Moisture is known to affect the flowing properties of powders. However, the quantification and the understanding of the observed effects are far to be obvious. To study air moisture influence on powder flowability in a laboratory, a conditioning system and a precise flowmeter are necessary. Simple flow testers, which are commonly used R&D laboratories, are not able to quantify precisely the effect of moisture on powders. The use of advanced techniques is necessary. In this paper, we present how a precise and user-friendly flowability test associated with a dynamic conditioning system can be used to quantify the influence of moisture on the flowability properties of lactose powders. The effect of residence time T in high humidity conditions is analyzed. Afterward, we show that the relative humidity range that optimizes the flowability of lactose powders is between 30% and 50%. For lower values of the relative humidity, the apparition of electric charges inside the bulk induces cohesive forces. For higher relative humidity, the condensation of the air humidity at the contact between the grains forms capillary bridges which favors also the cohesive interactions. A model taking both triboelectric and capillary effects into account is proposed to fit the experimental data.