On 7 July 1999, Ensign John Gay was on the highest deck of the aircraft carrier USS Constellation when an F/A-18C Hornet tore past wrapped in a white bell of cloud. The nose and canopy were clear. The rest of the jet seemed to be bursting out of a solid disc of fog.

The Navy released the frame with a caption saying the Hornet “breaks the sound barrier.” It went on to win a World Press Photo prize, and in 2007 NASA’s Astronomy Picture of the Day ran it under the title “A Sonic Boom.” Twenty-seven years later, that is still how pictures like it are usually read.

The cloud is real; the caption’s certainty is the problem. A vapor cone (also spelled vapour cone, and nicknamed shock collar or shock egg) is weather made by the airplane: humid air that is stretched, chilled into droplets and then evaporated again by a shock wave as the jet slides through it. It can form below Mach 1, and its sharp back edge shows that some of the air around the jet went supersonic. It does not show that the jet itself did.

The 1999 Photo Everyone Calls the Sound Barrier

The Hornet belonged to Strike Fighter Squadron 151 (VFA-151), the Vigilantes, embarked on Constellation, and the Navy’s caption puts it over the sea off Pusan, South Korea. The carrier had left San Diego on 18 June 1999 for the Korean Peninsula, before moving on to the Persian Gulf in late August. As a U.S. Navy image, photo 990707-N-6483G-001 is in the public domain.

It took first prize for science and technology in the 2000 World Press Photo contest, credited to John Gay, U.S. Navy/Sports Illustrated, and shot, the contest archive notes, from “the highest vantage point on the ship.” NASA’s 2007 write-up called the cloud’s origin “still debated” and offered “a leading theory” built on something called the Prandtl-Glauert singularity. Hold that thought.

So was the Hornet supersonic? No speed for that pass appears in the Navy caption, the contest entry or NASA’s write-up, and the cloud cannot settle it. NASA Langley’s 1994 study of in-flight condensation, “Patterns in the Sky,” shows in its front pages an F-14A Tomcat wrapped in cloud as it “passes through speed of sound near carrier.” Later in the report, a second F-14A photo shows the same kind of cone in level flight at Mach 0.9.

Same jet type, same cloud, two speeds. Official captions waver too. On 22 June 2009 the Navy captioned one photo of an F-22 Raptor crossing USS John C. Stennis in the Gulf of Alaska as a “supersonic flyby,” and another frame from that day as a “transonic flyby.”

Weather Made by an Airplane

Air flowing around a fast jet speeds up over the canopy and the wings, and where it speeds up, its pressure and temperature fall. NASA’s report spells out the chain: once the local temperature drops to the dew point, “the water vapor can begin to condense and become visible.” When the flow slows and compresses again, the droplets “evaporate, or vaporize, and are no longer visible.”

Temperature is the real trigger. NASA’s analysis found relative humidity “more sensitive to temperature changes than to the changes in pressure,” which is why the effect needs air that is already humid. Seeds matter as well: Chris Combs, a high-speed aerodynamicist at the University of Texas at San Antonio, notes that droplets need particles such as dust, pollution or smoke to start forming.

Then comes the trick that makes a cone. NASA’s Glenn Research Center puts it plainly: “While the aircraft itself may be traveling less than the speed of sound, the air going around the aircraft exceeds the speed of sound at some locations on the aircraft.” Those supersonic pockets end in a shock wave, across which pressure and temperature jump “almost instantaneously.”

That shock is the cone’s back wall. Combs points out that a shock wave is only on the order of 100 nanometers thick and creates “a temperature jump that VAPORIZES water.” The droplets vanish along one razor-thin surface, which is why the rear of a vapor cone looks as if it was cut with a knife. The smaller puff over the 1999 Hornet’s canopy is the same effect in miniature, where the air expands over the cockpit.

The shape even carries a speed clue. Comparing an F-4J with an F/A-18, NASA’s authors noted that the Hornet’s shock “slopes forward, which is indicative of a lower subsonic Mach number” than the F-4’s. And if a cone flickers on and off during a pass, Combs blames patchy humidity, not a pilot repeatedly “breaking the sound barrier” (which, he adds, “isn’t a thing”).

Real Engineering explains why the cloud around a fast jet is condensation rather than a visible shock wave.

The Singularity That Never Was

That “leading theory” goes back to the 1920s, when Ludwig Prandtl and Hermann Glauert gave engineers a shortcut for compressibility: take pressures worked out for slow, incompressible flow and divide them by the square root of one minus the Mach number squared. At Mach 0.5 that inflates them by about 15 percent, and at Mach 0.9 it more than doubles them. At Mach 1 you divide by zero, and the formula predicts infinite pressure.

That infinity is the Prandtl-Glauert singularity, and it belongs to the math, not the air. The rule comes from a linearized theory that assumes small disturbances and no shock waves, so it works up to about Mach 0.7 and fails exactly where transonic flow begins. Near Mach 1, real pressure disturbances grow sharply but stay finite.

Aerodynamicist Chris Combs opens his 2020 thread on what vapor cones really are.

Combs took the idea apart in that 2020 thread, which is still pinned to his X profile. He called vapor cones the biggest point of confusion among aviation fans online, and he found just one online resource that explained them properly: NASA’s 1994 report. Wikipedia’s own article now calls the singularity a construct “often incorrectly used to explain vapor cones in transonic flows.”

Why did the myth stick? Wikipedia ties it to the early-20th-century belief that the sound barrier could not be crossed. NASA’s Glenn Research Center is blunter: “the sound barrier was only an increase in the drag near sonic conditions because of compressibility effects.” A formula that blows up at Mach 1 fits the old wall story perfectly; the cloud does not.

Ropes, Fog and Wing Clouds

The same physics works at far lower speeds wherever air is stretched and chilled. Every lifting wing sheds a pair of counter-rotating vortices from its tips, each with a low-pressure core, and NASA’s report says condensation from wingtip vortices is “seen more frequently than any other” pattern, partly because these vortices form on every configuration that produces lift.

Fighters show it as soon as they start to pull. NASA’s report puts it simply: “The tip vortex is usually the first part of the flow field to condense.” The result is two white ropes streaming back from the wingtips, like those behind an F-15E Strike Eagle leaving a tanker during Operation Iraqi Freedom.

Pull harder and the whole top of the wing can fog over. NASA calls this the gull pattern and caught it on its Transonic Aircraft Technology (TACT) research jet at about Mach 0.8 in a 3.3g turn, with flight data showing supercritical flow and a shock wave over the wing. “The gull type of condensation pattern can occur on any airplane if the lift conditions are right,” the authors wrote.

Air show crowds see the dramatic version when a Super Hornet dives for show center and pulls, as one did at Naval Air Station Oceana in 2004, with a cloud billowing off its upper surfaces. That cloud is lift made visible: low pressure over the wing, chilled air, droplets.

Airliners do it too. On humid days, wings on approach or climb-out can fill with cloud with no shock wave anywhere near them, and Wikipedia notes such clouds form even “in purely subsonic flow over wings.” A Boeing 777 sinking through fog toward Heathrow turned that into a clip with about 13,000 upvotes on r/aviation.

In the clip, the 777’s wingtip vortices drag long curls of fog down behind it while a sheet of condensation hugs the wing. It is the same physics as the Hornet’s cone, at a fraction of the speed.

Why the Best Cones Happen Over Water

Vapor cones seem to cluster at carrier decks, beach air shows and harbor fly-bys, and that is no coincidence. The cloud needs humid air, and NOAA notes that maritime air masses “originate over the oceans and are therefore moist air masses.” In air that wet, a fast, low pass only has to cool it a little to reach the dew point.

Look at the examples in this story: Gay’s Hornet off Korea, the F-22 over a carrier in the Gulf of Alaska, the Hornet at the top of this page passing USS Carl Vinson in the Pacific, and NASA’s F-14A near a carrier. NASA’s authors also warned that these patterns “often occur without warning because they are affected by atmospheric weather, lighting conditions, and airplane flight conditions.” The same jet at the same speed can be wrapped in cloud one day and clean the next.

A legacy F/A-18C Hornet roars past a carrier at sea in this 2013 clip; the uploader calls it a sonic boom, but the cloud alone cannot confirm that.

So, Was It the Sound Barrier?

Maybe. The Navy said the Hornet broke the sound barrier, and no photograph can prove otherwise. What the picture does show is humid air, a jet close to Mach 1 and pockets of supersonic flow ending in a shock. Combs puts the odds bluntly: a vapor cone “by no means says a vehicle is moving at M=1 (on the contrary it’s quite unlikely the speed is EXACTLY M=1).”

The sonic boom was never in the frame anyway. A boom is a pressure wave that trails an aircraft for as long as it flies faster than sound, and you hear it rather than see it. The white bell around Gay’s Hornet is something better: a cloud made and unmade by the airplane in the instant it takes to pass.

Sources: NASA SP-514, Patterns in the Sky (James F. Campbell and Joseph R. Chambers, NASA Langley Research Center), 1994; NASA Glenn Research Center, Beginner's Guide to Aeronautics (Transonic Aircraft, Normal Shock Wave Equations, Mach Number); NASA Astronomy Picture of the Day, 19 August 2007; World Press Photo, 2000 Photo Contest archive; U.S. Navy photo 990707-N-6483G-001 (Wikimedia Commons), 7 July 1999; DVIDS, 22 June 2009; Chris Combs (University of Texas at San Antonio) on X, 1 September 2020; NOAA JetStream, Air Masses; Real Engineering (YouTube), 28 August 2021; Wikipedia: Vapor cone, Prandtl-Glauert singularity, Prandtl-Glauert transformation, USS Constellation (CV-64), VFA-151