Although meteorological observation are made in figures, it is usuaal, when making calculations involving aircraft performance or correctionn to instuments, to consider them relative to the ISA. These are known as "ISA deviations".
If for instance, the observed temperature were 5degC warmer than that expected in the ISA, then the deviation would be +5degC.
For calculation example; let say we are flying at FL170, the outside air temperature is -18degC (after consider the friction effect):
the ISA temperature: 15degC + (-1.98x17) = 15 - 33.66 = -18.66
ISA deviation woulde be: difference between ISA Temp and OAT = (-18.66) - (+5) = -23.66
That means, duiring that flight, the actual outside air temperature is WARMER compared to the FL170's ISA temperature by 23.66degC. Normally in performance charts they'll write kinda like this... "ISA Deviations +23degC".
HAPPY FLYING!!!
Ref: JAA ATPL BOOK 9 - METEOROLOGY
Teach-Train-Educate
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