What is the environmental lapse rate and how does it relate to stability?

• A. The actual rate of temperature decrease with height; if the ELR is steeper than the dry adiabatic rate, the atmosphere is unstable; if it is shallower, stable; near the ELR is neutral
• B. The rate of humidity change with height
• C. The rate of pressure change with height
• D. The rate of wind speed change with height

Answer: (A)

The main idea here is how the environment’s actual temperature fall with height (the environmental lapse rate) determines whether rising air is buoyant. The environmental lapse rate is the real rate at which air temperature drops with height at a given place and time. To judge stability, compare this rate to how fast an air parcel would cool simply by expanding as it rises: the dry adiabatic lapse rate is about 9.8°C per kilometer, and the moist adiabatic rate is around 6°C per kilometer depending on moisture.

If the environmental lapse rate is steeper (a larger temperature drop with height) than the dry adiabatic rate, a parcel that starts rising stays warmer than its surroundings as it ascends and continues to rise — the atmosphere is unstable. If the environmental lapse rate is shallower (a smaller drop) than the dry adiabatic rate, the rising parcel becomes cooler than its surroundings and it won’t rise further — the atmosphere is stable. If the environmental lapse rate roughly matches the adiabatic rate, buoyancy is near zero and the layer is neutral.

So the statement correctly describes the environmental lapse rate as the actual temperature change with height and links a steeper ELR to instability, a shallower ELR to stability, and near equality to neutral.