We’re all familiar with the clouds that follow behind airplanes when warm jet exhaust meets the cold upper atmosphere, but if you’re lucky enough to find yourself near an airport on a humid day, you can experience a cloudy, visual treat that never fails to dazzle.
A couple of days ago, YouTube user flugsnug uploaded an incredible video showing an Emirates Boeing 777 landing and taking off on a gray, drizzly day. The bursts of clouds that follow the aircraft on approach and departure are phenomenal, with spectacular explosions of condensation forming above the wing and behind the flaps, as well as long streamers of moisture that extend off the wingtips of the jet. The wake that follows the aircraft spins existing clouds into intricate swirling patterns that radiate from the flight’s path.
If you have a seat overlooking the wing, it’s pretty common to see sudden bursts of condensation out your window as the aircraft ascends through a deck of clouds. The process that generates these clouds is the same one that allows the plane to fly in the first place.
The shape of an airplane’s wing allows it to generate lift by creating low pressure above the wing and high pressure below the wing. The lower pressure allows the air temperature to drop below that of the environment around it, often cooling it to the dew point on moist days; the dew point is the temperature at which the relative humidity will reach 100% and the air is completely saturated.
We see this effect all the time when we open a bottle of soda—the sudden drop in pressure inside the bottle allows the air temperature to drop as well, allowing a sudden cloud to form. When an airplane enters an area of very high humidity with a high angle of attack (the angle between the wing and the direction of the airflow—think of a plane landing with its nose tilted up a bit), the low pressure created on top of and around the wing can easily condense the moisture into short-lived clouds like those we see in the video.
The most noticeable formation is from the wingtip vortices behind the aircraft, which creates both streamers of condensation on either side of the plane and the dramatic swirls in the stratus clouds that are already looming above the ground.
While beautiful, these vortices create a dangerous phenomenon known as “wake turbulence,” which can be hazardous to aircraft following too close behind. The crash of American Airlines Flight 587 in Queens, New York, back in November 2001 occurred as a result of the copilot trying (too hard) to stabilize the aircraft after it flew through the wake turbulence of a Boeing 747 a few miles ahead.
If you ever have a chance to go watch planes one day and you’re directly beneath the path of landing aircraft, you can sometimes hear these vortices as a few seconds after the plane passes overhead. The noise they make can range anywhere from a hissing to a crackling to a loud whipping sound.