the airplane shown in the diagram is flying at 20,000' above sea level. What is the probable temperature at that height?

The temperature decreases steadily with increasing altitude from sea level to 36,000 feet, after which the temperature remains constant at about -69.7°F up to an elevation of 65, 600 ft.
The probable temperature at the airplane height of 20,000 feet is about -32°F
The above value is correct due to the following reasons:
The parameters given in the diagram are;
The location of the airplane = 50°N Latitude
The elevation of the airplane above sea level = 20,000ft. above sea level
The variation of temperature with elevation in the lower troposphere is presented as follows
The variation of temperature with height = The standard average lapse rate = 6.5°C per kilometer 3.5°F (2 °C) per 1,000 ft. up to or 36,000 ft.
Therefore, in the given diagram, given that the surface level temperature, t₀ = 0°C, we have the temperature, t at the 20,000 ft. height given as follows;
t = 32°F - 20,000 ft. × 3.5 °F/1,000 ft. = -32°F
The probable temperature at the height of the airplane, 20,000 ft. is approximately -32 °F
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