What is lapse rate and what is the standard environmental lapse rate used in higher geography?

• A. The rate at which air pressure decreases with height; about 12 hPa per kilometre
• B. The rate at which air temperature decreases with height; standard environmental lapse rate is about 6.5°C per kilometre (6.5°C/km)
• C. The rate at which humidity decreases with height; about 1 g/kg per kilometre
• D. The rate at which wind speed increases with height; about 5 m/s per kilometre

Answer: (B)

Lapse rate describes how temperature changes with height in the atmosphere. Specifically, the environmental lapse rate is the rate at which the surrounding air cools as you rise, and the standard value used in higher geography is about 6.5°C per kilometre. This 6.5°C/km is a conventional average, providing a reference even though actual conditions vary from place to place and day to day.

Understanding this helps you assess atmospheric stability: the environmental lapse rate is compared with adiabatic lapse rates (the rates at which a rising air parcel cools on its own, without heat exchange). When the environment cools more slowly with height (a smaller ELR, around 6.5°C/km) than the parcel would if it rose dry adiabatically (about 9.8°C/km), the parcel tends to stay cooler and denser than its surroundings, indicating stability. If the ELR were steeper (larger) than the dry adiabatic rate, rising air would become warmer than its surroundings and convection would be more likely.

The other choices mix up what is being measured: temperature change with height is not about air pressure, humidity, or wind speed.