A seasoned soldier aka The weathermaster

[Gilrandir]

While you can get some information about the terrain of the province you are going to invade, there is no way you can be sure of the season your battle will be landed in. Or is there? Can you influence the choice of the season?
A connected question is about the weather you are going to get. While attacking you may choose weather (within certain limits: even if the promised weather is fine it starts to drizzle whenever the AI spots that I have Naphta). What about defending? Is the Naptha curse at work here as well?

[Karl08]

There is no way to influence which season you'll be fighting in, as far as I'm aware. And come to think of it, I think the only time I've fought in snow is when I've fought in Sweden or Norway.

You can choose to wait in hopes of more favourable weather when attacking (though you will only be able to wait 4-5 days at most, not till the next season or anything), but you have no choice whatsoever when defending.

[Gilrandir]

In my experience, winter battles happen not only in northern provinces, but also in most of what is now France, Germany, Poland and Russia. I don't like it as the troops get exhausted even if they stay put (much the same as in the desert). But I hoped someone has a recipe to avoid winter battles.

[gollum]

You can tell which season it is by the music that plays on the camp map. Every tune corresponds to a season, and that season has its own weather parameters. Weather effects are most harsh in Temperate terrain and that in vanilla is far more rare than it is in reality, hence you get less harsh weather overall.

Harsh weather has its own way of fighting, be it heavy rain or snow. Just remember that the enemy is also affected by fatigue and so you can use this to your advantage. In addition bowstrings, xbows/arbs and guns fire less well in bad weather conditions and this means you would take less casualties in a stright attack than otherwise you would have.

[Gilrandir]

In addition bowstrings, xbows/arbs and guns fire less well in bad weather conditions and this means you would take less casualties in a stright attack than otherwise you would have.

But the Guide says that arbalesters (unlike xbows) have metal bowstrings, thus they are not affected by moisture in the air.

[gollum]

Hello Girlandir, i don't have the game installed at the moment and hence i cannot check files or through custom battles, however neither the unit descriptions in-game, nor frogbeastegg's guide are without mistakes*. Experience suggests that volleys by all missile weapons are slower and impact less in bad weather. Yet, as i said i cannot confirm or not that conclusively at the moment.

Be that as it may, the point is that bad weather gives you in general missile cover that favors straight melee attacks aka rush tactics. This is all the more if you aim your line of approach through a wood or similar that provides additional cover.

*for example the arbalesters unit description says iirc that they take longer to reload than xbows, yet that is not so - they take the exact same time if memory serves.