Air becomes dry when it contains very little water vapor relative to how much it could hold. The single biggest factor is temperature: the atmosphere’s capacity for moisture increases by about 7% for every 1°C rise in temperature, so cold air naturally holds far less water vapor than warm air. But temperature is only the starting point. Geography, weather patterns, indoor heating, and air conditioning all play roles in stripping moisture from the air you breathe.
Temperature Controls How Much Moisture Air Can Hold
The relationship between temperature and moisture capacity follows a principle in physics called the Clausius-Clapeyron equation. In practical terms, it means that as air warms up, it can hold exponentially more water vapor. Air at 30°C can hold roughly four times as much moisture as air at 10°C. This is why tropical climates feel so humid and why winter air feels bone-dry even when it’s foggy outside.
This relationship also explains why “dry” is relative. There are two ways to measure humidity. Absolute humidity is the actual amount of water vapor in the air, measured in grams per cubic meter. Relative humidity compares that actual amount to the maximum the air could hold at its current temperature, expressed as a percentage. A winter day at 5°C with 100% relative humidity still contains far less total moisture than a summer day at 30°C with 50% relative humidity. When people describe air as “dry,” they’re usually reacting to low relative humidity, which is what determines how quickly moisture evaporates from your skin, sinuses, and throat.
Why Winter Air Is So Dry Indoors
Cold winter air starts with very little absolute moisture. Continental polar air masses, the ones responsible for cold snaps across much of North America and northern Europe, are defined by their low temperatures and minimal moisture content. When that air gets pulled into your home and heated, its temperature rises but its water vapor stays the same. Since warm air can hold so much more moisture, the relative humidity plummets.
Consider air that enters your home at 0°C and 80% relative humidity. Once your furnace heats it to 22°C, the same parcel of air now sits at roughly 20% relative humidity, without a single molecule of water being removed. The air didn’t lose moisture. It just gained the capacity for much more, making the existing moisture seem like almost nothing. This is why indoor air in winter often drops well below the 40% to 60% relative humidity range that research links to optimal comfort, immune function, and virus resistance.
High-Pressure Weather Systems
Large high-pressure systems are one of the most common causes of prolonged dry spells outdoors. In a high-pressure zone, air sinks from higher altitudes toward the ground. As it descends, increasing atmospheric pressure compresses it, which raises its temperature. That warming effect, just like indoor heating, drops the relative humidity. The air mass essentially dries itself out on the way down. This is why clear, sunny days under high pressure often come with noticeably dry air, and why prolonged high-pressure systems can lead to drought conditions.
Mountains and the Rain Shadow Effect
Geography plays a major role in regional dryness. When moist air from an ocean or large body of water encounters a mountain range, it’s forced upward. As it rises, it cools, and its moisture condenses into clouds and precipitation on the windward side. By the time the air crosses the peaks and descends on the other side, it has already dumped most of its water. The descending air then compresses and warms, further lowering its relative humidity.
This process, called the rain shadow effect, is responsible for some of the driest landscapes on Earth. The deserts of eastern Washington and Oregon sit in the rain shadow of the Cascade Range. Nevada’s arid climate exists largely because the Sierra Nevada intercepts Pacific moisture. Patagonia in South America is dry because the Andes block westerly moisture from the Pacific. In each case, the mountains act as a moisture barrier, wringing water out of the air before it can reach the land beyond.
Desert and Continental Climates
Distance from water matters. Coastal areas benefit from constant evaporation off the ocean, which feeds moisture into the air. The farther inland you go, the fewer opportunities there are for the air to pick up new water vapor. Continental interiors, particularly those surrounded by large landmasses with no nearby lakes or seas, tend to have drier air year-round. Central Asia, interior Australia, and the American Great Plains all experience this effect.
Deserts compound the problem. Sandy and rocky surfaces absorb heat quickly and reflect it back, raising air temperature without adding any moisture. Sparse vegetation means very little water is released through plant transpiration. The result is air that is both hot and extremely dry, sometimes dropping below 10% relative humidity.
How Air Conditioning Dries Indoor Air
While winter heating dries air by warming it without adding moisture, air conditioning removes moisture directly. An air conditioner works by passing indoor air over a cold evaporator coil. When the coil’s surface temperature drops below the air’s dew point, water vapor condenses on the coil and drains away as liquid. The air that returns to your room is both cooler and drier. This is why you’ll sometimes see water dripping from an outdoor AC unit on a humid summer day.
In hot, humid climates, this dehumidification is a welcome side effect. But in milder conditions, air conditioning can strip too much moisture from indoor air, leaving it uncomfortably dry even in summer. Some modern systems include reheat coils or variable-speed compressors to control how aggressively they dehumidify, but standard units offer no way to dial in a specific humidity level.
How Dry Air Affects You
You feel dry air primarily through your skin, eyes, nose, and throat. When relative humidity drops below about 40%, moisture evaporates from your body’s surfaces faster than usual. Lips crack, nasal passages dry out and become more vulnerable to irritation, and skin can feel tight or itchy. Contact lens wearers often notice the discomfort first.
Research on indoor environments has found that maintaining 40% to 60% relative humidity minimizes the transmission and survival of many airborne viruses, supports healthy immune function, and avoids the mold growth that starts when humidity climbs above 60% to 75%. Below that range, dry conditions can also damage wooden furniture, musical instruments, and paint. Static electricity becomes more of a problem too: low humidity allows electrostatic charge to build up on surfaces and clothing, leading to those annoying shocks when you touch a doorknob or another person.
If your indoor air consistently feels dry, a hygrometer (available for a few dollars at most hardware stores) can tell you exactly where your humidity stands. A portable humidifier or a whole-house humidifier integrated into your HVAC system can bring levels back into that comfortable 40% to 60% range during the months when heating or geography works against you.