Ace the 2026 Australia CASA Instrument Proficiency Exam – Soar High and Navigate Your Success!

The standard temperature lapse rate in clear air is defined as the rate at which temperature decreases with an increase in altitude. In the troposphere, which is the lowest layer of Earth's atmosphere, this rate is approximately 6.5 degrees Celsius per kilometer (or 3 degrees Celsius per 1000 feet). This means that as you ascend in clear air, the temperature typically drops about 3 degrees Celsius for every 1000 feet of altitude gain.

This lapse rate is important for aviation, as it helps pilots understand how temperature changes with altitude can affect aircraft performance and weather patterns. In general, the lapse rate indicates that as altitude increases, the density of the air decreases, which can influence lift and engine performance.

The other options presented do not accurately reflect the standard environmental lapse rate. They either represent rates that are too high or too low compared to established meteorological data.