Air temperature dictates the capacity of the atmosphere to hold water vapor, directly influencing the percentage known as relative humidity (RH). If the actual amount of moisture in the air remains constant, an increase in temperature causes the relative humidity to decrease, and a decrease in temperature causes the relative humidity to rise. This inverse connection is central to understanding how moisture behaves in the environment.
Defining the Key Components of Humidity
To understand this dynamic, it is necessary to distinguish between the two primary ways humidity is measured. Absolute humidity (AH) is the actual mass of water vapor present in a fixed volume of air, typically expressed in grams per cubic meter (g/m³). This value does not change when the air temperature changes.
Relative humidity (RH) is a percentage that compares the amount of water vapor currently in the air to the maximum amount the air can hold at that specific temperature. It is a ratio indicating the air’s degree of saturation. Because the air’s maximum holding capacity depends on temperature, the RH percentage changes constantly even if the actual moisture content remains the same.
The Physics of Water Vapor Capacity
The mechanism linking temperature and air’s moisture capacity is saturated vapor pressure (SVP), the maximum pressure water vapor can exert before it condenses. Temperature measures the average kinetic energy of air and water molecules. Warmer temperatures mean faster-moving water molecules, making it easier for them to escape a liquid surface and remain in the vapor phase.
This increased molecular energy means that warmer air requires a much higher vapor pressure to force the water molecules to condense back into liquid. Consequently, the SVP increases exponentially with temperature, allowing warm air to hold significantly more water vapor before reaching 100% saturation. For example, the maximum absolute humidity roughly doubles for every 11°C (20°F) increase in temperature.
Conversely, colder air has a much lower SVP because its water molecules move slower and require less pressure to return to the liquid state. This lower capacity means that a small amount of water vapor in cold air can represent a high percentage of the maximum possible, resulting in a high relative humidity. This physical principle explains why simply heating a parcel of air, without adding or removing any moisture, immediately lowers the relative humidity.
The Critical Role of the Dew Point
The dew point is a temperature measurement that provides a direct, absolute measure of the moisture content in the air. It is defined as the temperature to which air must be cooled, at constant pressure and moisture content, to reach 100% relative humidity. When the air temperature drops to the dew point, the air becomes saturated, and further cooling causes water vapor to condense into liquid droplets, forming dew, fog, or clouds.
The difference between the current air temperature and the dew point determines the relative humidity. A large difference indicates low relative humidity and dry air, because the air must be cooled significantly to reach saturation. A small difference between the air temperature and the dew point means the air is close to saturation, resulting in a high relative humidity. Because the dew point is independent of the air temperature, it is often a better indicator of how “muggy” the air feels than relative humidity alone.
Real-World Effects and Applications
The temperature-relative humidity relationship affects everyday life, particularly within indoor environments. In cold climates, when frigid outdoor air with low absolute moisture content is brought inside and heated, its temperature rises dramatically. This sharp temperature increase causes the air’s capacity to hold water vapor to soar, which causes the relative humidity to plunge, often leading to very dry indoor air, static electricity, and dry skin.
Air conditioning systems in warm, humid climates exploit this relationship to dehumidify spaces. An air conditioner cools the air below its dew point temperature, causing the excess water vapor to condense into liquid on the cold evaporator coils. This process removes moisture from the air, simultaneously lowering the temperature and the dew point, making the air feel less sticky and more comfortable.