The common term “humidity” and the scientific measurement “relative humidity” are often used interchangeably, but they refer to distinct properties of the air. Both measurements relate to the water vapor present in the atmosphere, but their definitions are fundamentally different. Understanding this distinction, especially the role of temperature, is necessary for accurately gauging weather conditions, human comfort levels, and potential issues like condensation. The core difference lies in whether the measurement expresses the actual amount of water present or a percentage of the air’s saturation point.
Absolute Humidity: Measuring Water Content
Absolute humidity (AH) is a straightforward measure of the actual mass of water vapor contained within a fixed volume of air. It quantifies the pure water content and is typically expressed in units such as grams of water vapor per cubic meter of air (g/m³). This measurement represents the total moisture available in the air, regardless of the air temperature or pressure.
The value of absolute humidity can range from nearly zero in dry conditions to approximately 30 g/m³ in fully saturated, warm air. Because it is a measure of mass per volume, AH is a direct indicator of the total water vapor that would be extracted if condensed out of the air sample. This measurement remains constant unless water vapor is physically added to or removed from the air parcel.
Relative Humidity: Measuring Saturation
Relative humidity (RH), in contrast, is a percentage that expresses the air’s current state of saturation. It compares the actual amount of water vapor present to the maximum amount the air could possibly hold at that specific temperature. A reading of 50% RH signifies that the air is holding exactly half the moisture it is capable of holding.
This percentage provides a dynamic and useful picture of the air’s condition. A 100% RH reading means the air is completely saturated and cannot accept any more water vapor. At this point, water vapor will begin to condense into liquid water, potentially forming dew, fog, or precipitation.
Why Temperature Changes the Relationship
The key distinction between these two measurements is the role of temperature, which directly controls the air’s capacity to hold moisture. Warmer air can hold significantly more water vapor than colder air, sometimes doubling its capacity for roughly every 10 degrees Celsius increase in temperature. This relationship explains why absolute humidity and relative humidity differ.
If the absolute amount of water vapor remains the same, a change in temperature will cause a shift in the relative humidity. For example, if air is cooled, its maximum holding capacity decreases, causing the RH percentage to rise sharply. Conversely, if that same air is warmed, its capacity increases, and the RH drops, making the air feel drier, even though no water has been removed.
Real-World Impact
The difference between these two measurements has significant consequences for human experience and building science. Relative humidity most affects human comfort because it dictates the rate at which sweat evaporates from the skin. When RH is high, sweat evaporates slowly, impairing the body’s natural cooling mechanism and making the air feel muggy and warmer.
RH also indicates the likelihood of condensation and precipitation for weather forecasting. Managing RH is important for preventing material damage. High RH levels promote the growth of mold and mildew and can cause warping in wood and paper products. The dew point, the temperature at which RH reaches 100%, is often a more useful metric for assessing moisture hazard.