Wx Watch: Bustin' Caps

Warm air trying to reach cool

September 1, 2008

I’ll bet you remember this summertime scenario: The day dawns oppressively humid, with low stratocumulus, maybe some light rain, and perhaps mist or fog in low-lying areas. The temperatures are in the mid-70s, but the dew point matches it. Preflighting becomes a sweaty task, even though little exertion may be involved. By late morning, surface temperatures hit the mid-80s, and keep on rising. Why? Breaks are appearing in the overcast, you can see blue sky above, and the sun’s now doing a great job of burning off any fog and lifting the cloud bases. “It doesn’t look too bad,” you may say to yourself. “Maybe I’ll grab some lunch, then take off in the early afternoon.” Bad idea. You should have left already.

That’s because there’s most likely to be a convective powder keg brewing aloft. Many times, there’s a layer of warmer, drier air above all the murk near the surface. This inversion—usually at around 5,000 to 6,000 feet agl—prevents that steamy air below from rising and forming convective clouds. This is why meteorologists call such an inversion a “cap.” Sometimes you can see these inversion caps in atmospheric soundings, as shown on Skew-T charts. Go online and enter “Skew-T” in the “type of plot” drop-down menu. (See “ Wx Watch: Skew-T Basics,” July AOPA Pilot.)

Above the cap, temperatures aloft make the typical drop with altitude. What do you think would happen if all that warm, humid air below moved upward and managed to reach all that cooler air? You’ve got it: It would keep on rising with a passion, and create thunderstorms by mid-afternoon.

Meteorologists call this delayed upward motion “breaking the cap.” It’s a slang term that well illustrates how pent-up low-level, hot-and-humid air can eventually erode the inversion layer, and then break through to form towering cumulus and cumulonimbus clouds later in the day.

Exactly when the cap breaks and the low-level air earns its “freedom” depends on the strength of the inversion. If the inversion layer/cap is only two or three degrees Celsius warmer than the low-level air, then the sun doesn’t have to work long at all to raise low-level temperatures enough to surpass those in the cap. But it’s a different story if the cap is a strong one, say, 10 or more degrees Celsius warmer than the air below it. It may take all day for the sub-cap air to break through. When a strong cap is broken, look out. All of that muggy air skyrockets to the flight levels, owing to its tremendous positive bouyancy.

It all boils down to the rising air parcel’s reaching the altitude at which it becomes warmer (and less dense) than the surrounding air. This altitude is called the level of free convection (LFC).

The moral of this story? Take off early in the morning on days when air mass thunderstorms are forecast. You want to climb up through any inversion caps—even if it means filing IFR—and cruise in the cooler, clearer on-top conditions. Your flight will be smoother, and there’s the additional advantage of being able to spot any building cumulus down the road. You might even note the outside air temperatures as you climb up through the inversion, to see how strong it is, and to see where it tops out. And monitor surface temperatures from ASOS/AWOS/ATIS broadcasts en route, to see if they are about to bust through the cap temperatures you saw on climb-out. Geeky? Perhaps. But it beats just waiting for the cumulus to pop up!

On one flight, I had a front-row seat at a cap-breakage. I was flying over North Carolina, in the warm sector of a frontal complex. The surface weather was as described in the first paragraph, and I was cruising at 9,000 feet with a friend—above an undercast at 9,000 feet. It was early afternoon. Up ahead, there was a break in the clouds. I flew over it and watched the ground below, bathed in sunlight.

“Well, if something happens, at least we can spiral down through that break,” said my right-seater. About 15 minutes later, with the cloud break behind us, I turned to get a better look at it. In its place was a wall of fast-growing cumulus. That sunlight had broken the cap, more cumuli were heading for the heavens, and a convection-free 180-degree turn was no longer possible. Breaks in the overcast aren’t always a good sign! It’s something to think about when cloud watching on summer days—from the cockpit or the ground.

E-mail the author at tom.horne@aopa.org.

Thomas A. Horne

Thomas A. Horne | AOPA Pilot Editor at Large, AOPA

AOPA Pilot Editor at Large Tom Horne has worked at AOPA since the early 1980s. He began flying in 1975 and has an airline transport pilot and flight instructor certificates. He’s flown everything from ultralights to Gulfstreams and ferried numerous piston airplanes across the Atlantic.