Snow

Snow is precipitation in the form of frozen water crystals, falling as several snowflakes. When snowfall begins at the start of a turn in an Advance Wars game, it creates movement difficulties for most Commanding Officers (COs).

Advance Wars

Snow falling on an Advance Wars 2 battlefield

Snow causes movement costs to increase for several types of Terrain, as shown below.
During snowfall, units under Olaf's command can move as if it was Clear weather. His CO Power (Blizzard) also causes it to snow for a turn.

Advance Wars 2: Black Hole Rising

Snow's effects weren't changed from AW1 to AW2, but snow could also be invoked by Olaf's Super CO Power, Winter Fury.

Advance Wars: Dual Strike

Snow no longer restricts movement, but instead doubles units' fuel consumption when moving. Olaf is immune to this effect and gains a 20% attack boost in snow. A CO with the Icebreaker skill rank also gets a 20% attack boost in snow. This can stack with Olaf's ability.

Advance Wars: Days of Ruin

Snow was changed again in Days of Ruin, where it reduced the movement range of all units by one space. Penny's units were immune to this effect.