Well, there are really three kinds of trace. The ripples look just like a line of heat mirage, but are concentrated around the trajectory. At Camp Perry, standing behind shooters at the 600-yard line on a clear day, with blue sky in the background, the firing trajectory is arcing high enough that you can easily watch these lines from every gun while the bullet is above the target with the blue sky in the background. Some are fairly straight, while others make a clear helix due to the bullet from those having greater initial yaw to recover from. Coaches will look for these in their large coach spotting scopes to see where misses are going.
This video shows one pretty clearly at 1:28.
Another type of trace is the brief vapor condensation trail behind the bullet when fired in humid air. This is due to the partial vacuum behind it that exists until air can move in to fill it. In humid air, along with dropping pressure, that partial vacuum drops temperature below the dew point of the water, so you get what looks like a gray streak behind it. It's the same phenomenon that makes vapor cones around aircraft shockwaves or just behind the leading edge of the spherical shockwave of a large explosion.
This poor-quality video appears to show some of both of the above trace types. The mirage trace is visible in all of the shots, but I believe I am picking up the short-lived vapor trace just behind the bullet as well.
The third kind of trace is seen only by the shooter AFAIK. If you are shooting with bright sun at your back and low in the sky (just an hour or so after sunrise shooting westward or later in the afternoon shooting eastward), and you fire a bullet with a solid copper base, the sun can reflect off the back of it and you see a clear almost instantaneous glimpse of a round copper disc as the bullet angle lines up to reflect the solar glint back to your eye. This last one has always been called "bullet glint" to the best of my knowledge, but I don't think it is an officially recognized term.