Altimeter

[redblue]

Can someone explain the difference between Isothermal Atmosphere and International Standard Atmosphere?

[Mad (Flt) Scientist]

The isothermal atmosphere assumes that the temperature in the atmosphere is invariant with height. [hence 'isothermal', just like 'isobars' are lines of equal (iso) pressure (bar)]

The ISA is an atmospheric model with a defined temperature profile, not a constant temperature, and is a better model of the real atmosphere.

[hercboy]

this is taken out of Bob taits BAK theroy book,

" the ISA is a set of atmospheric conditions which have been accepted as representing the average or standard conditions for the whole earth. Sea-level pressure in ISA is 1013 hectopacals (hPa), while sea-level temperture is 15 degrees C"

and you would use the ISA if you had to figure out the pressure hight. the equations pretty easy

(1013- QNH) X 30 + the height of the airfield. and thats how you figgure out the pressure hight (in feet) for your P charts

[enicalyth]

ISA is simply an agreement that best expresses the average atmosphere.

Isothermal means "equal temperature (with height)".

The first isothermal region is defined in ISA terms way up at 11-20 kilometres above the ground. It starts at the "tropopause". Way way up beyond that are two more isothermal regions I can think of starting at the stratopause and the mesopause.

So you live in the troposhere where the weather is, above 11km you work in the stratosphere. A few work in the mesosphere. Dan Dare and Digby work beyond that.

But that'll do for now.

[john_tullamarine]

Important to keep in mind that any "standard atmosphere" is not intended to be a strict representation of the atmosphere as such ... rather an idealised set of numbers which, coincidentally, may bear some relationship to what might be seen in the real atmosphere from time to time in particular places.

As we all understand, the real atmosphere varies regularly and significantly from the ISA idealisation. ISA, if my recollection is correct, is based on North American continental met records from many years ago.

It is worth noting that there is a number of such standard atmospheres defined and in use within the test fraternity.

Two considerations to observe ..

(a) for test or certification work, one might want to use a particular standard atmosphere to suit a defined requirement. This is more likely to be seen in the military world.

(b) for civil work, the main significance of the ISA is that it provides a reference atmosphere for resolving actual test data and comparing aircraft on an apples vs apples basis