SEATTLE - If you've ever seen sheens of rain that appear to fall from clouds, but then just disappear about halfway down, that's known as virga. It's basically rain that evaporates before it reaches the ground.
Virga occurs when you have a layer of drier air near the surface. As the rain falls out of the cloud, it reaches this dry air layer, and just evaporates back into water vapor before it can reach the ground -- although if you're under an area of virga, you'll still likely notice a drop or two that survived the fall.
Virga is quite common in desert areas, where the humidities are generally so low, that most of the rain will just evaporate right out.