GATE 2022 AE – Question 27
For International Standard Atmosphere (ISA) up to 11 km, which of the following statement(s) is/are true?
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Correct answer: (A) The hydrostatic/aerostatic equation is used.; (D) Air is treated as a perfect gas.
Explanation
The **International Standard Atmosphere (ISA)** is a defined model of the atmosphere.
- **A. Uses the hydrostatic (aerostatic) equation.** True. The variation of pressure with height follows $dp=-\rho g\,dh$. ✓
- **B. Lapse rate is $-10^{-2}$ K/m.** False. The ISA lapse rate in the troposphere is −6.5 K/km $=-6.5\times10^{-3}$ K/m.
- **C. Sea level has $T_s=300$ K.** False. The ISA sea-level temperature is **288.15 K** ($15\,^\circ$C). (The pressure 101 325 Pa and density 1.225 kg/m³ are correct.)
- **D. Air is a perfect gas.** True. The model uses $p=\rho RT$. ✓
Answer **A and D**.