What looks like a harmless cumulus cloud can quickly become a thunderstorm in the right conditions. You probably understand that thunderstorms need 3 ingredients to form: moisture, instability, and lifting action.
As you lift air from the surface, it cools. The temperature keeps dropping and approaching the air’s dew point. Once it hits the dew point, moisture starts to condense out of the air, forming clouds. This altitude is the convective condensation level. It’s the lowest altitude that condensation happens due to convection from surface heating.
As moisture condenses out of the air, it releases energy. (It takes energy to turn water into a gas, and that energy releases as heat as the gas condenses back to water). Currently, that moisture is condensing out of the lifted air, it’s much warmer than the surrounding air. as it rises, that temperature gap grows, and the air continues to accelerate upward, forming a strong updraft. This creates a high cumulus cloud or TCU. And with that, you’ve got the developing stage of a thunderstorm.
In the summer and early fall, you’ll notice pop-up thunderstorms all over the U.S. It’s typically completely clear below the cloud base layer right before precipitation begins. And even after light precip has started, the bottom of the cloud base doesn’t always look that bad.
But, you must never fly beneath a build-up that could become a thunderstorm or one that already is. Here’s a good example of why..
NTSB Report: Pilot Flies 172 under a thunderstorm
The pilot didn’t receive a weather briefing before starting the cross-country flight. after takeoff, the pilot requested from air traffic control to fly below 500 ft above ground level on the ocean shoreline. The controller approved the request but advised of heavy precipitation (a thunderstorm) at the airplane’s 12-o’clock position and four miles ahead. The controller additional advised that the pilot ought to turn left and fly offshore three miles to avoid the thunderstorm. though the pilot acknowledged the instructions, a review of radar and GPS data for the flight revealed that he continued on course.
About three minutes later, the pilot reported he was reversing direction, and no more communications were received from the pilot. A review of the airplane’s GPS track overlaid on weather radar plots revealed that the airplane flew into an area of extreme intensity precipitation and then entered a rapid descent and impacted the ocean. Examination of the wreckage didn’t reveal any proof of preimpact mechanical malfunctions. it’s probably that the pilot lost control of the airplane once it encountered strong downdrafts and heavy rain associated with the thunderstorm.
Mature Stage = Mixed Updrafts and Downdrafts
During the second stage of thunderstorm growth, cool falling raindrops pull air down, creating cold downdrafts. This causes a mixing of updrafts and downdrafts, making extremely turbulent air within and beneath a thunderstorm. Eventually, the rain will begin to fall. And when it does, you’ll experience downdrafts exceeding 2,500 FPM, even once you’re under the cloud base.
Dissipating Stage = strong Downdrafts
If you get caught below a thunderstorm as it reaches the dissipating stage, you could encounter severe downdrafts exceeding 6,000 FPM. As rain becomes heavy, additional air is pulled down with it. At this point, the thunderstorm will begin to die quickly. However, it’s also the most dangerous time to be caught underneath it.
With the correct conditions, it only takes a couple of minutes for a seemingly benign cloud to become a full-blown thunderstorm. so what’s the best advice to stay safe? If the cloud looks like something you wouldn’t want to fly through, you shouldn’t fly under it either.
Keep Your Distance
The Federal Aviation Agency (FAA) recommends you stay at least five miles from any visible storm cloud, but they strongly recommend increasing the distance to twenty miles or more if you can. Hail, violent turbulence, and strong downdrafts can extend miles far from a thunderstorm.
Use the same logic when flying underneath clouds, and you’ll keep yourself out of harm’s way.