The common perception suggests that a rain shower “clears the air,” providing a refreshing break from stifling weather. However, the true relationship between rain and atmospheric moisture is not a simple reduction of humidity. The impact of precipitation on how humid the air feels is a complex, two-part process governed by changes in air temperature and the subsequent evaporation of water. Depending on whether you are experiencing the rain or the period immediately following, the air can feel dramatically different, often shifting from merely wet to oppressively sticky. Understanding this dynamic requires looking closely at the two distinct ways scientists measure moisture in the atmosphere.
Defining Humidity: Relative vs. Absolute
Humidity is a general term for the amount of water vapor present in the air, but meteorologists use two distinct measurements to quantify it. Absolute humidity is a straightforward measure that quantifies the actual mass of water vapor within a given volume of air, typically expressed in grams per cubic meter (g/m³). This value does not change simply because the air temperature rises or falls; it only changes if water vapor is physically added to or removed from that volume of air. Absolute humidity gives a direct indication of the total moisture available in the atmosphere.
The more commonly cited measurement 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 to the maximum amount of water vapor the air can hold at its current temperature. Warm air has a greater capacity to hold moisture than cold air, so temperature is the dominant factor determining the relative humidity. If the air cools while the absolute moisture content stays the same, the relative humidity percentage rises because the air’s holding capacity has decreased. Conversely, if the air warms, the relative humidity drops.
The Immediate Impact of Rainfall on Air Moisture
The process of rain falling almost always causes a temporary, but significant, increase in relative humidity. As raindrops descend from the cloud layer, they encounter air that is not saturated with moisture. Some of the liquid water from the falling drops evaporates into the surrounding air, which is a process known as evaporative cooling. This phase change requires energy, which is drawn from the ambient air, causing the air temperature near the ground to drop.
Since cooler air can hold less water vapor, this drop in temperature causes the air’s moisture capacity to shrink. Even if the actual amount of water vapor—the absolute humidity—remains constant or increases only slightly from the evaporation of the raindrops, the relative humidity percentage rises dramatically. It is common for the air to reach 100% relative humidity, or saturation, during a steady rainfall. This is why the air feels heavy and damp while the rain is occurring.
Why Humidity Often Rises After the Rain Stops
The feeling of oppressive mugginess that often follows a rain shower is largely due to the subsequent process of evaporation. Once the precipitation stops, the ground, pavement, vegetation, and other surfaces are covered with standing water. If the sun returns or the air temperature begins to rise, this pooled water starts to evaporate rapidly, adding a substantial amount of new water vapor to the air in the lower atmosphere, known as the boundary layer.
This influx of new water vapor directly increases the absolute humidity of the air. Even if the temperature is rising, increasing the air’s capacity to hold moisture, the sheer volume of water vapor being added from the wet surfaces often outpaces this capacity increase. The resulting combination of high temperature and high absolute moisture content leads to the feeling of a sticky environment. Furthermore, a lack of strong wind can trap this newly evaporated moisture near the surface, allowing the high humidity to persist.