What looks like a harmless cumulus cloud can quickly become a thunderstorm in the right conditions.
Thunderstorm Formation
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 occurs because of 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 into water.) Now 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 towering cumulus cloud, or TCU. And with that, you have the developing stage of a thunderstorm.
From the spring through early fall, you'll find pop-up thunderstorms all over the United States. It's usually completely clear under 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 should never fly underneath 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 Thunderstorm
The pilot did not receive a weather briefing before beginning the cross-country flight. After takeoff,
the pilot requested from air traffic control to fly below 500 ft above ground level along the ocean
shoreline. The controller approved the request but advised of heavy precipitation (a thunderstorm) at
the airplane's 12-o'clock position and 4 miles ahead. The controller further advised that the pilot
should turn left and fly offshore 3 miles to avoid the thunderstorm. Although the pilot acknowledged
the instructions, a review of radar and GPS data for the flight revealed that he continued on course.
About 3 minutes later, the pilot reported he was reversing direction, and no further
communications were received from the pilot. 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 did not reveal any
evidence of preimpact mechanical malfunctions. It is likely that the pilot lost control of the airplane
when 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, creating extremely turbulent air within and beneath a thunderstorm. Eventually, rain will begin to fall. And when it does, you can experience downdrafts exceeding 2,500 FPM, even when you're under the cloud base.
Dissipating Stage = Strong Downdrafts
If you get caught under a thunderstorm as it reaches the dissipating stage, you could encounter severe downdrafts exceeding 6,000 FPM. As rain becomes heavy, more air is pulled down with it. At this point, the thunderstorm will begin to die quickly, but it's also the most dangerous time to be caught underneath it.
With the right conditions, it only takes a few 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 FAA recommends you stay at least 5 miles from any visible storm cloud, but they strongly recommend increasing the distance to 20 miles or more if you can. Hail, violent turbulence, and strong downdrafts can extend miles away from a thunderstorm.
Use the same logic when flying underneath clouds, and you'll keep yourself out of harm's way.
These rules-of-thumb are even more important in hilly and mountainous terrain, where rising ground elevation gives you fewer outs to escape developing weather.
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