Nights often feel more humid than days. Understanding why humidity increases after sunset involves recognizing how temperature and moisture interact in the atmosphere.
What Humidity Really Means
Humidity is measured in different ways. Absolute humidity quantifies the actual mass of water vapor present within a given volume of air, typically expressed in grams per cubic meter. In contrast, relative humidity expresses the percentage of water vapor the air currently holds compared to the maximum amount it can hold at that specific temperature. Warmer air possesses a greater capacity to hold water vapor than cooler air. As air temperature changes, its capacity for moisture also changes, directly impacting the relative humidity.
Why Nights Get Cooler
Nights feel more humid due to the Earth’s cooling process after sunset. This cooling occurs through radiational cooling, where the Earth’s surface emits heat into space. Cooler air has a reduced capacity to hold water vapor compared to warmer air. Even if the actual amount of water vapor in the air (absolute humidity) remains constant, the decrease in the air’s moisture-holding capacity causes the relative humidity to rise significantly.
The Dew Point
As the night progresses and temperatures continue to drop, the air may reach the dew point. This is the temperature at which the air becomes saturated with water vapor, reaching 100% relative humidity. When the air temperature cools to or below this point, water vapor can no longer remain in its gaseous state and begins to condense into liquid water droplets. This condensation is visible as dew on surfaces, fog in the air, or even frost if temperatures fall below freezing.
How Your Location Affects Nighttime Humidity
Local geographical features influence nighttime humidity. Areas near large bodies of water, such as oceans or lakes, often experience higher nighttime humidity because these water sources provide a continuous supply of moisture to the air through evaporation. Regions with dense vegetation contribute to atmospheric moisture through transpiration, where plants release water vapor into the air. Urban areas, due to the “urban heat island” effect, retain heat longer, which can influence how efficiently air cools and retains moisture. Humidity generally decreases with increasing altitude, meaning mountainous regions typically experience drier conditions compared to lower elevations.