During campaigns and matches where the weather is set to random, a random game mechanic called weather might occur, causing different types of effects on the battlefield for the duration of the weather. Random weather effects only last for 1 day.
Some COs can actively increase the frequency of a specific type of weather occurring with their CO ability, or outright change the weather using their CO Power.
When the weather is clear, no extra penalties are imposed on units, making it a usual and fair game.
In Advance Wars and Advance Wars 2: Black Hole Rising, movement of all land vehicular units will be inhibited by one when crossing through terrains that impose a movement penalty. For example, cities don't have a movement penalty, so during rain, all units still only use 1 movement point to go through it. For plains, which normally require 2 and 1 movement point for units traveling on tires and tread respectively to pass through, now use up 3 and 2 movement points. The vision range for all units during Fog of War is also reduced by 1.
For the Blue Moon CO Olaf, movement points required for all his air and naval units to travel across all terrains is doubled, while his Infantries' movement through all non-property terrains, and his Mechs' across mountains is halved.
In Advance Wars: Dual Strike, rain imparts no movement penalties, but Fog of War will always be enabled with reduced vision, even when it is turned off.
Rain could greatly affect your vehicular units in both deployment and retreat. When Drake is present in a match, rain does not affect his units' movement and the chance of rain occurring during random weather is increased by 7%.
In Advance Wars: Days of Ruin, it reduces all units' visibility to 1 and all building visibility to 0.
In Advance Wars and Advance Wars 2: Black Hole Rising, the movement penalty during snow is even more severe than rain. As well as increasing the movement penalty of all land vehicular units traveling across non-road and non-property terrains by one, Infantry, air and naval units require double the normal movement points needed to traverse all terrains. For Mechs, they require one extra move to go through mountains. Units' vision range in Fog of War is unaffected during snow.
Snow makes deploying your units to the frontline all the more difficult, with most units' movement range reduced one way or another. Non-air units going through properties don't suffer any movement penalties in snow as well as rain, so make use of them when moving your units.
Olaf's units don't suffer any penalties during snow, meaning his units' movement and fuel usage is unimpaired. This gives Olaf a momentous advantage in snowy maps, or matches in which the weather condition is set to random. Olaf could also use his CO Power or Super CO Power to cause snow to fall on the map for 1 turn, or 2 turns in Dual Strike. His units gain a 20% attack boost in Dual Strike when snow is present.
In Advance Wars: Days of Ruin, snow flatly reduces all unit movement by 1.
In Advance Wars: Dual Strike, sandstorms reduce the maximum range of indirect units by 1.
Sandstorms make holding chokepoints a lot harder, making it easier for the opponent to push through your lines, since your indirect units cannot fire on enemy units due to their reduced range. This makes Artillery units quite useless.
When sandstorms are in effect, Max's indirect-combat units suffer from a total of -2 range. This makes them pathetic, especially his Rockets and Missiles, where they can only fire 3 spaces away.
In Advance Wars: Days of Ruin, sandstorms reduce the attack power of all units by 30%.