Relative humidity describes the amount of water vapor held in the air compared to the maximum amount the air can hold at that specific temperature. Expressed as a percentage, this value indicates how close the air is to the saturation point. Since warm air holds more moisture than cold air, a fixed amount of water vapor can result in low relative humidity on a hot summer day but high relative humidity on a cold winter morning. Pennsylvania’s climate shifts between muggy summers and frigid winters, requiring residents to manage indoor moisture levels throughout the year due to constant fluctuations in air moisture.
Geographical and Seasonal Variation Across Pennsylvania
Pennsylvania’s climate features significant regional differences, especially concerning summer moisture. The eastern and southeastern regions, including Philadelphia, experience the highest incidence of oppressive summer humidity. This is measured by the dew point, which is an absolute measure of moisture in the air; conditions are uncomfortable when the dew point rises above 65°F. In summer, eastern areas frequently experience dew points exceeding 70°F, making the air feel thick and heavy.
The Great Lakes influence moisture levels, with the Erie region in the northwest exhibiting higher average afternoon relative humidity (around 65%) compared to inland cities like Pittsburgh (around 55%). Central mountainous regions, such as the Alleghenies, often see slightly lower summer humidity and cooler temperatures. However, the entire state remains subject to the humid continental climate, with morning relative humidity readings frequently high.
The humidity challenge shifts with the onset of winter when outdoor temperatures drop significantly. Cold air contains very little water vapor, a concept known as low absolute humidity. When this cold, low-moisture air is drawn into a home and heated, its relative humidity plummets, often falling below 20%.
The outdoor relative humidity percentage during a Pennsylvania winter may appear high, particularly during fog or snow. This high percentage means the cold air outside is close to its saturation point, but the absolute volume of water is minimal. Running a furnace or central heating system only warms the existing, low-moisture air, creating bone-dry indoor conditions.
Strategies for Managing Indoor Humidity Levels
To maintain the ideal relative humidity range of 40% to 60%, seasonal mechanical solutions are necessary. In summer, the primary goal is moisture removal, partially accomplished by air conditioning systems. When warm, humid air passes over the AC’s cold evaporator coil, excess water vapor condenses into liquid droplets and is drained away.
However, an air conditioner designed primarily for cooling may not run long enough to adequately dehumidify, often leaving a clammy feeling. In these instances, a stand-alone or whole-house dehumidifier is useful, particularly in moisture-prone areas like basements. Dehumidifiers actively condense water vapor, ensuring the indoor moisture level remains below the 60% threshold necessary to inhibit mold and dust mite growth.
Year-round, proper ventilation controls localized moisture sources inside the home:
- Exhaust fans in the kitchen should be used while cooking.
- Bathroom fans should run for at least 15 to 20 minutes after showering to vent steam directly outside.
- Moisture produced by clothes dryers must be directed out of the living space.
- Moisture produced by dishwashers must be directed out of the living space.
When winter arrives and indoor relative humidity drops below 30%, the focus shifts to adding moisture using a humidifier. Whole-house humidifiers connect directly to the HVAC system, distributing consistent moisture throughout the entire home via ductwork. These systems are more effective than portable units, which only treat the air in a single room. Monitoring adjustments with a simple hygrometer ensures the indoor relative humidity remains within the beneficial 30% to 50% range during the heating season.
Mitigating Health and Personal Discomfort
The human body reacts to extremes in air moisture, leading to various health and comfort issues associated with Pennsylvania’s seasonal humidity swings. During the humid summer months, the body’s natural cooling mechanism, which is the evaporation of sweat, becomes inefficient because the air is already near saturation. This lack of evaporation causes a person to feel hotter than the actual ambient temperature, increasing the risk of heat exhaustion. High moisture also promotes the growth of mold and dust mites, which can trigger respiratory irritation and exacerbate symptoms for individuals with allergies or asthma.
In contrast, the low indoor humidity of winter can cause a different set of physical problems. Air that is too dry pulls moisture from the body, leading to dry, flaky skin, chapped lips, and irritation of the eyes. Low humidity also dries out the mucous membranes lining the nasal passages and throat, reducing the body’s natural defenses and potentially increasing susceptibility to airborne viruses.
Personal strategies offer immediate relief that complements mechanical humidity control. Maintaining adequate hydration is important year-round, as the body requires water to manage temperature in summer and to keep mucous membranes moist in winter. Applying topical emollients and lotions can counteract the drying effects of low winter air moisture on the skin. Appropriate clothing choices, such as lightweight, breathable fabrics in summer, help facilitate sweat evaporation and manage personal comfort levels during the state’s muggiest periods.