Atmospheric humidity is the concentration of water vapor present in the air. This invisible, gaseous form of water is a fundamental component of the atmosphere, influencing weather patterns and human comfort. Many people actively seek environments with low humidity due to the reduced physical strain and increased feeling of coolness it provides, especially during warm weather. Low-humidity climates offer a distinct physical comfort compared to humid regions where the air feels heavy or “muggy.”
How Humidity is Measured
Relative Humidity (RH) is often misunderstood because it is a temperature-dependent measure. Relative humidity is expressed as a percentage, indicating how close the air is to being fully saturated with water vapor at its current temperature. Warm air has a greater capacity to hold moisture than cold air, meaning a high RH in cold weather can still involve a small actual amount of water, while a moderate RH in hot weather can feel oppressive.
The Dew Point, however, is a direct and absolute measure of the actual amount of water vapor in the air. It is the temperature to which air must be cooled for it to reach 100% relative humidity, causing condensation. Because the dew point temperature is a consistent indicator of moisture content regardless of the air temperature, it is the more accurate gauge of true air dryness and human comfort. Generally, a dew point below 55°F is considered dry and comfortable, while levels above 65°F feel muggy and oppressive.
The States with the Lowest Annual Humidity
The states with the driest air are predominantly located in the high-altitude and desert regions of the Western United States. The lowest overall annual mean dew point belongs to Alaska, though its low temperature is the main factor. For the contiguous United States, the driest states are concentrated in the Mountain West.
Nevada is often cited as having extremely low dew points, particularly its vast desert regions. New Mexico also ranks highly, with its northern and central areas like Albuquerque experiencing very low afternoon relative humidity, sometimes dropping below 30% on an annual average. Arizona follows closely, with cities like Tucson recording a very low average afternoon RH of 25%.
Wyoming and Colorado, with their high-elevation plains and basins, also feature prominently among the driest states. Idaho and Utah round out the list of the driest states, especially in their high-desert and plateau regions. These states share low dew points because of their geographical placement away from major oceanic moisture sources.
Geographic Factors That Create Dry Climates
The dryness in the Western United States is largely a consequence of massive geographical barriers and high elevation. The most influential factor is the “rain shadow effect” created by the major mountain ranges that run north-south, such as the Sierra Nevada and the Rocky Mountains. As moist air masses from the Pacific Ocean move eastward, they are forced to rise over these mountains.
The rising air cools, causing the water vapor to condense and fall as precipitation on the windward, western side of the range. By the time the air descends on the leeward, eastern side, it has lost most of its moisture content, becoming warm and extremely dry. This dry, sinking air creates the expansive, arid landscapes seen in the Great Basin and the deserts of the Southwest.
High altitude also plays a significant role in reducing air moisture, as the air naturally holds less water vapor at lower atmospheric pressures. States like Colorado and Wyoming, with average elevations significantly higher than the rest of the country, experience lower dew points. Furthermore, the sheer distance of these interior states from the Gulf of Mexico and the Pacific Ocean limits the influx of warm, humid air masses that affect the eastern and southern parts of the country.
Practical Implications of Living in Low Humidity
Living in a persistently low-humidity environment presents considerations for health and property maintenance. The lack of moisture in the air can lead to physical discomfort, primarily affecting the body’s mucous membranes and skin. Dry air tends to wick moisture away from the body, resulting in symptoms like dry, itchy skin, chapped lips, and irritation in the nasal passages and throat.
This dryness can also increase the frequency of nosebleeds and exacerbate respiratory conditions by drying out the protective mucus lining of the airways. Low humidity affects the indoor environment as well, causing materials to shrink and crack as they lose moisture. Wooden furniture, floors, and musical instruments can warp, and paper goods may become brittle. The buildup of static electricity is common in dry air. Many residents in these regions choose to install whole-house humidification systems to maintain indoor air moisture within a comfortable and healthier range, often between 40% and 60% relative humidity.