Humidity is the concentration of water vapor present in the air. Many people observe that the air often feels heavier or stickier after the sun sets. This common perception of increased nighttime humidity is accurate, but the underlying reason is a change in how we measure moisture, not a significant addition of water vapor to the air mass. The actual amount of moisture in the air often remains relatively constant from day to night, but the air’s capacity to hold that moisture changes dramatically.
The Inverse Relationship Between Air Temperature and Relative Humidity
To understand why the air feels more humid at night, it is necessary to distinguish between two measurements of atmospheric moisture. The actual amount of water vapor is absolute humidity, typically measured in grams per cubic meter. This value changes slowly and does not fluctuate significantly overnight. The measurement reported most often in weather forecasts is relative humidity, 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. Imagine the air as a sponge with a capacity that shrinks as the temperature drops. Warmer air has a much larger capacity to hold water vapor, meaning that a given amount of moisture results in a lower relative humidity percentage. Conversely, cooler air has a smaller capacity for moisture.
As air temperature decreases at night, the maximum amount of water vapor the air can hold also decreases. If the absolute humidity stays the same, the relative humidity percentage must increase because the air is getting closer to its reduced saturation limit. For instance, air holding half its capacity at 80°F might be 50% relative humidity; if the temperature drops to 60°F, that same moisture could represent 75% or more of the air’s reduced capacity. This inverse relationship is the primary reason for the sensation of high humidity after dark.
How Radiational Cooling Lowers Nighttime Temperatures
The mechanism responsible for the temperature drop is radiational cooling. During the day, the Earth’s surface absorbs energy from solar radiation, warming the ground and the air immediately above it. Once the sun sets, this incoming energy ceases, and the Earth radiates its stored heat back into space as infrared energy.
This outward flow of heat causes the ground temperature to fall, and the air layer in contact with the surface is cooled by conduction. Radiational cooling is most effective on clear and calm nights. Clouds act like a blanket, reflecting outgoing infrared radiation back toward the ground, which slows the rate of cooling.
A lack of wind also prevents the colder air near the surface from mixing with the warmer air higher up in the atmosphere, allowing the temperature drop to be concentrated near the ground. This cooling process is a direct physical consequence of the energy balance changing after sunset. The resulting lower air temperature then sets the stage for the observed increase in relative humidity.
Reaching the Dew Point and Atmospheric Saturation
The dew point is defined as the temperature to which a parcel of air must be cooled to achieve a relative humidity of 100%. At this point of atmospheric saturation, the air can no longer hold any more water vapor, and any further cooling will result in the water vapor changing state.
When the air temperature reaches the dew point, excess water vapor begins to condense into liquid droplets. This condensation is the physical manifestation of the air reaching its saturation limit. Common signs of this saturation are the formation of dew on surfaces, or the appearance of ground fog and mist.
Fog and dew are visible outcomes of the cooling process, confirming that relative humidity has reached its maximum value. The closer the nighttime air temperature is to the dew point, the more humid the air feels, even if the absolute amount of moisture has not changed since midday. This cycle of cooling, rising relative humidity, and eventual saturation explains why the air feels damper at night.