If at an altitude of 3 km the air temperature is minus 18 degrees. What is it equal to at the surface of the Earth?

The air temperature drops by about 6 degrees Celsius for every 1 kilometer of climb. Consequently:
1) 3 km / 1 km = 3, that is, at an altitude of 3 km, the air temperature is 3 times lower than at the Earth’s surface
2) Let’s find out how many degrees the temperature dropped with a rise of 3 km:
3 * 6 = 18 degrees.
3) If at an altitude of 3 km the air temperature is 18 degrees Celsius, then at the surface of the Earth it is less by 18 degrees, that is:
18 – 18 = 0 degrees.
Answer: 0 degrees Celsius.