnocturnal
cooling. The highest density altitudes are most common during

the warmest hours. Air density decreases and creates a higher
density altitude if there are increases in altitude, temperature, or

by Tom Hamilton

Density altitude: The yardstick to measure performance
in warmer temperatures and higher elevations.

Thin Air

Density Altitude Chart: To determine density altitude, find
intersection of pressure altitude (diagonal lines) and temperature
(vertical lines). Draw a horizontal line to intersect density altitude
on left hand vertical axis.

21

Balloon Life,July2000

The first step in computing density altitude is to determine the pressure altitude by setting 29.92 in the Kollsman window
of the balloonaltimeter, if equipment is anolder type, or bydialing in 29.92 in the digital display of the altimeter of the
Ballor other electronic instrument pack. The indicatedaltitude isthe pressure altitude.
The second step in computing density altitude is to determine the effect ofthe actual air temperature on the airdensity.
The standard temperature of the atmosphere is 15ºC (59ºF) at sea level with a decrease of 2ºC (3.5ºF) per 1,000 feet (This
is the standard temperature lapse rate.). Each degree Celsius variation from the standard temperature changes the density
altitude approximately 120 feet. If the actual temperature is below standard for the pressure altitude, the density altitude
islowered; if the temperature is above standardfor the pressure altitude,the densityaltitude is raised.
Temperature variation is incorporated into a formula for obtaining density altitude from a known pressure altitude:

PA + (120X V)= DA

Where PA is pressure altitude;
120 is the temperature constant;
V isthe variation of the actual temperature from standardatthe pressure altitude; and
DA isthe densityaltitude.

moisture content. Anychange inair densityaffectstheperfor-
manceof a balloon.Itisnotuncommonduringsummer after-
noons in the southeast for launch fields withanelevationof 40
feet MSL tohave a density altitude of 5,000feet.
Recognizingthesechangesandcalculatingyourdensity
altitude will provide a valuable tool for anticipating the perfor-

mance of the balloon. Nearby is an explanation for computing
density altitude and an approximate density altitude chart. Work
through some examples and add another workhorse to your
arsenal of information. Like most potentially threatening avia-
tion hazards, the effects of density altitude can be handled
through pilot awareness of and respect for the phenomenon.