What is the temperature around an aeroplane flying at an altitude of 4000 metres if the temperature on the earth surface is 40 oC?

Analyze the question

This is about the normal (environmental) lapse rate: temperature in the troposphere falls as altitude increases. We need the temperature at 4000 m when the surface temperature is 40 °C.

Concept

The standard lapse rate used in WASSCE Geography is 1 °C per 150 m of ascent (some textbooks use 6.5 °C per 1000 m, or 1 °C per 165 m).

Solution

1. Temperature drop = 4000 m ÷ 150 m per °C = 26.67 °C, about 27 °C
2. Temperature at 4000 m = 40 − 26.67 = 13.33 °C, about 13 °C

Using the other common rate, 6.5 °C per 1000 m:

1. Drop = 4 × 6.5 = 26 °C
2. Temperature = 40 − 26 = 14 °C

Which option fits?

• A (26 °C) is the temperature drop in the 6.5 °C/km method, not the final temperature. This is a common trap.
• B (66 °C) is wrong, because temperature does not rise with altitude here.
• C (14 °C) matches the 6.5 °C per 1000 m method exactly.
• D (12 °C) does not match either method closely.

The 1 °C per 150 m method gives about 13 °C, which is closest to C (14 °C). The 6.5 °C per 1000 m method gives exactly 14 °C.

Answer: C. 14 °C

Exam tip: Always subtract the drop from the surface temperature. Option A is the "drop" and is placed there to catch students who stop one step early. Remember: 6.5 °C per 1000 m, so multiply by the number of kilometres and then subtract.