As a flavor effect, or as a serious impediment? 

I'm not a big fan of losing because I was forced to fight in inclement weather. Almost certainly, if it were real life, my troopers would have just gone back to bed. 

At the very least, if inclement weather is in the offing I should have a chance to delay the battle -- either by waiting in a camp or running around -- until conditions improve. 

Obviously that wouldn't hold for ambushes or magically induced weather.

I'd prefer weather effects giving some unpredictable spice to battles. Going into a fight knowing you'll win is as boring as hell.

You know I'd rather be sure that I wasn't about to throw away a thousand men or my sovereigns life.

im just hoping for terrain bonuses or negative effects.

Trees block arrows or hinder cavalry movement. etc

Hmm... I've been thinking about this the last few minutes since I posted and I'm liking it more and more. 

Weather should have other effects as well, to make you decision to use those spells a bigger deal.  You call up a hurricane to negate your enemies archery advantage, but hurricanes (being massive wind and rain storms) have a chance to damage buildings and terrain improvements.  If you throw up a hurricane to help yourself in battle, you may end up destroying some of your local infrastructure (unless you are invading, then that's a good thing). 

Say you lose a big battle at the mountain pass fort you had guarding the route into the heart of your territory.  Your reinforcements are still being built and the enemy army has free access to rape and pillage their way around your territory or can make a beeline for the soft interior of your country.  You throw up a blizzard, which reduces movement speed and has some other effects (reduces morale/combat stats, small chance of killing off soldiers from freezing to death).  The blizzard buys you time to get your reinforcements in play.  Unfortunately, your blizzard also kills off some of your citizens, destroys some improvements, and halts caravan travel in the area for the duration (because that's what blizzards to do medieval societies). 

You could even tie the strength of the storm partially to how close it is to the caster's seat of power.  Since I am the one that literally brought my land back to life I am more in tune with local nature and weather and so my weather spells are more powerful within my territory. 

I think they should just add some weather magic into the game so you could summon some rain clouds when you facing tons of archers.

If I cast weather spells, I'd like for it to affect magic, too.

• Summoning a thunderstorm makes my lightning bolts more powerful, gusts of wind stronger, and that fire trap my enemy had going? Yeah, that's gonna be put out.
• Or if I summon intense heat over an area, making the grasses burn easier, throwing up a firewall should spread and quickly grow out of my control.
• Or summoning wind with fire going on should spread the fire in the direction of the wind, if feasible.