A Rain Cape is like a poncho. It forms a tent over your bike and body. It is specifically designed to cover your handle bars, your back, and your saddle, when in the riding position. Rain Capes have loops at the front, which you can hook over your thumbs, brake levers, or wrists to keep your hands and handle bars dry. Any rain that hits your upper body drips off the edge of the cape rather than run down onto your legs and into your shoes.
Since the Rain Cape is completely open underneath, it provides plenty of ventilation. Allowing air to circulate around the body while still shedding rain, means you stay completely dry. Good air circulation prevents body heat from building up, insuring that sweat will not soak you from the inside out... a problem with many "waterproof" jackets and pants.
A waist strap holds down the back of the cape to keep it from blowing up as you cycle. The strap works well in moderate and light winds, however, a cape should not be worn in heavy winds.
To keep dirty water and mud from flying up under your cape, you need a pair of full-length fenders and mud-flaps on your bicycle. If you will be touring, or riding in the rain on a continuing basis, fenders are worth the extra few ounces of weight. Fenders shed the water that otherwise would soak you and your gear from underneath the cape. Fenders also keep wet sand from the road off your derailluers and chain.
If you do not want to put fenders on your bike, then you will need to wear waterproof pants and shoe covers to prevent water and mud from splashing up from the tires, soaking your feet and legs.
To maintain good visibility you will need a visor or other waterproof head covering. A hood is not recommended for cycling, as it can limit a cyclists visibility. It is safer to ride in a cape with no hood, and add a waterproof helmet cover.
Rain Capes are usually bright yellow for visibility.
They are great for touring. Rain Capes are small, lightweight, and easy to pack. They provide excellent protection from sudden showers.