Rain often appears to lower the muggy feeling in the air, but the relationship between precipitation and atmospheric moisture is complex. The immediate effect of rain is to reduce the total amount of water vapor in the atmosphere, as moisture physically falls out of the sky. This process is quickly countered by other meteorological factors, including a drop in temperature and the subsequent evaporation of rainwater from surfaces. The temporary relief people feel is less about a massive reduction in moisture and more about a change in how that moisture is distributed and how the surrounding air temperature shifts. Understanding whether rain “gets rid” of humidity requires distinguishing between the two primary ways we measure water content in the air.
Defining Humidity: Relative vs. Absolute
Humidity is the concentration of water vapor present in the air, referring to two distinct measurements. Absolute humidity (AH) is the actual mass of water vapor contained within a specific volume of air, typically expressed in grams per cubic meter (g/m³). This value is not dependent on the air’s temperature and represents the true amount of moisture present.
Relative humidity (RH), conversely, is the ratio of the current absolute humidity to the maximum amount of moisture the air could hold at that specific temperature. It is expressed as a percentage and is the metric people most often hear in weather reports and use to gauge comfort. Since warmer air has a greater capacity to hold water vapor than cooler air, the relative humidity will change significantly with temperature, even if the absolute moisture content remains constant.
For example, air containing 10 grams of water vapor per cubic meter has a fixed absolute humidity of 10 g/m³. If that air is warm, its capacity to hold moisture is high, resulting in a low relative humidity, perhaps 50%. If the air cools down, its capacity to hold vapor shrinks, and the relative humidity percentage will rise, even though the total amount of water (AH) has not changed.
The Immediate Effect of Rain on Air Saturation
Rain is the result of air reaching 100% relative humidity, or the dew point, at a higher altitude, causing water vapor to condense into liquid droplets. This condensation and subsequent precipitation is a physical removal of water vapor from the air mass. As the rain falls, the air mass that generated the storm effectively “wrings out” its excess moisture.
This atmospheric cleansing is often referred to as a “washout” effect, where the water droplets physically collect and deposit water vapor and other particles as they descend. The air directly beneath and within the rain-producing cloud mass is therefore drier immediately after the event because the liquid water has fallen out. This reduction in the actual water content means the absolute humidity (AH) of the air mass has decreased.
However, this drying effect is localized to the original air mass and is quickly modified by other processes at the surface. While the overhead air is technically drier in absolute terms, the effect on ground-level conditions is complicated by the presence of the rainfall itself.
The Role of Temperature and Evaporative Cooling
The most significant factor contributing to post-rain comfort is the drop in temperature. Rain clouds block incoming solar radiation, and the process of evaporative cooling further lowers the air temperature. Evaporative cooling occurs when water on the ground and surfaces transitions from a liquid to a gas, a phase change that requires heat pulled directly from the surrounding air.
A drop in air temperature has an inverse effect on relative humidity (RH). Since cooler air has a reduced capacity to hold water vapor, a sudden temperature decrease can cause the RH percentage to spike, sometimes reaching 100% during the rain. However, the temperature drop is often substantial enough to make the air feel less oppressive, even with high RH.
The temperature at which the air must be cooled to achieve 100% relative humidity is called the dew point. The dew point is a reliable indicator of how comfortable the air will feel, as it directly reflects the absolute moisture content. While rain may temporarily increase the dew point locally through evaporation from the ground, the overall cooling effect provides temporary relief until the ground moisture evaporates and the air warms again.