Low temperatures and low humidity can reduce the speed of sound in air, making previously subsonic rounds supersonic. For example, at 68 degrees, the round has to be slower than 1125 fps, but at 32 degrees, the round has to be slower than 1086 fps.
Even if the actual bullet is subsonic, it may be possible for the expanding gases or mini shockwaves coming off the bullet to be faster than the speed of sound.
