They are actually directly proportional to each other. Air has a higher relative humidity when it is colder. So in other words, the colder your grow room is, the less humidity you'll be able to withstand. Google VPD charts.. they well point you in the right direction. It basically explains to you where your humidity should be based on your temperature. That way you can adjust whatever one you CAN control (ie. Humidifier) based on the other variable.