What Is Relative Humidity and How Is It Measured?
Relative humidity indicates the amount of moisture present in the air compared to the maximum amount it could hold at a specific temperature. It measures how saturated the air is with water vapor.
How Relative Humidity is Determined
Air’s capacity to hold water vapor increases as its temperature rises. When air holds the maximum possible amount of water vapor for its temperature, it is considered saturated, reaching its dew point.
The dew point is the temperature at which air becomes saturated with water vapor, causing condensation to occur. At this temperature, relative humidity is 100%. When air cools to its dew point, water vapor transforms into liquid water, forming dew, fog, or clouds. The difference between the air temperature and the dew point indicates how much moisture the air can still absorb before becoming saturated.
Factors Influencing Relative Humidity
Temperature plays a primary role in influencing relative humidity. Warmer air possesses a greater capacity to hold water vapor than cooler air. If the actual amount of water vapor in the air remains constant, a decrease in temperature will cause relative humidity to rise. Conversely, an increase in temperature will lead to a decrease in relative humidity, as the air’s capacity to hold moisture expands.
Atmospheric pressure can also exert a minor influence on relative humidity. While less impactful than temperature, changes in pressure can slightly alter the air’s density and its ability to hold water vapor. Temperature remains the predominant factor dictating fluctuations in relative humidity.
Why Relative Humidity is Important
Relative humidity significantly affects human comfort. When humidity levels are high, the air feels warmer because sweat evaporates more slowly from the skin, hindering the body’s natural cooling process. Conversely, very low humidity can cause discomfort by drying out skin, eyes, and mucous membranes, and can lead to static electricity.
Health is also influenced by appropriate humidity levels. High relative humidity can promote the growth of mold and dust mites, which can exacerbate respiratory conditions like asthma. Maintaining indoor humidity between 40% and 60% is recommended to minimize these health risks. Very dry air can irritate the throat and nasal passages, potentially increasing susceptibility to respiratory infections.
Beyond human health, relative humidity impacts physical environments. Wooden furniture and structures can expand in high humidity and contract in low humidity, potentially leading to warping or cracking. Sensitive electronics can also be affected, with high humidity risking condensation and corrosion, while low humidity can increase static discharge, which may damage components. Plants generally thrive within specific humidity ranges, as it influences their transpiration rates and overall growth.
How Relative Humidity is Measured
Relative humidity is measured using instruments called hygrometers. One common type is the psychrometer, which uses two thermometers: a dry-bulb thermometer measuring the ambient air temperature and a wet-bulb thermometer with its bulb covered by a water-soaked wick.
As water evaporates from the wet wick, it causes a cooling effect, making the wet-bulb thermometer read a lower temperature than the dry-bulb. The rate of this evaporative cooling depends on the humidity of the surrounding air; drier air allows for more evaporation and thus a greater temperature difference between the two thermometers. By comparing the readings from both thermometers, relative humidity can be calculated using a psychrometric chart or formula.
Modern electronic hygrometers utilize sensors that measure changes in electrical properties due to moisture absorption. Capacitive hygrometers, for example, measure how humidity affects a material’s electrical charge storage capacity. Resistive hygrometers detect changes in a material’s electrical resistance as it absorbs water vapor. These digital devices provide direct readings of relative humidity.