FAA Ground School Practice Test

Which measurement is used to determine the stability of the atmosphere?

1. Relative humidity

2. Actual lapse rate

3. Absolute temperature

4. Pressure gradient

The correct answer is: Actual lapse rate

The measurement used to determine the stability of the atmosphere is the actual lapse rate. The actual lapse rate refers to the rate at which the temperature of the atmosphere decreases with an increase in altitude. This is crucial in assessing atmospheric stability because it allows meteorologists to compare the actual lapse rate with the adiabatic lapse rates (the dry and moist adiabatic lapse rates) to determine whether the atmosphere is stable, unstable, or neutral. When the actual lapse rate is less than the moist adiabatic lapse rate, it indicates that the atmosphere is stable, meaning that air parcels that are lifted will be cooler and denser than their surroundings, causing them to sink back down. Conversely, if the actual lapse rate is greater than the dry adiabatic lapse rate, the atmosphere is unstable, leading to upward motion and potential convection. Relative humidity measures the amount of moisture in the air compared to the maximum it can hold at a given temperature, which can influence cloud formation but does not directly indicate stability. Absolute temperature, while important for determining atmospheric conditions, does not provide information on how temperature changes with altitude. Pressure gradient refers to the change in pressure over distance and influences wind but is not a direct measure of atmospheric stability.