Colorado is known for its mountains and sunshine, but its climate is also characterized by extremely low humidity. The air in the state is dry, especially when compared to locations near the coasts or in the central United States. Humidity refers to the amount of water vapor in the air, most commonly measured as relative humidity. Relative humidity expresses the current moisture content as a percentage of the maximum amount of moisture the air can hold at a specific temperature. This measurement often drops significantly low in Colorado, sometimes reaching single-digit percentages, defining the region’s semi-arid environment.
The Mechanisms Creating Colorado’s Dry Climate
The primary reason for Colorado’s arid conditions is the rain shadow effect. Prevailing weather systems move from west to east, originating over the Pacific Ocean, and encounter the barrier of the Rocky Mountains. As moist air is forced upward over the peaks of the Continental Divide, it expands and cools, a process called orographic lifting. This cooling causes the water vapor to condense and fall as rain or snow on the western, or windward, side of the mountains.
Once the air passes over the high peaks, it descends the eastern slope, which is the leeward side, having been stripped of most of its moisture. As this air sinks, it is compressed, causing it to warm significantly in a process known as adiabatic heating. This warm, dry air acts like a sponge, absorbing any remaining surface moisture and creating the notably dry conditions experienced across the Front Range and Eastern Plains.
The state’s high elevation further exacerbates the dryness. Cooler air at high altitudes naturally holds less absolute water vapor than warmer air at sea level. This low absolute moisture content, combined with the rain shadow effect, results in consistently low dew points and the sensation of parched air across the region.
Regional and Seasonal Variations in Moisture
While the entire state is generally dry, humidity levels show regional and seasonal differences. The mountain valleys and the Western Slope, already affected by the rain shadow, often experience the most arid conditions year-round. In contrast, the Eastern Plains and the Front Range can occasionally draw moisture northward from the Gulf of Mexico, leading to slightly higher humidity and precipitation totals than the western areas.
Seasonal moisture increases during the summer months due to the North American Monsoon (NAM), typically ramping up from July through August. This pattern involves a shift in wind flow that pulls moisture from the Gulf of California and the Gulf of Mexico into the Southwest. The influx of this tropical air leads to daily afternoon thunderstorms, particularly over the mountains, which temporarily raises dew points.
The driest period, however, is often the winter, when the cold, dry air is further depleted of moisture by home heating systems. Running forced-air heat can drive indoor relative humidity levels well below 20 percent, creating desert-like conditions inside homes. This combination of naturally arid air and indoor heating makes maintaining comfortable humidity a challenge during the colder months.
Practical Implications of Low Humidity
The lack of moisture in the air has several direct effects on health, personal comfort, and property maintenance. Low humidity promotes rapid dehydration, which can be particularly problematic at high altitudes where the body is already working harder to process oxygen. Common physical reactions include dry skin, chapped lips, irritated eyes, and an increase in nosebleeds due to dried-out nasal passages.
Dry air also compromises the body’s natural defenses, as the mucous membranes in the respiratory system dry out, becoming less effective at trapping pathogens. This reduced functionality can increase susceptibility to common respiratory infections, such as colds and the flu. Many residents mitigate this by using a humidifier, targeting an indoor relative humidity level between 30 and 50 percent for health and comfort.
The moisture-depleted environment also impacts property, particularly wooden structures and furnishings. Extremely dry air causes materials to shrink, leading to damage such as:
- Cracks in hardwood floors.
- Warping of door frames.
- Damage to wood furniture and trim.
- Damage to musical instruments.
Excessive static electricity is another common consequence of air with a low moisture content, which can be a nuisance and potentially damage sensitive electronic equipment. Maintaining adequate indoor humidity is therefore a necessary preventative measure to protect both health and home.