A super-adiabatic lapse rate occurs when the temperature decreases with height at a rate of greater than 10 degrees Celsius per kilometer. A super-adiabatic lapse rate is usually caused by intense solar heating at the surface. Especially when the winds are light and the soils are dry, heat from the sun will build at the surface. A super-adiabatic lapse rate is common in the Southwest U.S. in the summer, but can occur in most regions of the U.S. in the summer when the skies are clear (maximum insolation), wind speeds are low (limited vertical mixing) and soils are dry (no evaporational cooling). A super-adiabatic lapse rate is labeled as absolute instability. The super-adiabatic layer is generally shallow and located near the earth's surface. Whether a super-adiabatic lapse rate at the surface leads to precipitation is a function of the moisture content of the air, the cap strength, trigger mechanisms, and upper level forcing mechanisms, etc. A super-adiabatic lapse rate in the middle and upper troposphere is rare. Another situation a super-adiabatic lapse rate can occur is over a warm lake. When a cold air mass moves over a large lake (i.e. Great Lakes) the lake warms the air nearest to the lake surface. This can result in instability and a large temperature decrease with height above the lake. In heavy lake-effect snow situations there will often be a super-adiabatic lapse rate above the lake. A downsloping wind is another situation it can occur. With a downsloping wind, air is warmed at the dry adiabatic lapse rate as it sinks. This combined with surface heating can produce a super-adiabatic lapse rate in the lower troposphere in the afternoon. If a dry adiabatic temperature profile is heated on its lower end it will become super-adiabatic (lapse rate greater than 10 C/km). |