What Is Relative Humidity
Relative humidity is the ratio between the amount of moisture in the air and the greatest amount of moisture the air could hold at the same temperature.
In other words the hotter it is, the more water the air can hold. at that is because air holds different quantities of water at different temperatures.
When the temperatures in your grow room climb the humidity levels will climb with it and moisture starts to condense.
Where does the water go when temperatures drop?
Once the temperatures drop, at night time for example, it condenses onto the surface of plants and grow-room walls, just as dew condenses out side.
The greater the temperature variation, the greater the humidity variation will be, the more variation can increase stress to your plants.
Extra heat or ventilation at night can be necessary if the temperature variation exceeds 8/10°C (15°F)