When one goes to high Y values (in excess of 20,000) and it is snowing, snowflakes will take on an unusual appearance- primarily in that:

1- they will fall in a more disjointed, glitch-y manner, although their overall motion is not slowed, increasing in glitchiness with Y value.
2- they will become extended vertically in a seemingly static pattern, increasing in height with Y value.

One can reproduce this effect by traveling to any area where it is snowing, making it snow, and traveling to the same X and Z location, with a Y location of at least 20,000, but most easily noticeable above Y=50,000.

The below images are from 1.9 and 1.9.2, although it has also been tested in 16w14a, 16w15b, and 1.9.3 PR1.