Relative humidity levels represent the amount of water vapor circulating in the air. (In a sense, relative humidity is the mirror opposite of dissolved oxygen levels in the water.)
Unlike some weather phenomena, humidity has little effect on the Half Moon itself, but the same cannot be said of its crew -- particularly when high humidity tips over into precipitation.
To study humidity levels, we use a sling psychrometer (pictured) and/or a whirling hydrometer. The sling psychrometer contains both wet and dry bulb thermometers. By twirling the psychrometer around for 60 seconds, its user induces evaporation from the wet bulb, cooling it. The resulting temperature differential between the wet and dry bulbs can be used to measure humidity -- the drier the air, the cooler the wet bulb becomes. The whirling hydrometer uses a similar method, but uses a wind-up motor to automate the process.
By combining relative humidity and barometric changes, a barometer allows us to accurately predict coming weather fronts, the frequent bearer of stormy weather conditions.