Humidity measures the water vapor present in the atmosphere. Water vapor is the gaseous form of water, and its concentration determines whether the air feels muggy or dry. Understanding the lowest recorded humidity requires exploring the complex physics that drive moisture out of the air.
Understanding Relative Humidity
The most common measurement reported in weather forecasts is relative humidity (RH), which is expressed as a percentage. Relative humidity represents the ratio of the amount of water vapor currently in the air compared to the maximum amount the air can hold at that specific temperature and pressure. The air’s capacity to hold water vapor increases significantly as the temperature rises.
A different measure, absolute humidity, quantifies the actual mass of water vapor present in a given volume of air, regardless of temperature. For example, cold air at 100% relative humidity contains far less total water vapor (absolute humidity) than hot air at a much lower 50% relative humidity. Instruments called hygrometers are used to measure these moisture levels.
The Lowest Recorded Humidity Measurement
While the theoretical lowest possible value for relative humidity is 0.0%, this reading has never been officially recorded at a surface weather station on Earth. The air always contains at least trace amounts of water vapor, making a true zero reading practically impossible to achieve outside of a laboratory environment.
One of the lowest officially documented relative humidity readings was 0.36%, recorded in Safi-Abad Dezful, Iran, on June 20, 2017. This low value occurred when the air temperature reached 115.7°F (46.5°C). The extreme heat dramatically increased the air’s capacity to hold water, pushing the relative percentage close to zero despite the minuscule amount of moisture present. The previous verifiable low was 0.6% in Las Vegas, Nevada, in 2011.
Atmospheric Processes That Create Extreme Dryness
Extreme dryness is typically caused by a combination of atmospheric forces and geographical factors. Adiabatic heating occurs when air descends rapidly through the atmosphere. As air sinks, the increasing atmospheric pressure compresses it, causing the air mass to heat up substantially.
This dramatic temperature increase raises the air’s capacity to hold water vapor, thus drastically lowering the relative humidity. This effect is common in areas where strong high-pressure systems persist, forcing air downward. High-pressure systems create stable conditions that suppress cloud formation and precipitation, allowing the air to dry out over extended periods.
Rain Shadows
Geographical features like rain shadows also contribute to extreme dryness. When moist air is forced up and over a mountain range, it cools, and the water vapor condenses into rain or snow on the windward side. By the time the air descends on the leeward side of the mountains, it is significantly warmer and depleted of moisture, a process that further lowers the relative humidity.
Cold Ocean Currents
Cold ocean currents along coastal deserts, such as the Humboldt Current off the coast of Chile, also play a role. They cool the air near the surface, which creates stable inversions that block the formation of rain-bearing clouds.