The rate at which atmospheric temperature decreases with increasing altitude, averaging approximately 6.5°C per kilometre in the standard atmosphere.
The lapse rate is the rate of decrease of atmospheric temperature with increasing altitude. The ICAO Standard Atmosphere (ICAO Annex 3) defines an average environmental lapse rate of approximately 6.5°C per kilometre in the troposphere, though the actual lapse rate at any given time and place varies considerably. When the actual lapse rate exceeds the dry adiabatic lapse rate, the atmosphere is said to be unstable; rising air parcels continue to accelerate upward, promoting convective cloud development, cumulus towers, and ultimately cumulonimbus. When the temperature actually increases with height — a temperature inversion — the atmosphere is stable and vertical mixing is suppressed.
For a mariner, the lapse rate has practical relevance in several contexts. A steep lapse rate indicates an unstable atmosphere prone to convective squalls and sudden wind shifts, relevant to small-craft safety and to operations at anchor or in harbour. A strong temperature inversion near the surface can trap moisture and produce persistent advection fog that is difficult to disperse, affecting radar propagation and visual range. Sub-refractive and super-refractive conditions caused by abnormal lapse rates alter the effective range of shipborne radar, which is a consideration under SOLAS equipment standards.
Officers do not typically measure lapse rate directly, but they can infer atmospheric stability from observed cloud types, squall frequency, and the vertical extent of cloud development. Numerical weather prediction forecasts and SIGMET information from meteorological authorities provide stability indices relevant to passage planning.