Definition of Environmental Lapse Rate
The Environmental Lapse Rate (ELR) describes how the actual temperature of the atmosphere changes with increasing altitude at a given time and location. It is typically expressed in degrees Celsius per kilometer or Fahrenheit per 1,000 feet. Unlike adiabatic lapse rates, which represent theoretical temperature changes for rising or sinking air parcels, the ELR measures the existing vertical temperature profile of the ambient air.
Key Principles and Factors
The ELR is influenced by various factors including solar radiation, surface heating, cloud cover, and air mass movement. On average, the troposphere (the lowest layer of Earth's atmosphere) cools by about 6.5°C for every 1-kilometer increase in altitude. However, the ELR can vary significantly from this average, leading to different atmospheric stability conditions. For instance, a very steep ELR indicates unstable air prone to convection and storms, while a shallow or inverted ELR (temperature inversion) signifies stable conditions.
A Practical Example
Imagine a hiker ascending a mountain. As they climb, they experience a drop in air temperature. If the temperature at the base is 20°C and it drops to 13°C after climbing 1 kilometer, the ELR for that specific kilometer is 7°C/km. This actual measured rate informs meteorologists about the potential for cloud formation, precipitation, or clear skies, as it determines how rising air parcels will behave relative to the surrounding air.
Importance in Meteorology and Aviation
The Environmental Lapse Rate is fundamental to understanding atmospheric stability and predicting weather patterns. Meteorologists use it to forecast severe weather, determine the height of cloud bases, and assess conditions for atmospheric dispersion of pollutants. In aviation, pilots rely on ELR data to understand air density, which affects aircraft performance, and to anticipate turbulence or clear-air conditions during flight.