To study humidity levels, we typically use a sling psychrometer 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.

Combining relative humidity and barometric changes, the barometer allows us to accurately predict coming weather fronts, the frequent bearer of stormy weather conditions.