Relative humidity typically reaches its daily peak around sunrise. This daily cycle is not due to a sudden increase in the amount of water vapor overnight, but rather a direct consequence of the relationship between air temperature and its ability to hold moisture.
Understanding Relative Humidity
To understand why morning air is so humid, it is important to distinguish between two terms: absolute and relative humidity. Absolute humidity is a measure of the actual mass of water vapor present in a given volume of air, often expressed in grams per cubic meter. This value is a direct indicator of the total moisture content and does not change with temperature or pressure.
Relative humidity (RH), however, is the metric that changes dramatically throughout the day and is what people perceive as “dampness.” Relative humidity is expressed as a percentage, representing the ratio of the amount of water vapor currently in the air to the maximum amount the air could hold at that specific temperature. When this percentage is high, the air feels saturated because it is close to its limit for holding moisture.
The Inverse Relationship Between Temperature and Moisture Capacity
The core of the morning humidity effect lies in the inverse relationship between air temperature and relative humidity. Warmer air has a significantly greater capacity to hold water vapor than cooler air. For instance, the moisture-holding capacity of saturated air roughly doubles with every 20°F increase in temperature.
When the temperature drops, the air’s maximum capacity for water vapor shrinks rapidly. If the absolute humidity remains constant overnight, any decrease in air temperature will cause the relative humidity to rise. This occurs because the same amount of moisture now represents a much larger percentage of the air’s reduced capacity.
The Daily Temperature Cycle and Humidity Peaks
The earth undergoes a 24-hour meteorological cycle, known as the diurnal cycle, which dictates the timing of the humidity peak. Air temperature typically reaches its lowest point of the day just before or right around sunrise, after hours of radiative cooling throughout the night. This cooling is most effective on clear, calm nights when there is little cloud cover to trap heat near the surface.
Because the air’s moisture capacity is at its lowest when the temperature is at its minimum, the relative humidity simultaneously reaches its daily maximum during this early morning period. As the sun rises, solar radiation begins to warm the ground and the air above it, causing the air’s capacity to hold water vapor to quickly expand. This increase in capacity leads to a rapid and significant drop in the relative humidity percentage, which is why the air feels much drier by the afternoon.
Dew and Fog Formation
The high relative humidity of the morning often results in visible water formations like dew and fog. The dew point is the temperature at which a parcel of air, holding a specific amount of moisture, becomes completely saturated, meaning the relative humidity reaches 100%.
When the air temperature cools down to meet the dew point, the excess water vapor condenses into liquid water droplets. If this condensation occurs on surfaces like grass or cars, it is called dew. If the condensation happens in the air itself, it forms a suspension of tiny droplets known as fog, which is essentially a cloud at ground level.