This is the end result of playing with the new weather system in K.I.S.A. Below is a brief explanation of each element.
The image used for fog/clouds has been updated to one that blends better when overlaid with itself multiple times. The clouds behind the mountains are only one layer in most levels, but the fog uses 2-4 layers. This gives a more realistic simulation of fog by making it appear like the patches of opacity and transparency are shifting positions.
The snow particles now use a softer sprite, and scale their size and speed based on distance from the camera. They also use a new system to trace paths through the air, reacting individually to the wind to form more interesting paths than their previously straight lines through the air.
Thanks to GMC user petenka for his snow and fog examples.