Relativity humidity is the amount of moisture in the air as a percentage of the maximum amount possible at a given temperature. Relative humidity is given in units of %RH. It is important to measure and control humidity within greenhouses and indoor hydroponic gardens. As part of photosynthesis plants transpire water through pores in their leaves, called stomata, into the air. This process also keeps plants cool, similar to sweat, and prevents them from overheating and drying out. Having the correct humidity helps plants to grow faster. It also prevents the growth of mold and pests.