WBOC’s, Dan Satterfield, Talks Jet Vapor at the OC Air Show
Many people think this effect happens when a plane breaks the sound barrier. However, at air shows, planes like the Blue Angels or the F-22 Raptor are often restricted to 700 mph for their highest speed. The speed of sound is 767 mph, and other than the designated high-speed passes, the planes at shows go a lot slower than 700 mph. How then do clouds appear at the oddest times, like when planes are turning or pulling G-forces at slow speed?
The clouds are caused by changes in air pressure… not speed. In the video, above, Dan walks us through what is happening. When a plane creates a sudden change in air pressure around the wing. This changes the dew point in the air, and the water vapor suddenly condenses into a puff of a cloud. It’s a spectacular effect, and it comes down to a fundamental and frequently used meteorological equation, The Ideal Gas Law.
Pressure multiplied by volume = amount of gas multiplied by temperature and a constant.
When air pressure drops suddenly, it changes the amount of water vapor which can fit in the same space as just moment before. And thus… a cloud appears…temporarily.
A high flying plane creates something different, essentially a cirrus cloud, from the vapor in the exhaust turning into ice crystals. For example, in the picture below.
(Photo credit: Aviationstirling – Own work. Licensed under CC BY-SA 4.0 via Wikimedia Commons)
A plane hitting the sound barrier creates the iconic “vapor cone” or “shock collar” which is a visual manifestation of the shock waves creates by supersonic speeds, and what ends up being a sonic boom. The shock waves themselves are creating the pressure variance in that case, in a uniform wave pattern. Because many modern air shows avoid sonic booms, this means spectators also miss the “vapor cones.”
(Photo credit: Realbigtaco Licensed under Attribution-ShareAlike 3.0 Unported Creative Commons)
Yet another effect is not a cloud nor vapor at all! It’s airshow smoke. To create this billowy plume, low viscosity oil is atomized and inserted into the exhaust. This is the same smoke used for skywriting, and it’s not to be confused with the cirrus clouds of commercial aircraft.
Luckily, the Ideal Gas Law allows to enjoy a truly magical, yet completely natural effect at airshows!