Hmm
Rain:
-Reduced Sight Range (After all you cant see that far in Rain)
-Reduced Movement Speed (Its harder to move in Wet Terrain)
-Reduced Reload (Its wet after all so gripping stuff is harder)
-Reduced Range of Projectiles (Rain means much more resistance for an projectile)
-Increased loss of Stamina when Running (Running on Wet Ground is pretty)
-Increased Healing if staying still (For some Reason Rain feels pretty Good on Wounds
)
-Increased Armor against Arrows (Wet Armor is more likely to glitch of Arrows and the Arrow also is slower in the rain)
-Increased Resistance to Fire Damage (well this is self explaining I hope)
-Decreased Resistance to Ice Magic and Electric Damage (See above ^^)
Winter:
-Reduced Movement Speed (Frozen Ground is hard to move on)
-Reduced Reload (Fingers and Limbs tend to be more sluggish when theyre cold)
-Reduced Regain of Stamina (In this Cold its tiring to just stay awake)
-Reduced Self Healing (Having Your Blood and Body slowed down also means that your healing slower)
-Reduced Armor (Armor gets more and more like sandstone when its cold ^^)
-Increase in Sightrange ( In the White of the Winter you can See anything that Moves on two Legs on 100 miles away :P )
-Increased Resistance to Fire Damage (Nothing Burns here to begin with ^^)
-Decreased Resistance to Ice and Electric Damage (Well thats logical I guess)
Night:
-Reduced Sightrange (We Aint Vampires after all ^^)
-Reduced Reload (Its hard doing this when you dont see your weapon)
-Increase of Healing when staying still (The Night tends to give a calm feeling to people resting which allows to heal up faster)
-Increase of Stamina Regain when staying still (See above)
-Increase of Armor (Hitting the Weakspots in the Darkness is hard so your likely to hit the better Armored Plates)
-Reduce of Accuracy on Ranged Weapons (Aiming when you cant clearly see your target is an skill in itself ^^)
Just to name some possibilitys :P