How Do Meteorologists Use the Hydrometer?
Meteorologists use sophisticated tools to measure atmospheric phenomena and make predictions. One tool -- the hydrometer -- contains a long stem of mercury in a glass tube; in appearance it is similar to a laboratory thermometer. A weight is located on the bottom of the stem, and a graduated scale is printed on the external side of the device. It also contains two thermometers -- one kept wet and the other dry -- whose temperatures are compared to measure humidity.
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Measuring Humidity
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Hydrometers measure humidity, indicating the percentage of water content in the air. The hydrometer is lowered into a liquid and allowed to settle. A reading is taken from the top surface of the liquid. One thermometer must be kept wet at all times. Evaporation gives the wet bulb thermometer a lower temperature. Humidity is computed by comparing the temperature difference of the dry- and wet-bulb thermometers.
Predicting Precipitation
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Chances of rain increase as relative humidity rises; the hydrometer measures relative humidity. The hydrometer also measures the moisture content of warm air that is rising. A meteorologist compares the humidity of the rising air to temperatures and conditions at higher elevations. That information allows him to make a prediction of precipitation.
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Predicting Acid Rain
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A hydrometer placed in liquid sinks at a rate dependent on the composition of the liquid. For example, the device sinks deeper in a non-acidic substance than in an acidic one. Meteorologist use this sink-rate to measure the acidity of moisture in the air. When acidity is high enough --and rain is predicted -- an acid-rain storm is forecast.
Measuring Indoor Humidity
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Some indoor spaces -- such as greenhouses -- must be kept at a high level of humidity. Other locations -- such as museums and humidors -- must be kept at low levels of humidity. Hydrometers measure the humidity in these locations via the same method used for measuring outdoor weather.
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