Technical question: Capping inversion?

[Portastorm]

Been reading a fair amount this week in the Texas forecast discussions about "capping inversions." I understand that an inversion means a more stable layer of air exists at the upper levels, but is a "capping inversion" when the air temps rise the higher you get in the atmosphere, this creating more stability?

[Johnny]

Cap (or Capping Inversion) - A layer of relatively warm air aloft (usually several thousand feet above the ground) which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability. However if the cap is removed or weakened, then explosive thunderstorm development can occur.

[Hoosierwxdude]

Parcels of air that are positively buoyant (rising) really can't break through a cap and continue to rise, unless there is enough of a lifting mechanism to allow them to do so. IF enough lift is present to allow them to rise to the level of free convection, they will continue to rise on their own (as well as cool) from that point until they reach equilibrium, where the temperature of that cooling parcel is equal to the temperature of the surrounding environment.

Overall, the cap can be a good thing or a bad thing. On the one hand, it can suppress thunderstorm development completely if it's strong enough. But on the other hand, if the cap can locally be broken then it means that you will only have isolated storms which are great from a chasing perspective!

[isobar]

Actually capping inversion and stability are two different things, although the inversion can cause that portion of the atmosphere to be "stable." Typically air parcels cool with height, and since it must be warmer than the environment in order to continue to rise (which eventually could lead to convective weather), if it encounters a layer of air warmer than itself, it's kind of like hitting a ceiling.

NWS in Norman can explain it much better than I can. http://www.srh.noaa.gov/oun/severewx/glossary.php#Introduction

Cap (or Capping Inversion) - A layer of relatively warm air aloft (usually several thousand feet above the ground) which suppresses or delays the development of thunderstorms. Air parcels rising into this layer become cooler than the surrounding air, which inhibits their ability to rise further. As such, the cap often prevents or delays thunderstorm development even in the presence of extreme instability. However if the cap is removed or weakened, then explosive thunderstorm development can occur. See CIN and Fig. 6, sounding.

The cap is an important ingredient in most severe thunderstorm episodes, as it serves to separate warm, moist air below and cooler, drier air above. With the cap in place, air below it can continue to warm and/or moisten, thus increasing the amount of potential instability. Or, air above it can cool, which also increases potential instability. But without a cap, either process (warming/moistening at low levels or cooling aloft) results in a faster release of available instability - often before instability levels become large enough to support severe weather development.

Edit: Just saw the other replies. Oh well, 3 answers at once is better than none.

[Portastorm]

Thanks guys! I truly appreciate your help. Makes sense now.

... yet another reason -- knowledgeable and helpful folks -- why I love S2K!

[jeff]

As posted above an inversion is where the air temp increases instead of decreases with height. Such inversions are usually very noticeable on Skew-T diagrams from sounding locations and aircraft ascents. An inversion can be eroded through surface heating which moves the temp. line right on the Skew-T and eliminates (for lack of a better desciption) the warm layer above. Lifting can erode and lift a cap allowing surface parcels to break through. Cooling in the capping layer from short wave disturbances, and surface forcing along a front, dryline, ect. Usually a combination of these factors results in the plains cap breaking during the spring time during the afternoon and early evening hours.

The central and southern plains are unique with respect to that a warm layer is typically found due to SW winds at 2000ft-5000ft transporting warm air from northern Mexico or the higher terrain of the Rockies E and ENE across the plains due to the rotation of storm systems from west to east and counterclockwise over the US. Significant amounts of SBCAPE are allowed to develop through insolation under the cap during the day if it is not broken early and convective development does not occur until late afternoon. This is why most severe weather events and tornadoes occur during the afternoon and evening hours across the plains after clear skies for most of the day....the cap finally breaks allowing surface parcels to rise vigorously on a high CAPE and at sometimes high shear envirnoment.

One thing to note, regardless of CAPE amounts and instability under the cap, there are times when the factors do not come together usually due to timing when the cap will not break and the surface based energy is not realized...in chasers lingo...a BUST.