The right clothing can often mean the difference between life and death, and often, the clothing that seems the most logical is actually the worst option.
In an arid environment like a desert, it may seem like less clothing would be better, but that isn’t the case. Look at what natives wear; long flowing robes and head-wear. Why? One word—sweat. If you paid attention in high school physics, you remember that a liquid’s temperature is directly related to the average kinetic energy of its molecules so it cools as it evaporates. But your water supply is finite; especially in a desert, so you don’t want this to happen too quickly or you’ll run out of water, dehydrate, and die. Loose, lightweight clothing covering most of your body blocks the sun and ensures that your sweat evaporates more slowly, keeping you cooler and helping you stay hydrated longer.
Tropical environments present similar problems with heat, however, due to the humidity, your sweat won’t evaporate as readily. While the thick vegetation, especially in a triple canopy jungle, generally blocks much of the sun, you should still wear long sleeves, long pants, and if possible, a scarf and hat. This will help your sweat evaporate more slowly, but will also protect you from thorny vines and biting insects. It’s also critical in a tropical environment to keep your feet dry.
People tend to dress the most inappropriately in cold environments. I was once guilty of this myself. I’ll give you a hint—if you look like Ralphie from A Christmas Story, you’re doing it wrong. Dress for the activity first, then the temperature; bundle up if you won’t have to move much, but if you’ll be active, dress lighter. Movement will increase your body temperature and wearing too much clothing will increase it even more, causing you to sweat and possibly even overheat. Then you’ll be wet and dehydrated, which leads to hypothermia. The key is to dress in layers, with a thin wicking base layer (I like Under Armour), followed by an insulating layer, followed by a breathable weather-resistant outer layer, like a GORE-TEX® Jacket. Depending on the temperature and your activity, you can add or remove insulating layers to suit your comfort. You should also wear a hat and a pair of Thinsulate gloves. Replace any wet clothing immediately.
You probably never thought about how to dress in the water because 99.9% of the time, you’ll just be in a swimsuit, but if your airplane crashes into the water (and you happen to survive) or your boat capsizes, it matters. It will feel awkward trying to swim in clothing, but keep it on; it will help you retain heat, which is critical because you can suffer from hypothermia in any environment cooler than 98.6 F—which is nearly every body of water. You should take your shoes off though, to make swimming easier. A good way to hold on to them is tie the laces together and hang them over your neck.