The feeling of high humidity often seems most noticeable late at night or in the early morning hours. Humidity is the amount of water vapor present in the atmosphere. The reason for the nighttime spike in mugginess is not that new moisture is added to the air, but that the air’s ability to hold existing moisture changes. This mechanism is a physics principle related to temperature, which controls the atmosphere’s capacity for water.
Relative Humidity and the Air’s Capacity for Water Vapor
The key to understanding the nighttime humidity increase lies in the distinction between absolute humidity and relative humidity. Absolute humidity measures the actual mass of water vapor in a specific volume of air, often expressed in grams per cubic meter (g/m³). This value typically remains constant throughout the night.
Relative humidity (RH) is a percentage comparing the current water vapor in the air to the maximum amount the air could hold at that specific temperature. This maximum capacity is directly linked to temperature; warmer air holds more water vapor than cooler air. For example, air at 30°C can hold more than three times the water vapor of air at 10°C before becoming saturated.
As the temperature decreases after sunset, the air’s maximum capacity to hold water vapor shrinks. Since the absolute amount of water vapor remains the same, this fixed amount represents a larger percentage of the air’s reduced capacity. If air containing 10 grams of water vapor has 50% RH at 25°C, dropping the temperature to 15°C could cause the relative humidity to jump to 90% or more, even with no change in moisture content. This inverse relationship between temperature and capacity drives the perceived increase in nighttime humidity.
How the Earth Cools After Sunset
The temperature drop necessary to shrink the air’s moisture capacity is driven by radiative cooling. During the day, the Earth’s surface absorbs solar radiation. Once the sun sets, this incoming energy ceases, but the ground continues to emit its stored heat back into the atmosphere and toward space as infrared radiation.
On nights with clear skies, this radiation escapes efficiently because clouds do not reflect the heat back toward the surface, resulting in a drop in ground temperature. The air layer adjacent to the cooled ground then loses heat through conduction, transferring energy directly to the colder surface. This cooling of the lowest layer is further distributed through convection and mixing.
This loss of heat from the surface causes the air temperature near the ground to fall steadily throughout the night, often reaching its minimum just before sunrise. As this air cools, its capacity to hold water vapor decreases, causing the relative humidity percentage to climb. The ground drives the temperature reduction, which dictates the rise in relative humidity.
The Formation of Dew and Fog
The rise in relative humidity due to cooling eventually leads to saturation, which has visible consequences. The dew point is defined as the temperature to which air must be cooled for the relative humidity to reach 100%. At this point, the air holds the maximum amount of water vapor possible, and any further cooling forces the excess moisture out of the gaseous state.
When the air temperature drops to the dew point, condensation begins, changing the water vapor back into liquid water droplets. If this condensation occurs directly on surfaces cooled by radiation, such as grass blades, car roofs, or pavement, it forms dew. This formation confirms that the air surrounding the surface has reached saturation.
If the air layer cools to the dew point slightly above the ground, the condensed water droplets remain suspended, creating fog. This is often called radiation fog because it is caused by the same nocturnal radiative cooling that causes dew. Dew and fog are thus the physical, visible evidence that the air’s capacity has been overwhelmed by the falling temperature.