Foul weather can change your trip considerably, especially if the rain persists. However, if you’re dressed for the conditions you’ll have a better chance of physically (and mentally) dealing with what nature throws your way. If you’re not prepared for bad weather, your trip can soon become an ordeal to survive rather than a positive time spent out in the woods.
First, consider what type of conditions you could face while out in the elements and the length of your stay. If it’s mid-summer and you’re merely going out for a day hike, a light-weight, seam-sealed, breathable rain jacket will probably do. It’s easy to throw something like this in your pack even if there is only the slight chance of showers. In all likelihood it will be warm rain you’re dealing with and you’ll be counting on water protection (and not warmth) from your chosen jacket.
If you’re dealing with more moderate all-day rain you’ll have to step it up to a more rugged stormproof (waterproof) and breathable jacket. You need a jack that’s reinforced in the shoulders, side and back, to reduce wear and tear. You will also need a large hood so you can wear a cap underneath to keep the hood brim up. Good wrist and waist enclosures are also an asset. But most important, you need zippers under the armpits to help ventilation. If you sweat then you’ll get wet from the inside out. Ventilation is key to stopping that.
Rain pants need zippers as well. They provide ventilation, but also allow you to pull your rain pants on and off over your boots (no one wants to take their boots off while it’s raining).
Rain hats, like one of those southwesters, can make a great addition. It allows you more free movement to look around than a rain jacket hood.
For colder weather you’re going to have to layer up underneath the rain jacket as well. Use a light weight synthetic (not cotton) water repelling fabric that will wick water, and your sweat, away from your body.
And the colder the weather the more rugged the rain pants as well. For off-season trekking you’ll have to upgrade to a thicker (but still lightweight) material, with zippers, large leg openings and reinforced material along the lower inner leg.
Gear shown in video: