Isa Environmental Lapse Rate

[utopia]

What is the provenance of the 6.5 K/km environmental lapse rate in the troposphere under ISA?

[johnholmes]

what do you mean 'provenance' ?

[utopia]

what do you mean 'provenance' ?

Origin / source. Can the number be produced by an equation such as can be done for adiabatic and pseudoadiabatic lapse rates? All I can find is a wiki article stating it is an average chosen for the ICAO ISA model.

[johnholmes]

only just in will try and answer tomorrow

[Rob]

somebody please explain

[johnholmes]

will be in my reply tomorrow

[johnholmes]

The ISA and what it means

First ISA=International Standard Atmosphere and was, in the main, set up by ICAO=The International Civil Aviation Organisation.

One of its chief aims is to enable a set of rules to allow aircraft to fly safely from A to B in spite of the fact that air temperatures change by quite large amounts in the distance travelled. Aircraft fly on altimeters which are dependent on pressure which is dependent on temperature. The temperature, at say 30,000ft, being the temperature of the column of air from the surface to that height. It will NOT be constant, thus an aircraft flying from an area of ‘cold’ air to one of ‘warm’ air will not fly ‘level’. Perhaps it might be easier to change the idea of temperature to one of pressure. Flying from an area of low pressure at the surface to an area of high pressure at the surface at a constant height of, say 30,000ft, can ONLY be done IF the aircraft seems to fly uphill? This might seem odd but is at the base of all the ISA idea.

Pressure falls more quickly with height in cold air than it does in warm air.

ISA is given as, at mean sea level (msl) an air temperature of 15C, pressure of 1013.25 millibars, and a density of 1225 grammes per cubic metre; the rate of fall of temperature with height is taken as 6.5C per kilometre (1.98C per 1000ft) up to 11 km (36,090 feet) and from there to 20 Km (65,617 feet) is assumed constant at -56.5C. Changes occur above this which I am not quoting but the full text can be found below

http://en.wikipedia.org/wiki/International...dard_Atmosphere

although Wilkipedia is not always my favourite source of definitions.

In your original question ‘Origin / source. Can the number be produced by an equation such as can be done for adiabatic and pseudoadiabatic lapse rates? All I can find is a wiki article stating it is an average chosen for the ICAO ISA model.

If you wish to become very involved then yes there are technical papers which will discuss this if you ‘Google’ but essentially the system is an arbitrary set of definitions, partly for flight safety as I’ve tried to explain above, partly to enable calibration of altimeters and to assess, in the testing stage, the performance of aircraft.

I hope this answers your query and for anyone else that wondered what ISA meant?

[chionomaniac]

I didn't know what ISA stood for but do now, so thanks John.

[utopia]

John thanks for the reply. If you have a technical publication on precisely how the environmental lapse rate (-6.5 K/km) is derived I'd be very grateful.

I don't have a copy of ISO 2533:1975 or ICAO Doc 7488 but the freely available US Standard Atmosphere 1976 appear not to explain the figure (obviously all the usual hydrostatic and hypsometric equations are there but I can't see how the 6.5 K/km comes into existence).

Thanks.

[johnholmes]

no time tonight but I'll have a search round in my old paper files from Met days and try other sources

[johnholmes]

unable to find any references at the moment but I think you are getting a bit 'wound' up with no need about the temperature.

Its a STANDARD, or average, just the same as the average/standard msl pressure is taken to be 1013.25mb or the temperature at msl is 15C.

Once you have a standard then the deviations from it allow calibrations to be done respective to that standard.

it allows aircraft to fly safely as ALL aircraft fly with that STANDARD as the base line.

hope that helps you or anyone else. If I find a readable reference then I'll post it.

Edited October 8, 2008 by johnholmes