The resistance to vertical air motion is called atmospheric stability. Stable air tends to resist a vertical lifting force. If we measure the temperature at various altitudes we can get some idea of the general stability of the airmass. The change in temperature with altitude is known as the lapse rate. The vertical lifting force may either be orographic (i.e. lifted by terrain) or convectional (i.e. air lifted when heated from below). Heating at the surface or cooling aloft can make air unstable whereas cooling at the surface or warming aloft can create greater stability. The temperature Lapse Rate is the rate of decrease of temperature with increasing altitude. The Standard Lapse Rate is 2 degrees Centigrade (C) per 1000 feet of altitude in the troposphere. When a layer of air has temperature increasing with altitude we have an inverted lapse rate or an Inversion is formed. Rising air is cooled by expansion and sinking air is heated by compression. A rapid decrease in temperature with altitude promotes unstable air while no change or slight decrease with altitude is an indication of stable air. Increases in temperature with altitude also signal the presence of stable air. Moist air is lighter than dry air so that warmmoist air near the surface will likely be unstable. Surface heating, cooling aloft, upslope airflow, inflowof cooler air will lead to instability. For reference purposes, certain conditions are defined for a “StandardAtmosphere”. The ICAO (International Civil Aviation Organization) Standard Atmosphere:
1. It is referenced to Mean Sea Level.
2. Based on temperature 15 degrees C. 3. Has a lapse rate of 1.98 degrees C/1000 ft.