Humidity: It's All Relative

     *Relative humidity* is the amount of moisture in the air compared to what the air can "hold" at that temperature. Technically, air doesn't "hold" water. It has to do with vapor densities, and thermal /barometric changes. As air temperature and pressure changes, it's relative humidity changes. 
     Although cannabis is a hearty plant, for the best medicinal results, keep your humidity in the plants' optimal range.  This range varies depending on the stage of growth that your plants are in. Here is a list of ideal humidity ranges for different life stages of cannabis:

80-90% - ROOTING CLONES AND CUTTINGS

60-70% - VEGETATIVE GROWTH

40-60% - FLOWERING

Higher humidity is needed for your precious babies as they do not have the root support to obtain the proper amount of water from their grow medium.  When your plants bud, too much moisture can accumulate in  the clusters of buds. Lowering the humidity helps control mold and other fungal contaminants from developing on your plant.  
     I caution you to not be reckless and go all "Death Valley" on your growroom.  Too little moisture will stress your plants!  However, I have heard of some growers deliberately stressing their plants by drastically dropping the humidity during the final few days before harvest to increase resin production. They claim that the  resin acts as the plant's natural "moisture-lock" on the buds in an attempt to protect the flowers for future pollination.  Little does the plant know that pollination is not in its future!