ISA lapse rate
 

I'm wondering if anybody here knows why the ISA temperature lapse rate is set as a standard at -1.98 degrees C per 1000 feet?

The ISA is said to be made up of dry air but the dry adiabatic lapse rate is -3 degrees C per 1000 feet. Does anybody know the reason for the discrepancy?

The temperature in the ISA reduces with altitude at a rate of 1.98C per 1000ft.
The air is not moving up or down; it is stationary. If an observer travels up within the atmosphere, he/she will experience colder temperatures as the altitude increases.

If a parcel of air is lifted through the atmosphere, the parcel will expand and cool. The rate of cooling depends on the humidity of the parcel. These rates of cooling are known as lapse rates and have nothing to do with the ISA model of temperature reduction of the atmosphere itself.

The resulting temperature of the parcel at its new higher altitude may differ from the temperature of the surrounding atmosphere. If the parcel is warmer than its surroundings, it will continue to rise. If the parcel is colder, it will want to descend.

I think Suade is confusing the two concepts:
1. The reduction in temperature with altitude of the stationary atmosphere; and
2. The cooling of a parcel of air as it is lifted within the atmosphere.

+1 to what eckard said, you're confusing two different things.

Another way of describing it would be a "thought" experiment..

If you took a thermometer and simply moved it vertically upwards through the air in a static atmosphere (no wind, no updrafts/down drafts) in an ideal world you would see the temperature drop at the ISA rate ( until you reach the top of the tropopause). The thermometer is measuring the temperature of different molecules as it moves upwards.

OTOH if, somehow, you could tie the thermometer inside the well known imaginary parcel/ballon of air at sea level, and then displace the parcel of air itself upwards the air whose temperature you are measuring is no longer static, the molecules are now moving vertically (as is the thermometer) and are subject to cooling caused by expansion as the pressure drops and ballon/parcel expands. The air would cool at the dry or wet rate, depending on humdity/dew point , etc.

P.S. For the avoidance of doubt and nit-picking I am well aware the above is very much a first approximation/ hand waving explanation....

The ISA doesn't explicitly define humidity. There is a perfect gas assumption in the pressure lapse rate that implies dry air, but that's not relevant to calculations of performance etc based on ISA.

It nominally represents the average worldwide atmospheric profile in terms of pressure and temperature vs altitude to provide a normalising parameter for performance calculations etc.

If you actually had ISA conditions, then the air would be stable if it remained unsaturated as the actual lapse rate is lower than the convective lapse rate, as you describe.

ISA is a set of average conditions based upon a nominal 45 degrees latitude around the vernal equinox, with some standard figure for humidity at sea level, and a tropopause at 11km. Virtually never will you ever actually see those conditions. It's based upon a set of balloon soundings and aircraft observations done by NACA and NASA between the 1920s and 1960s.

They're a hellishly useful baseline standard, because they provide something that we can reference everything else - everything is ISA +/- and people like me use standard ISA figures for designing and certifying aeroplanes.

The actual equations used for defining ISA are exponential equations with a lot of digits. However, below 10,000ft it looks remarkably like a straight line, and you can approximate it with 1.98deg/1000ft.

Generally speaking, it's based upon a stationary column of air at ISA S/L conditions at the bottom, with of course all that matter on top, so the pressure reduces with altitude. That of course affects the values.

Once upon a time I could derive all these equations from scratch with a pencil and paper. Please don't ask me to now!

G

Yep, wot Genghis said. I don't know what the QFE is at the moment but is sure isn't 1013.2 and the air temperature is not 15 degrees. Those are the mean values for an average point on planet earth.