Humidity is a measure of the invisible water vapor mixed with the air, representing the atmosphere’s moisture content. This atmospheric moisture is a fundamental component of weather and climate, regulating air temperature and contributing the latent energy that drives storms. Measuring humidity is important for human comfort, as high moisture levels interfere with the body’s natural cooling process of sweat evaporation. Humidity measurement is used widely in weather forecasting, indoor climate control, and industrial processes where moisture can impact materials and product quality.
The Standard Measurement: Relative Humidity
Relative Humidity (RH) is the most common measurement of atmospheric moisture, frequently reported in weather forecasts. It is expressed as a percentage, indicating the ratio of the amount of water vapor currently in the air compared to the maximum amount the air could hold at that specific temperature. The “relative” aspect means the measurement is dependent on the air’s temperature, since warmer air has a greater capacity to hold water vapor than cooler air.
An RH reading of 100% signifies that the air is completely saturated and cannot hold any more water vapor at that temperature. When this saturation point is reached, the air is at its dew point, and further cooling will cause the excess water vapor to condense into liquid, forming dew, fog, or clouds. Conversely, an RH of 0% would mean the air contains no water vapor, a condition rarely seen in the natural atmosphere.
A constant amount of water vapor in the air will yield a lower RH percentage if the air temperature rises, but a higher RH percentage if the temperature drops. This explains why a cold morning can feel damp with a high RH, even if the total amount of water vapor is less than on a warm afternoon. Relative humidity is a direct indicator of the potential for condensation and is frequently monitored in residential and commercial settings to prevent mold growth and maintain comfort.
Quantifying Mass: Absolute and Specific Humidity
While Relative Humidity describes the air’s saturation level, other measurements are used to quantify the actual mass of water vapor present. Absolute Humidity (AH) is a direct measure of the total mass of water vapor in a given volume of air. It is typically expressed in grams of water vapor per cubic meter of air (\(g/m^3\)).
Absolute humidity can range from near zero in very dry, cold air to roughly 30 \(g/m^3\) in saturated air. This measurement is straightforward, but it changes if the air expands or contracts due to temperature or pressure variations, even if no moisture is added or removed. Because the volume of air is not stable, absolute humidity is less useful for atmospheric science calculations than other mass-based measurements.
Specific Humidity, sometimes called the mixing ratio, is the preferred mass-based measurement used by meteorologists. It is defined as the ratio of the mass of water vapor to the total mass of the moist air parcel. This is commonly expressed in grams of water vapor per kilogram of air (\(g/kg\)).
Specific humidity is a stable measurement because it compares mass to mass, meaning its value does not change as the air temperature or pressure fluctuates. This stability makes it invaluable for tracking air masses and moisture transport in weather models. It provides a direct quantification of the actual moisture content.
Instruments for Measuring Water Vapor
The general term for an instrument used to measure humidity is a hygrometer. Hygrometers do not measure water vapor directly but instead measure a physical or electrical property that changes in response to moisture content. The resulting data is then converted into the appropriate units, such as percentage, mass/volume, or mass/mass.
One common type is the psychrometer, which uses two thermometers: a dry-bulb and a wet-bulb. The wet-bulb thermometer has its bulb covered in a moist cloth; as water evaporates from the cloth, it cools the bulb. The difference between the dry-bulb temperature and the lower wet-bulb temperature is used to calculate Relative Humidity.
Modern electronic hygrometers often use capacitive or resistive sensors to determine moisture level. Capacitive hygrometers measure the change in electrical capacitance of a thin material as it absorbs water vapor. Resistive hygrometers measure the change in electrical resistance of a moisture-sensitive material, providing quick and accurate readings for digital weather stations and home use.
The hair tension hygrometer is an older, analog device that uses organic material, such as a strand of hair, to measure moisture. Human hair lengthens as humidity increases and shortens as it decreases, and this mechanical change is linked to a dial that displays the Relative Humidity. High-precision instruments, such as the chilled-mirror hygrometer, determine the dew point by cooling a mirror until condensation forms, and this temperature is used to derive other humidity values.