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Name: Nathan
Status: educator
Age: 40s
Location: N/A
Country: N/A
Date: 1999 

In a recent Current Science magazine (October 22, 1999 Volume 85, Issue 4; page 2), There is a photo of a F/A-18 Hornet jet taken over the Pacific Ocean. The caption states that the picture was taken at the moment the jet broke the sound barrier. The picture depicts a disk-like cloud perpendicular to the tail of the plane, and has the same axis as the fuselage. It is at the rear of the plane. A small cloud also appears just behind the canopy. Where does the water come from to form this structure? What is causing this phenomenon? If it is water vaporizing, is it due to a pressure change? . . . temperature change?

Without seeing the photo I am just making an educated guess, but I suspect what you are seeing is a function of the lowered pressure (due to Bernoulli's principle) causing a lowering of the temperature (adiabatic cooling). When the air is cooled to the dew point, a cloud can form.

This can often be seen as trails coming off the wing tips (or sometimes off whole wings) when a heavy aircraft is landing. When a pilot sets up for a landing, she (sorry for the gender bias) increases the angle of attack which decreases the pressure further on the top of the wing. When the air accelerates around the wing tip - moving form an area of higher pressure to an area of lower pressure - this series of events occurs. I suspect this is what was happening in your F-18 photo... though not at the wing tip.

Larry Krengel

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