Question

if the air temperature @ sea level were 23 C what would the air temperature be at a height of 2 km under avg conditions?

Answer 1

try:
http://www.engineeringtoolbox.com/air-altitude-temperature-d_461.html
or
http://www.digitaldutch.com/atmoscalc/

Answer 2

10 degrees C

Answer 3

Since the normal lapse rate is 6.5°C per kilometer, at a height of 2 kilometers the temperature would decrease 13°C (6.5°C × 2 = 13°C). The air temperature at 2 kilometers would be 23°C - 13°C = 10°C.