The Lapse Rate is the rate at which temperature changes with height in the Atmosphere. Lapse rate nomenclature is inversely related to the change itself: if the lapse rate is positive, the temperature decreases with height; conversely if negative, the temperature increases with height.
When air is forced to rise up in the atmosphere, the pressure reduces with height. For a given volume of gas, the pressure divided by the temperature remains constant (Boyle's Law). Therefore, as the air pressure reduces, so does the temperature.
If no heat is exchanged with the surrounding air during this process, which is called “adiabatic cooling”, the rate at which the air cools, the Adiabatic Lapse Rate (ALR) is a constant.
For unsaturated air, the lapse rate is 3°C per 1000 feet; this is called the Dry Adiabatic Lapse Rate (DALR). However, when the parcel of air reaches the Dew Point and becomes saturated, water vapour condenses, latent heat is released during the condensation process, which warms the air, and the lapse rate reduces. The Saturated Adiabatic Lapse Rate (SALR) is therefore the rate at which saturated air cools with height and is, at low levels and latitudes, 1.5°C per thousand feet. At higher altitudes and latitudes, where there is generally less water content in the air, and therefore less latent heat to release, the SALR is closer to 3°C per thousand feet.
The ELR (Environmental Lapse Rate) is the actual rate at which the ambient temperature changes with height.
Considering the parcel of air as before and utilizing the DALR and SALR for that parcel of air in contrast to the surrounding air:
If the ELR is greater than the DALR, rising air will be warmer than the surrounding air and therefore keep rising; the atmosphere is then said to be unstable. If ELR is greater than SALR, the air is said to be absolutely unstable, since the air, whether saturated or unsaturated, will always have a higher temperature than it surroundings.
When the ELR is less than the SALR and greater than the DALR, then the air is considered conditionally unstable: the condition being whether the air is saturated or not.
If the ELR is less than the DALR, then the rising air will be cooler than the surrounding air and will sink - the atmosphere is said to be stable. If the ELR is less than the SALR, the air is said to be absolutely stable, since the air, whether saturated or unsaturated, will always be cooler than the surrounding air.
The distinctions between stability and instability as noted above are the foundation of weather analysis, in particular for afternoon airmass thunderstorm development or stable conditions. The most complex situation is when the troposphere is considered conditionally unstable, since a trigger may be needed to enhance either the stability or instabiliy factor.