Also, when the aircraft is approaching show center, the pilot is feathering the throttle to make sure to keep it just at the threshold. You'll see the vapor cone appear/disappear several times as the pilot is adjusting their airspeed to the wind conditions. High humidity (yeay Portland, Oregon) helps set up a perfect environment and the F/A-18 really produces some nice cones. 
I don't have a camera with a continuous shooting option, but have caught vapor cones on the F/A-18 several times just by eyeballin and lucky timing 
If you have a continuous shutter setting, let 'er rip.
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