Page 69 - Aeronautical Engineer Data Book

56 Aeronautical Engineer’s Data Book
3.5 The International Standard
Atmosphere (ISA)
The ISA is an internationally agreed set of
assumptions for conditions at mean sea level
and the variations of atmosphere conditions
with altitude. In the troposphere (up to
11 000 m), temperature varies with altitude at
a standard lapse rate L, measured in K (or °C)
per metre. Above 11 000 m, it is assumed that
temperature does not vary with height
(Figure 3.1).
So, in the troposphere:
Temperature variation is given by:
– Lh
T = T 0
p 2 5.256
T 2
Pressure is given by: =
p 1 T 1
where T = temperature at an altitude h (m)
= absolute temperature at mean
T 0
sea level (K)
L = lapse rate in K/m
p = pressure at an altitude
The lapse rate L in the ISA is 6.5 K/km.

16 The stratosphere:
temperature does not
14
vary with height
12 The ‘tropopause’
Altitude in ’000 m 8 The troposphere:
10
temperature lapse
rate L = 6.5˚C/km
6
4
2

–60 –40 –20 0 20 40 60
Temperature, ˚C

Fig. 3.1 The ISA; variation of temperature with altitude