Density Altitude: A Review, Plus Much More
There are four important factors that affect air density; Altitude (MSL), Barometric
Pressure, Temperature, and Humidity.
The effect of these factors varies: Temperature has the most significant
influence on DA when compared to other factors. Barometric pressure has the
second greatest influence since Pressure Altitude (PA) is greater than field
elevation anytime the barometric pressure falls BELOW “Standard Pressure”
(29.92). i.e. When the barometric pressure at an air base at 3,000 ft. is 29.2” the
PA is 3,673. It is from THIS number that we can begin to calculate the effects of
temperature to arrive at a Density Altitude.
If we have an aircraft parked at the base, we can determine Pressure Altitude by
setting 29.92 into the Kollsman window of the Altimeter. If we do not have an
altimeter handy, we can determine the barometric pressure from METARs or by
calling one of the 70 Automated Surface Observation Station (ASOS) or
Automated Weather Observation Station (AWOS) phone numbers in
Washington/Oregon.
METARs are the hourly weather reports derived from AWOS and ASOS stations
that are found at the NOAA Aviation Weather website: http://aviationweather.gov/
or in the right margin of any airport’s information at www.airnav.com or at
http://www.aopa.org/airports/ or http://www.navmonster.com/.
Sources for AWOS/ASOS phone numbers include:
The FAA Airport Facility Directory (AFD) available on line at:
http://aeronav.faa.gov/index.asp?xml=aeronav/applications/d_afd or
http://www.faa.gov/air_traffic/weather/asos/
A list of AWOS/ASOS frequencies and phone numbers is included at the end of
this document.
ASOS vs. AWOS: What is the difference?
The basic difference between these two automated weather systems is that:
--ASOS is a product of a National Weather Service (NWS), Department of Defense (DoD) and
Federal Aviation Administration (FAA) joint venture. ASOS is comprised of a standard suite of
weather sensors (with several exceptions) all procured from one contractor.
--AWOS is a suite of weather sensors of many different configurations with different capabilities
that were either procured by the FAA or purchased by individuals, groups, airports, etc. They are
required to meet FAA standards to be able to report standard weather parameters.
ASOS is more sophisticated than AWOS and is designed to provide the necessary information to
generate actual METAR reports and TAF weather forecasts. All ASOS systems can determine
type and intensity of precipitation, thunderstorms, and obstructions to visibility as well as peak
wind speeds, gusts, directional shifts, etc.
Any time barometric pressure is lower than 29.92 inches of Hg, or the
temperature is warmer than “Standard Temperature” for a given elevation, then
Density Altitude will be HIGHER than True Altitude.
What is STANDARD TEMPERATURE?
At Sea Level, and at zero percent humidity, “standard temperature” is the overall
global average as determined by International Organization for Standards (ISO)
and adopted by International Civil Aviation Organization (ICAO), of which the
FAA is a member, for all international scientific purposes and aviation operations.
Standard temperature at sea level is 15C, or 59F.
The Standard Adiabatic Lapse Rate is 2 degrees C (or approximately 3.5 deg. F)
per thousand feet of altitude. This is the “average” rate that the air cools as
altitude increases. The chart below establishes “Standard Temperatures” for
each thousand foot increment of altitude.
10,000 ft.
-5 C
9,000 ft.
-3 C
8,000 ft.
-1 C
7,000 ft.
1C
6,000 ft.
3C
5,000 ft.
5C
4,000 ft.
7C
3,000 ft.
9C
2,000 ft.
11 C
1,000 ft.
13 C
Sea Level
15 C
The Density Altitude tables and calculations are based on the following formula.
To determine Density Altitude, ADD 117.4 ft. to the
current Pressure Altitude for each degree of ambient
temperature greater than “Standard Temperature.”
Example:
You do not have an aircraft or altimeter handy, and you could only make one
phone call (to an ASOS recording) from a remote air strip location. You were
able to locate the field elevation on a map. Determine the Density Altitude using
the following known information and the “117.4 ft.” rule above. Then check your
answer against using the DA graph to compare results.
Field Elevation:
3,015 ft. MSL
(from a GPS or map)
Current Barometric pressure:
29.4 inches Hg
(from ASOS, METAR, etc.)
Current Temperature:
21 deg. C
“
“
Step 1: Determine pressure altitude from the pressure altitude conversion chart.
Since the current barometric pressure is 29.4, add 485 ft. to your field
elevation: 3015 + 485 = 3,500 ft. which is your PRESSURE ALTITUDE.
Step 2: How many degrees warmer than standard temperature is it right now?
From the table on the previous page you determine that “standard
temperature” at 3,500 ft. MSL is: 8 C. The current temperature of 21 C, is
13 degrees C warmer than Standard Temp.
Step 3: Multiply 13 X 117.4 ft to determine the number of feet to add to the
pressure altitude to determine the Density Altitude.
13 X 117.4 = 1,526
(PA) 3,500 ft. + 1,526 ft. = 5,026 ft. Density Altitude
(Hint: It is easier to calculate DA without using a calculator by rounding the 117.4
ft. up to 120 ft. Some pilots just use 100 ft. per deg. C as a rule of thumb, and
then add a 200-300 ft. fudge factor to get a ballpark estimate; 1300 + 200 + PA.)
Plot the same calculation on the chart to see the pencil line on the left edge of
the graph almost exactly intersects the 5,000 ft. mark.
The FAA Airplane Flying Handbook, Pilot’s Handbook of Aeronautical
Knowledge, and Aviation Weather publications as well as the Cessna, Jepessen
and other private pilot study publications define density altitude as, “pressure
altitude corrected for non-standard temperature.” It is simple and correct.
The S-271 Chapter 5 slides, instructor guide and student workbook state that
“Density Altitude is pressure altitude corrected for temperature and
humidity.” Humidity? Where did that come from?
Here lies a problem: The FAA handbooks, pilot training publications,
and aircraft manufacturer Aircraft Flight Manuals (AFM) and Pilot
Operating Handbooks (POH) make NO MENTION of Relative
Humidity, much less provide any sort of charts to factor in the effects
of RH on Density Altitude and aircraft performance! How do we work
with and teach a definition in an industry that has no tables or charts
available to factor in this additional environmental factor? Should we
throw RH out of the S-271 definition and stick with concepts that are
widely accepted by the FAA and the aviation industry? Are the
effects of RH truly miniscule and insignificant or are we missing
something? If we can’t “correct for RH” using the tools we have
available, why should we allow the concept in the S-271 definition
and course materials since it only seems to encourage and create
confusion and uncertainty in the minds of students? We are the 271
Instructors. What should we really tell them about RH? What do WE
need to know and understand?
A web search on the topic reveals the following explanation, and several
automated on-line conversion tools reveal some interesting results!
Humidity and air density
Humidity is not generally considered a major factor in density altitude computations
because the effect of humidity is related to engine power rather than aerodynamic
efficiency. At high ambient temperatures, the atmosphere can retain a high water vapor
content. For example, at 96 degrees F, the water vapor content of the air can be eight
(8) times as great as at 42 degrees F. High density altitude and high humidity do not
often go hand-in-hand.
Most people who haven't studied physics or chemistry find it hard to believe that humid
air is lighter, or less dense, than dry air. One needs to merely look up to the sky for proof
to see clouds of water vapor suspended in the air, at least until they become so
saturated that they begin to precipitate rainfall. If clouds were indeed heavier than air,
they could never form in the sky, and our planet would be barren and lifeless.
To see why humid air is less dense than dry air, we need to turn to one of the laws of
nature the Italian physicist Amadeo Avogadro discovered in the early 1800s. In simple
terms, he found that a fixed volume of gas, say one cubic meter, at the same
temperature and pressure, would always have the same number of molecules no matter
what gas is in the container. Most beginning chemistry books explain how this works.
Imagine a cubic foot of perfectly dry air. It contains about 78% nitrogen molecules, which
each have a molecular weight of 28 (2 atoms with atomic weight 14). Another 21% of the
air is oxygen, with each molecule having a molecular weight of 32 (2 atoms with atomic
weight 16). The final one percent is a mixture of other trace gases.
Molecules are free to move about within our cubic foot of air. What Avogadro discovered
leads us to conclude that, if we added water vapor molecules to our cubic foot of air,
some of the nitrogen and oxygen molecules would leave — remember, the total number
of molecules in our cubic foot of air stays the same.
The water molecules, which replace nitrogen or oxygen, have a molecular weight of 18.
(One oxygen atom has an atomic weight of 16, and two hydrogen atoms each have an
atomic weight of 1.) This is lighter than both nitrogen and oxygen. In other words,
replacing nitrogen and oxygen with water vapor decreases the weight of the air in the
cubic foot; that is, it's density decreases.
Wait a minute, you might say, "I know water's heavier than air." True, liquid water has a
greater density and is heavier than air. But, the water that makes the air humid isn't
liquid. It's water vapor, which is a gas that is lighter than nitrogen or oxygen.
Compared to the differences made by temperature and air pressure, humidity has a
small effect on the air's density. But, humid air is lighter than dry air at the same
temperature and pressure.
Hot/Dry Day: 3,000 ft. elev., 29.92 in.
Temp. 32C (90F)
Elevation
feet
meters
Air Temperature
deg F
deg C
Altimeter Setting
inches Hg
Dew Point
deg F
DP -6C
RH 8%
3000
32
29.92
mb
-6
deg C
Reset
Density Altitude
5648
Absolute Pressure
26.816
Relative Density
84.49
1722
feet
inches Hg
908.09
84.49
%
meters
mb
%
Density Altitude: 5648 ft.
Hot/Humid Day: 3,000 ft. elev., 29.92 in. Temp. 32C (90F) DP 31C
Elevation
feet
meters
Air Temperature
deg F
deg C
Altimeter Setting
inches Hg
Dew Point
deg F
RH 94%
3000
32
29.92
mb
31
deg C
Reset
Density Altitude
6214
Absolute Pressure
26.816
Relative Density
83.05
Density Altitude:
feet
inches Hg
%
1894
908.09
83.05
meters
mb
%
6214 ft.
Conclusion:
ALL OTHER CONDITIONS EQUAL, A DAY WITH 94% RH vs. 8% RH YIELDS
AN INCREASE OF 566 FT. OF DENSITY ALTITUDE AT THE 3,000 FT. ELEV.
Hot/Dry Day: 8,000 ft. elev., 29.92 in.
Elevation
Air Temperature
Temp. 27C (80F)
feet
meters
deg F
deg C
Altimeter Setting
inches Hg
Dew Point
deg F
DP -9C
RH 8%
8000
27
29.92
mb
-9
deg C
Reset
Density Altitude
11162
Absolute Pressure
22.227
Relative Density
71.2
Density Altitude:
3402
feet
inches Hg
71.2
%
11,162 ft.
752.68
meters
mb
%
(3,162 ft. higher than the terrain elevation.)
Hot/Humid Day: 8,000 ft. elev., 29.92 in. Temp. 27C (80F), DP 26C,
Elevation
feet
meters
Air Temperature
deg F
deg C
Altimeter Setting
inches Hg
Dew Point
deg F
RH 94%
8000
27
29.92
mb
26
deg C
Reset
Density Altitude
11649
Absolute Pressure
22.227
Relative Density
70.11
Density Altitude:
11, 649 ft.
feet
inches Hg
%
3551
752.68
70.11
meters
mb
%
(3,649 ft. higher than the terrain elevation.)
Conclusion:
ALL OTHER CONDITIONS EQUAL, A DAY WITH 94% RH vs. 8% RH YIELDS
AN INCREASE OF 487 FT. OF DENSITY ALTITUDE AT THE 8,000 FT. ELEV.
Calculation Table outputs explained:
Input Values:
The elevation (or altitude) is the geometric elevation above mean sea level, and is the elevation at which the
altimeter setting, temperature and dew point have been measured.
The altimeter setting is the value in the altimeter's Kollsman window when the altimeter is set to correctly
read a known elevation. The altimeter setting is generally included in NWS reports. The altimeter setting is
not the same as the sea level corrected barometric pressure.
This calculator uses dew-point rather than relative humidity because the dew point is fairly constant for a
given air mass, while the relative humidity varies greatly as the temperature changes.
Output Values:
The density altitude is the altitude in the International Standard Atmosphere that has the same density as
the air being evaluated.
The absolute air pressure is the actual air pressure, not corrected for altitude, and is also called the station
pressure.
Relative density is the ratio of the actual air density to the standard sea level density, expressed as a
percentage.
The ICAO International Standard Atmosphere standard conditions for zero density altitude are 0 meters (0
feet) altitude, 15 deg C (59 deg F) air temp, 1013.25 mb (29.921 in Hg) pressure and 0 % relative humidity (
absolute zero dew point). The standard sea level air density is 1.225 kg/m 3 (0.002378 slugs/ft3).
Conclusion:
If you factor in the effects of RH using a table not readily known by or available to
pilots, you discover that RH can add an additional 450 to 600 ft. onto any DA
calculation on a humid day. It does not appear to be significantly different on a
day with hot vs. moderately warm temperatures, nor is there much difference on
the effects between aircraft operating at 3,000 and 8,000 ft. elevations. At 3,000
ft. or lower, however, high RH can account for up to 10% of the overall DA.
Some aviators have a habit of ignoring the effects of non-standard pressure as
well as having no knowledge of any effects by RH. If they are at a 6,000 ft.
airport and do not know the current altimeter setting, they often plot the
temperature on the DA Chart against an “assumed” Pressure Altitude that is
about the same as field elevation. On a low pressure day combined with high
humidity, they might be off in their DA estimation by well OVER 1,000 ft.
At the very least, we should use the tools commonly available, and then add in a
safety margin for all imprecise performance estimates. The good news is that
ASOS and AWOS have sensors for dew point and RH. The recordings are
updated every minute and end with a Density Altitude value that probably factors
in the measured dew point/RH values, much like the conversion tool above.
Oregon State ASOS/AWOS frequencies and phone numbers
KAST Astoria
Clatsop
OR 135.375
(503)
ASOS
861-1371
KUAO Aurora
Marion
OR 118.525
(503)
ASOS
678-3011
KBKE Baker City
Baker
OR 134.275
(541)
ASOS
523-5412
KBOK Brookings
Curry
OR 132.025
(541)
AWSS
412-8682
KBNO Burns
Harney
OR 135.575
(541)
ASOS
573-1382
KCVO Corvallis
Benton
OR 135.775
(541)
AWOS III
754-0081
KPDT
Eastern Oregon
Regional - Pendleton
Umatilla
OR 118.325
(541)
ASOS
278-2329
KEUG
Eugene - Mahlon
Sweet Field
Lane
OR
ATIS
125.225
(541)
ASOS
461-3114
K6S2 Florence
Lane
OR 118.225
(541)
AWOS
997-8664 IIIP
K4S1 Gold Beach
Curry
OR 118.15
(541)
AWOS
247-2518 IIIP
KHRI Hermiston
Umatilla
OR 135.225
(541)
ASOS
567-8580
K4S2 Hood River
Hood River OR 134.375
(541)
AWOS
386-2386 IIIP
K5J0
Grant
OR 118.375
(541)
AWOS
575-1122 IIIP/T
KLMT Klamath Falls
Klamath
OR
KLGD La Grande
Union
OR 135.075
(541)
AWOS III
963-6824
KLKV Lakeview
Lake
OR 135.525
(541)
AWOS III
947-5069
K9S9 Lexington
Morrow
OR 134.475
(541)
AWOS
989-8557 IIIP
KMMV Mcminnville
Yamhill
OR 135.675
(503)
ASOS
434-9153
KSLE McNary Field - Salem
Marion
OR
John Day
ATIS
126.5
ATIS
124.55
(541)
ASOS
883-8127
(503)
ASOS
371-1062
KONP Newport
Lincoln
OR 133.9
(541)
AWOS
867-4175 IIIP
KOTH North Bend
Coos
OR 135.075
(541)
AWOS III
756-0135
KONO Ontario
Malheur
OR 135.275
(541)
ASOS
889-7388
KHIO Portland - Hillsboro
Washington OR
KTTD Portland - Troutdale
Multnomah OR 135.625
KPDX Portland International
Multnomah OR
ATIS
127.65
ATIS
128.35
(503)
ASOS
640-2984
(503)
ASOS
492-2887
(503)
ASOS
284-6771
KRDM
Redmond - Roberts
Field
Deschutes OR 119.025
KMFR
Rogue Valley Intn'l Medford
Jackson
OR
KRBG Roseburg
Douglas
OR 135.475
(541)
ASOS
673-1483
KSPB Scappoose Industrial
Columbia
OR 135.875
(503)
ASOS
543-6401
KSXT Sexton Summit
Josephine
OR 118.375
(541)
ASOS
471-1460
KS47 Tillamook
Tillamook
OR 120
(503)
AWOS
842-8792 IIIP
ATIS
127.25
(541)
ASOS
504-8743
(541)
ASOS
776-1238
Washington State ASOS/AWOS frequencies and phone numbers
(360) 435AWOS III
8045
KAWO Arlington
Snohomish WA 135.625
KBLI
Whatcom
WA
KPWT Bremerton National
Kitsap
WA 121.20
(360) 674AWOS III
2811
KBVS Burlington/Skagit
Skagit
WA 121.125
(360) 757AWOS III
7767
Klickitat
WA 135.175
(509) 767ASOS
1726
KDEW Deer Park
Spokane
WA 135.175
(509) 276ASOS
2303
KORS East Sound/Orcas
San Juan
WA 135.425
(360) 376AWOS III
6045
KELN Ellensburg -Bowers Field Kittitas
WA 118.375
(509) 925ASOS
2040
KEPH
Ephrata
Grant
WA 135.775
(509) 754ASOS
3761
KPAE
Everett - Paine Field
Snohomish WA
KDLS
Bellingham
Columbia Gorge/The
Dalles
ATIS
134.45
ATIS
128.65
(360) 671ASOS
8688
(425) 355ASOS
6192
KFHR Friday Harbor
San Juan
WA 135.675
(360) 378ASOS
8491
KHQM Hoquiam - Bowerman
Grays
Harbor
WA 135.775
(360) 538ASOS
7021
KKLS
Cowlitz
WA 135.075
(360) 577AWOS III
1964
KMWH Moses Lake
Grant
WA
KOLM Olympia
Thurston
WA 135.725
(360) 943ASOS
1278
KOMK Omak
Okanogan
WA 118.325
(509) 826ASOS
2655
(509) 886ASOS
4226
Kelso Longview
ATIS
119.05
KEAT
Pangborn Memorial Wenatchee
Douglas
WA 119.925
KPSC
Pasco -Tri Cities
Franklin
WA
KCLM
Port Angeles-William H.
Fairchild
Clallam
WA 135.175
ATIS
125.65
(509) 762ASOS
5082
(509) 547ASOS
7379
(360) 457ASOS
1070
KPUW Pullman/Moscow
Whitman
WA 135.675
(509) 334ASOS
3222
KUIL
Clallam
WA 135.225
(360) 374ASOS
9731
King
WA
ATIS
126.95
(425) 255ASOS
6080
Quillayute
KRNT Renton
KBFI
Seattle - Boeing
Field/King County
King
WA
ATIS
127.75
(206) 763ASOS
6904
KSEA
Seattle - Tacoma
International
King
WA
ATIS
118.0
(206) 431ASOS
2834
KSHN Shelton
Mason
WA 119.275
(360) 427ASOS
3835
KGEG Spokane
Spokane
WA 123.325
(509) 624ASOS
4406
KSFF
Spokane
WA
KSMP Stampede Pass
Kittitas
WA 135.275
KTIW
Pierce
WA
Spokane -Felts Field
Tacoma Narrows
ATIS
120.55
ATIS
124.05
(509) 535ASOS
3290
(360) 886ASOS
2758
(253) 858ASOS
6507
KVUO Vancouver -Pearson Field Clark
WA 135.125
(360) 696ASOS
1280
KALW Walla Walla
Walla
Walla
WA 135.875
(509) 525ASOS
3014
KS52
Winthrop
Okanogan
WA 118.425
(509) 997AWOS III
0142
KFCT
Yakima
Yakima
WA N/A
(509) 577- AWOS
3516
IIIP/T
KYKM Yakima -McAllister Field Yakima
WA
ATIS
125.25
(509) 248ASOS
1502