[[Thermal Formation]] and development is closely related to atmospheric instability. ### Lapse Rates - Dry Adiabatic Lapse Rate (DALR): as a lifted parcel ascends, it will cool at the DALR until it becomes saturated; the DALR is 3C/1000ft - Saturation Adiabatic Lapse Rate (SALR): a saturated parcel will cool at the SALR as it ascends; the SALR varies from approximately 1.2C/1000ft at low altitudes, and approaches the DALR at high altitudes. At high altitude, there is less water vapor to condense, and less release of **latent heat**; additionally the colder air "can hold less water" as the vapor pressure of water increases and decreases with temperature. This may also be called the "moist adiabatic lapse rate". - Standard Lapse Rate: the **international standard atmosphere** defines the standard lapse rate in the **troposphere** as 2C/1000ft. This is simply a "standard" and does not define any stability processes. - Environmental Lapse Rate: is the actual change in temperature with altitude in the atmosphere at a given time and place. This may also be called the "ambient lapse rate". ### Stability and Instability A stable atmosphere suppresses vertical motion, while an unstable atmosphere enhances it. The greater the environmental lapse rate, the more unstable the atmosphere will be. An environmental lapse rate greater than the DALR is known as **absolute instability**, as a rising parcel will remain warmer (and therefore buoyant) with respect to the surrounding air. ![[absolute instability.png]] An environmental lapse rate between the DALR and SALR is known as **conditional instability**, meaning that a parcel will only exhibit buoyancy once cooling at the SALR. An environmental lapse rate less than the SALR will result in stability. Different stability characteristics may exist simultaneously at different levels in the atmosphere. **Skew-T Log-P** charts are the most direct way a pilot can determine the stability of the atmosphere at a given time and place. ### Conditions Conducive to Instability A parcel will continue to rise as long as it is warmer than the surrounding air. Conditions conducive to instability are higher surface temperature (ie strong solar heating), and cooler temperatures aloft.