Weather is an often overlooked aspect of the D&D environment. Sure, sometimes the DM will remember to mention that it's snowy, or that the rain is coming down hard; but to keep players immersed and cognizant of the weather conditions, attaching a mechanical effect can work wonders.

From blistering heat to driving rain; from slogging through snowdrifts to straining against the wind; from peering through fog to shielding your eyes against the bright sun. All of these things can add atmosphere to an encounter and be used by both characters and creatures in interesting tactical ways. Even better, you can mix and match these weather conditions - it can be foggy AND raining; it can be windy AND sunny. Or any combination thereof - it's a fantasy world, after all.