Rainout occurs when you are using a heated humidifier and start getting water in your hose. It's caused by the steamy air coming from the humidifier at a higher temperature than the surrounding room air, so it cools on the way to the mask. While it leaves the humidifier as "humid air" when it cools, it reaches the mask as "water" and you get a damp face.
People use a heated hose or a hose cover to help resolve this problem, or turn the heater on the humidifier down so that it is closer to room temperature. You could also try a hose hanger.
Simple Steps to Prevent Rainout
Place the machine at the same level as the bed. If you machine is higher, moisture is more likely to travel down to the mask as the air cools.
Simple Steps to Fix Rainout
Turn the humidifier down, or off.
Take as much of the hose as you can under the bed covers
Wrap the hose in a hose cover.
Purchase or make heated hoses.
Machine Rainout Reduction Features
Fisher & Paykel's Integrated Heated Hose