Density altitude
Encyclopedia
Density altitude is the altitude
Altitude
Altitude or height is defined based on the context in which it is used . As a general definition, altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The reference datum also often varies according to the context...

 in the International Standard Atmosphere
International Standard Atmosphere
The International Standard Atmosphere is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes. It has been established to provide a common reference for temperature and pressure and consists of tables of...

 at which the air density
Density of air
The density of air, ρ , is the mass per unit volume of Earth's atmosphere, and is a useful value in aeronautics and other sciences. Air density decreases with increasing altitude, as does air pressure. It also changes with variances in temperature or humidity...

 would be equal to the actual air density at the place of observation, or, in other words, the height when measured in terms of the density of the air rather than the distance from the ground. "Density Altitude" is the pressure altitude
Pressure altitude
In aviation, pressure altitude is the indicated altitude when an altimeter is set to an agreed baseline pressure setting. The baseline pressure is 1013.25 hPa, equivalent to 1013.25 millibar, or 29.92 inches of mercury. This setting is equivalent to the air pressure at mean sea level in the...

 adjusted for non-standard temperature.

Both an increase in temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 and, to a much lesser degree, humidity
Humidity
Humidity is a term for the amount of water vapor in the air, and can refer to any one of several measurements of humidity. Formally, humid air is not "moist air" but a mixture of water vapor and other constituents of air, and humidity is defined in terms of the water content of this mixture,...

 will cause an increase in density altitude. Thus, in hot and humid conditions, the density altitude at a particular location may be significantly higher than the true altitude.

Aircraft safety

Air density is perhaps the single most important factor affecting aircraft performance. It has a direct bearing on:
  • The lift
    Lift (force)
    A fluid flowing past the surface of a body exerts a surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction...

     generated by the wings — reduction in air density reduces the wing's lift.
  • The efficiency of the propeller or rotor — which for a propeller (effectively an airfoil
    Airfoil
    An airfoil or aerofoil is the shape of a wing or blade or sail as seen in cross-section....

    ) behaves similarly to lift on wings.
  • The power output of the engine — power output depends on oxygen intake, so the engine output is reduced as the equivalent "dry air" density decreases and produces even less power as moisture displaces oxygen in more humid conditions.


Aircraft taking off from a "hot and high
Hot and high
In aviation, hot and high is a condition of low air density due to high ambient temperature and high airport elevation. Air density decreases with increasing temperature and altitude. Lower air density reduces the amount of lift generated by the wings or the rotors of an aircraft, which may hamper...

" airport such as the Quito Airport
Mariscal Sucre International Airport
Mariscal Sucre International Airport is the international airport serving Quito, the capital city of Ecuador. It is named after Venezuelan born Antonio José de Sucre, known as the "Gran Mariscal de Ayacucho" , a hero of Ecuadorian and Latin American independence...

 or Mexico City
Mexico City International Airport
Benito Juárez International Airport , in Venustiano Carranza, one of the sixteen boroughs into which Mexico's Federal District is divided, is a commercial airport that serves Mexico City, the capital of Mexico...

 are at a significant aerodynamic disadvantage. The following effects result from a density altitude which is higher than the actual physical altitude:
  • The aircraft will accelerate slower on takeoff as a result of reduced power production.
  • The aircraft will need to achieve a higher true airspeed to attain the same lift - this implies both a longer takeoff roll and a higher true airspeed which must be maintained when airborne to avoid stalling
    Stall (flight)
    In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded...

    .
  • The aircraft will climb slower as the result of reduced power production and lift.


Due to these performance issues, a plane's takeoff weight may need to be lowered or takeoffs may need to be scheduled for cooler times of the day. Wind direction and runway
Runway
According to ICAO a runway is a "defined rectangular area on a land aerodrome prepared for the landing and take-off of aircraft." Runways may be a man-made surface or a natural surface .- Orientation and dimensions :Runways are named by a number between 01 and 36, which is generally one tenth...

 slope may need to be taken into account.

Calculation

Density altitude can be calculated from atmospheric pressure and temperature (assuming dry air).


where
density altitude in feet
atmospheric (static) pressure
standard sea level atmospheric pressure (1013.25 hPa ISA
International Standard Atmosphere
The International Standard Atmosphere is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes. It has been established to provide a common reference for temperature and pressure and consists of tables of...

 or 29.92126 US))
true (static) air temperature
Outside air temperature
In aviation terminology, the outside air temperature or static air temperature refers to the temperature of the air around an aircraft, but unaffected by the passage of the aircraft through it.-Aviation usage:...

 in kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

s (K) [add 273.15 to the Celsius (°C)] figure
ISA
International Standard Atmosphere
The International Standard Atmosphere is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes. It has been established to provide a common reference for temperature and pressure and consists of tables of...

 standard sea level air temperature in kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...

s (K) (288.15 K)
0.234969


Note that the leading coefficient is derived from the ratio of the ISA
International Standard Atmosphere
The International Standard Atmosphere is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes. It has been established to provide a common reference for temperature and pressure and consists of tables of...

 sea level temperature (288.15 K) to ISA
International Standard Atmosphere
The International Standard Atmosphere is an atmospheric model of how the pressure, temperature, density, and viscosity of the Earth's atmosphere change over a wide range of altitudes. It has been established to provide a common reference for temperature and pressure and consists of tables of...

 Temperature Lapse Rate (6.5 K/km), with the product converted into feet.

National Weather Service Equation

The National Weather Service
National Weather Service
The National Weather Service , once known as the Weather Bureau, is one of the six scientific agencies that make up the National Oceanic and Atmospheric Administration of the United States government...

 uses the following dry-air approximation of the above equation in their standards.


where
density altitude in feet
Is the station pressure (atmospheric static pressure) in inches of mercury (inHg)
T is the station temperature (atmospheric temperature) in Fahrenheit (F)


Note that the NWS standard specifies that the density altitude should be rounded to the nearest 100 feet.

Easy formula to calculate density altitude from pressure altitude

This is an easier formula to calculate (with great approximation) density altitude from pressure altitude ..and International Standard Atmosphere temperature deviation
Density altitude in feet = pressure altitude in feet + (120 x (OAT - ISA_temperature))


Where:
OAT = Outside air temperature
Outside air temperature
In aviation terminology, the outside air temperature or static air temperature refers to the temperature of the air around an aircraft, but unaffected by the passage of the aircraft through it.-Aviation usage:...

in °C
ISA_temperature = 15 °C - (1.98 °C/1000 ft × actual altitude in thousands of feet)

External links

The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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