What is the difference between humidity and wetness?

Some scholars differentiate between "balal" (wetness/dampness) and "rutubah" (moisture/humidity) in the context of purifying impurities. According to them, "balal" contaminates something dry that it comes into contact with an impurity, whereas "rutubah" does not transfer the dry impurity to itself. Thus, "rutubah" is considered less significant than "balal." They illustrate "balal" with a watermelon and "rutubah" with cheese.