Relative Humidity
The amount of moisture in the air relative to saturation, a minor air-density input that affects bullet drop far less than most shooters expect.
Relative humidity is the percentage of water vapor the air holds compared to the most it could hold at that temperature. Counterintuitively, humid air is slightly less dense than dry air, because water molecules are lighter than the nitrogen and oxygen they displace, so high humidity reduces drag a touch and trims drop a hair. The effect is real but small, usually swamped by changes in temperature and pressure across a normal day.
For trajectory work, humidity enters a solver alongside barometric pressure and temperature as one component of air density, and it folds into the density altitude figure many shooters use. Because its influence on a drag model is so modest, careful shooters track it but rarely let it drive a correction. Sound ballistics practice is to spend attention on the larger density inputs first and treat humidity as a fine adjustment.