We have a bad habit of throwing out weather terms here often without good explanation. On a day like today, we are not overly concerned about severe weather because of low CAPE values. What in the world is CAPE anyway?
CAPE stands for Convective Available Potential Energy. Here is the “geek” definition: A cumulative measure of the positive buoyant energy (in J/kg) a rising parcel of air would have between its Level Of Free Convection (LFC) and the Equilibrium Level (EL). CAPE is a vertically integrated quantity as well as a fundamental indicator of the potential intensity of deep, moist convection. So, what does all of that mean?
A high value of CAPE simply means the air is unstable, and air parcels are buoyant and can rise easily through the atmosphere. These rising air columns can mean rapid thunderstorm formation if there is no capping inversion and there is adequate moisture. So, what is a capping inversion? We often simply call it a cap; a layer of warmer air aloft which prevents air parcels from rising above the cap level. Caps can been easily seen on upper air soundings.
So, what is an upper air sounding? Weather balloons are launched from many locations around the nation at least twice daily (the Alabama location is the Shelby County Airport, about 20 miles south of downtown Birmingham). The balloons have a weather instrument packet attached called a radiosonde, and this measures weather parameters like temperature, humidity, and pressure on the balloon ascent.
As the balloon rises, the atmosphere thins and the pressure outside the balloon decreases allowing the balloon to expand and eventually break. This usually happens within two hours of the launch at elevations of 80,000 to 120,000 feet. Upper air data from the radiosonde reports serves as input for the computer models we use on a daily basis.
CAPE stands for Convective Available Potential Energy. Here is the “geek” definition: A cumulative measure of the positive buoyant energy (in J/kg) a rising parcel of air would have between its Level Of Free Convection (LFC) and the Equilibrium Level (EL). CAPE is a vertically integrated quantity as well as a fundamental indicator of the potential intensity of deep, moist convection. So, what does all of that mean?
A high value of CAPE simply means the air is unstable, and air parcels are buoyant and can rise easily through the atmosphere. These rising air columns can mean rapid thunderstorm formation if there is no capping inversion and there is adequate moisture. So, what is a capping inversion? We often simply call it a cap; a layer of warmer air aloft which prevents air parcels from rising above the cap level. Caps can been easily seen on upper air soundings.
So, what is an upper air sounding? Weather balloons are launched from many locations around the nation at least twice daily (the Alabama location is the Shelby County Airport, about 20 miles south of downtown Birmingham). The balloons have a weather instrument packet attached called a radiosonde, and this measures weather parameters like temperature, humidity, and pressure on the balloon ascent.
As the balloon rises, the atmosphere thins and the pressure outside the balloon decreases allowing the balloon to expand and eventually break. This usually happens within two hours of the launch at elevations of 80,000 to 120,000 feet. Upper air data from the radiosonde reports serves as input for the computer models we use on a daily basis.
on April 26, 2005, 4:55 am
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