In dry fly fishing a fluorocarbon leader will sink in the water very quickly. While this is a desirable quality in other fishing techniques, it is something to be avoided in dry fly fishing. The dry fly sits on the water and the fluorocarbon leader sinks, creating a loop underwater that inhibits the strike when a fish takes. On the other hand, a monofilament leader will actually float on the meniscus of the water and this can mean that the fish may be spooked by the leader.
Dry fly fishing is normally associated with those wonderful summer evenings when the fish rise to follow the emerging fly life and you try to match the hatch. The emerger patterns are designed to emulate the fly as it appears at the surface and has to force its way through the surface tension. Bright days demand the full dressed flies to compete with the adult that hatches, dries its wings and quickly flies off. Damper, cooler days will see the emerging fly taking longer to dry their wings and a fly that sits lower in the water will give better results. Finally, the spinners (egg layers) and spent (dying) flies require a similarly designed imitation, probably with open wings and a semi submerged presentation.
Dry flies, like their name implies are designed to be fished floating on top of the water. Obviously, therefore, they are fished on a floating line. They should be constructed of buoyant materials that ensure that they float, such as deer hair - which is hollow, cul de canard feathers - which have a natural oil content, or even manmade materials like closed cell foam or Antron. Furthermore, they should be constructed in such a way that they will float the right way up - the hook should hang in the water and act like a keel to give ballast and stability to the fly, otherwise they are likely to be blown off the water by the wind and give an unrealistic silhouette when seen from below.