My guess is that it's caused by the light source's alternating current, causing it to flicker (most HID and fluorescents are really pronounced) and act like a strobe.
I agree with will! Depending on the lighting an image break can happen. You also have to consider the background and reflective light on the subject. Other wise dark to light as the object moves. If the background lighting and shadows are not a constant enough the image merges and sections of it become as one related to the background especially in the realm of Digital. Digital Cameras run on a set parameters that are determined by its size and sensitivity processing parameters. An extrapolated 12.1 CCD will capture a moving image at a laser quality then a true 12.1 that is designed to take in the image as it captures it. Other wise an extrapolated fills in the holes deciding what belongs there.
If we knew the exposure time then it would be an easy matter to calculate the speed of the snow flakes. That would be a suitable cross-check on the 60Hz theory. If the speed comes out reasonable (as I think it would) then brphoto's idea is probably correct.
Actually, I'm going to have to correct myself. We can't find the speed from that information, but I can calculate the effective shutter speed. I count 11 dots in each snow streak, so the flashing is indeed at 60Hz.
I have another image taken from a moving vehicle where a road sign appears in this fashion. It is like I made multiple exposures and combined them into one all encompassing image. I hope my 4-ways weren't on;-)
yeah, i agree with brphoto and caedes, that the "speckled effect" is caused by the flickering light. depending on if you are in europe or in the us the pronounced parts in the movement would be further apart in europe than in the usa. 50 Hz vs 60 Hz. i never noticed that in motion photgraphy, but it makes perfect sense now that i think about it.
-Brad
Just as a side-side-side note: Only certain types of lights have this flashing effect (one that work like florescent tubes). Incandecent lamps don't do this because their light dies off too slowly when the current inverts.