would you like to walk around and fight in South Spain in full armour? Well, probably not in the summer, but since you can't influence the season of the battle, it might be a better idea to leave those armour 8 knights at home.
In northern Spain right now, it´s hot enough to discourage me from wearing light leather jackets, let alone parade in plate mail.

Jests aside: in MTW, Spain is Temperate, and there´s no heat penalty for units there (excluding camels, that is, so if you´re playing the almos, leave the bedouins at home and use horses)

Also: you CAN influence the weather to an extent. Whereas there´s no clear season change like in Shogun, the campaign map sounds point out whether it´s a "hot" or a "cold" year. I think this is relevant to mention, as it can be important if you´re plotting to invade Morocco or the Middle East with European armored units. Invading Morocco or Syria in winter will mean that the fight will take place in Temperate weather instead of Arid or Desert. That much I know, by testing. I do not know, however, if it applies to anything beyond those provinces (IE: AFAIK Egypt could be a permanent desert)