THIS IS WHAT IT LOOKS LIKE when an airplane passes the Speed of Sound. The cloud behind it is called the "Vapor Cone". It is a cloud of condensed water. It is caused by drop of air pressure, which causes the temperature to drop - - so when the temperature drops below the Dew-point temperature, a cloud forms. Now, I have never flown faster than 761.2 mph. {1,225 km / hr.} That is the Speed of Sound at sea level in a Standard Atmosphere {59 degrees F. / 15 degrees C.