Can't sleep because of a knee injury.
So I will attempt to bore myself to sleep.
I've noticed that a few people struggle with rh, as I've had a bit of training oh this (year's ago, early in my apprenticeship) I thought I'd share my bit of knowledge.
Humidity is an expression of the amount of water vapor held in the air, this is written as a percentage of relative humidity. What that means is, a percentage relative to the maximum amount of vapor the air can hold.
So if a room full of air at room temperature, can hold two glasses of water vapor before the air is saturated, and did hold all that vapor it's rh would be 100%, if the air only held 1 glass of water the rh would be 50%.
Just to make things even more confusing, this alters with temperature. Hot air holds more moisture than cold air. So, if our room above is heated up and it could hold 4 glasses of water vapor, the one glass it does hold is no longer 50% of the maximum because the max is now higher. So the rh is expressed as a lower value (25%). If the room is cooled down, the amount of vapor it can hold decreases and therefore in relative terms the air becomes more humid.
The temperature at which the air reaches 100% rh is called the dew point. This can be pretty much any temperature, outside in winter this occurs at low temps and can be seen as dew on any surface or as a morning mist. It occurs at higher temps in areas such as kitchen and bathrooms as condensation.
There are various ways to control humidity. The easiest way to control high rh is to chuck out the humid air and replace it (ventilation), most dehumidifiers work by drawing the air over a cooled surface so that condensation forms and drops out of the air. Low rh is easier to contol (generally) as you simply need to make the area wet.
Cheaper measuring equipment is pretty inaccurate regarding rh, but we only really need a rough figure. For the purpose of drying (assuming you don't hang the plants in a running grow or dry with artificial heat) the rh stated on the weather forecast is accurate enough.
Well, my knee still hurts and I'm still awake, so that didn't work!
