Hot air holds more water vapor than cold air.
High humidity leads to pest and disease problems and causes vapor to condense when warm water-filled air hits a cold surface. Low humidity, on the other hand, can dry out plants. Transpiration from plant leaves contributes greatly to greenhouse humidity. To avoid excessive humidity, water early in the day and only when needed, and make sure you have good greenhouse ventilation. If your humidity is too low, you can raise it by periodically wetting down the floor or misting plants.