This happens because of the vapour pressure of water. What this means is that no matter what the temperature or pressure, there will always be a tendency for some water molecules to want to "jump" out of the dish into the air to become gas. The vapour pressure depends on the temperature, the air pressure, and the nature of the substances involved. It's different for all substances. Water has a reasonably high vapour pressure; that's what gives us so much rain and cloud on Earth.