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River Science: Relative Humidity

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 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.

On this Voyage of Discovery, the students recorded relative humidity as a component of our AMVER SEAS weather reporting.

We use a variety of instruments to study humidity levels on the Half Moon, including sling psychrometers and whirling hydrometers. In this case, we used the hygrometer provided to us by NOAA. The hygrometer contains both wet and dry bulb thermometers. As water evaporates from the wet bulb, it cools that thermometer. 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 sling psychrometer and whirling hydrometer use similar methods, but involve manually twirling the device to induce evaporation for the former and a wind-up motor to automate the process for the latter.

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

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