It is a common belief that the air in winter is completely devoid of moisture, but this is a misconception. Water vapor, the gaseous form of water, is always present in the atmosphere, regardless of the temperature. The sensation of dry winter air is not due to a total absence of moisture, but rather the result of a significant shift in the physics of how air holds that moisture. Understanding the difference between the actual amount of moisture present and the air’s capacity to hold it explains why cold air feels so dry, especially once it moves indoors.
Understanding Absolute and Relative Humidity
To accurately understand winter air, it is necessary to distinguish between two specific measurements of water vapor: absolute humidity and relative humidity. Absolute humidity (AH) is a direct measure of the actual mass of water vapor contained within a specific volume of air, typically expressed in grams per cubic meter (g/m³). This value measures the total amount of water molecules present and does not change simply because the air’s temperature rises or falls.
Relative humidity (RH), on the other hand, is a percentage that expresses the air’s current moisture content compared to the maximum amount of moisture it could possibly hold at that specific temperature. Warmer air has a much greater capacity to hold water vapor than colder air, meaning that the air’s saturation point is directly tied to its temperature. If the air is holding half the moisture it is capable of holding at that temperature, the relative humidity is 50%. This measurement is what relates most closely to how humid or dry the air feels to people.
The Mechanism Behind Winter Dryness
The perception of dryness in winter stems from the fundamental relationship between temperature and the air’s capacity to hold water vapor. When outdoor temperatures drop significantly, the air’s maximum capacity to hold moisture also decreases substantially. For instance, air at a freezing temperature of 32°F (0°C) can hold only a small fraction of the moisture that air at 68°F (20°C) can hold.
Even if the cold outdoor air is highly saturated (e.g., 80% relative humidity), its corresponding absolute humidity is very low because the total capacity is small. When this cold, low-absolute-humidity air infiltrates a building and is heated, its temperature may rise dramatically from near freezing to 70°F (21°C). The amount of water vapor remains constant during this heating process. However, because the air’s capacity to hold moisture increases exponentially with the temperature rise, the relative humidity plummets. Air that was 80% RH outside might drop to 20% or less once heated indoors, creating an extremely dry indoor environment. This process essentially turns the indoor environment into a climate comparable to a desert.
Practical Impacts of Low Winter Moisture
The extremely low relative humidity leads to noticeable physical and environmental consequences. Low moisture levels cause rapid evaporation from surfaces, including human skin and mucous membranes. This excessive moisture loss results in physical symptoms such as dry skin, chapped lips, and eye irritation.
The dryness affects the respiratory system by drying out the protective mucous membranes lining the nose and throat. This reduces their effectiveness at trapping airborne pathogens and can increase susceptibility to respiratory issues and infections.
Impacts on the Home
Low humidity can have detrimental effects on materials that rely on moisture content for structural integrity.
- Wood floors, furniture, and cabinetry can shrink and crack as moisture evaporates into the dry air.
- Static electricity builds up when low moisture content prevents electrical charges from dissipating easily.
- The air’s dryness allows small airborne particles, including dust and viruses, to remain suspended longer, impacting indoor air quality.