The question of whether deserts are humid is complex, as the answer depends entirely on how humidity is measured. While the air in a desert often feels intensely dry during the day, this sensation relates to only one way of quantifying atmospheric moisture. To understand the air quality in arid regions, one must distinguish between the two primary methods used to quantify the water content in the atmosphere.
Understanding Humidity Measurement
The sensation of dryness in a desert is directly linked to Relative Humidity (RH), which expresses the amount of water vapor present as a percentage of the maximum amount the air can hold at that specific temperature. Because warm air has a much greater capacity to hold moisture than cold air, extreme daytime temperatures in hot deserts cause the relative humidity to plummet, sometimes to between 10 to 30%. This low percentage makes the air feel parched and causes rapid evaporation.
A different, more absolute measurement is Absolute Humidity (AH), which is the total mass of water vapor contained in a given volume of air, typically expressed in grams per cubic meter. Unlike Relative Humidity, AH does not change with temperature, providing a direct measure of the actual water content in the air parcel. A hot desert air mass with low RH might still contain more water vapor (higher AH) than an extremely cold Arctic air mass, which could have 100% RH but a very low AH. RH indicates how close the air is to saturation, while AH indicates the total amount of water available.
How Humidity Levels Change Daily
Desert environments are characterized by a massive fluctuation in temperature between day and night, known as the diurnal range, which is largely due to the dry air’s inability to trap heat. During the day, the lack of water vapor allows solar radiation to heat the ground intensely, and the heat escapes rapidly at night. This phenomenon results in temperature swings that can exceed 27°C.
This extreme temperature cycle drives a corresponding cycle in relative humidity. As the air temperature drops dramatically after sunset, the air’s capacity to hold water vapor shrinks rapidly. Since the absolute amount of water vapor remains constant, this cooling causes the relative humidity percentage to climb sharply. If the temperature drops low enough to reach the dew point, the air becomes saturated at 100% RH, leading to condensation. This process frequently results in the formation of dew or ground fog during the cooler pre-dawn hours.
Sources of Moisture in Extreme Environments
While many inland deserts have low atmospheric moisture, coastal fog deserts, such as the Atacama or the Namib, experience high frequency fogs that roll in from the ocean. In these hyper-arid regions, annual rainfall is virtually nonexistent, yet the relative humidity can often exceed 95% due to the dense, consistent marine fog. This fog forms when warm, humid ocean air meets cold coastal currents and serves as the primary water source for the entire ecosystem.
This atmospheric moisture sustains specialized plant and animal life through unique adaptations. For example, the Stenocara beetle in the Namib Desert has evolved specialized, bumpy surface structures on its back to capture and channel water droplets directly from the fog. These organisms rely on the physical interception of moisture from the air, a process known as fog harvesting. Furthermore, dew formation is a significant moisture source that supports vegetation in many desert locales, especially when temperature shifts are large.