When it rains, it will increase the relative humidity because of the evaporation. The air where the rain is falling may not be completely saturated with water vapor. However, the longer it rains, the more the humidity will increase because of the air constantly drawing the water. The evaporation will cool the air and increase the absolute moisture content of the air locally. On a larger scale, rain will remove water vapor through air condensation and deposit it on the surface. This means that across larger volumes, the average relative humidity reduces through rain. There are a variety of factors that need to be taken into consideration, including:
- Amount of rainfall
- Temperature
- Volume of space
When the air is hotter, it will cause the water to evaporate faster, thus creating a higher level of humidity. If the air is cooler, the water will reduce the humidity level and actually make it seem cooler than the temperature outside.
There is also the matter of the dewpoint temperature, which talks more heavily about moisture and the amount of water vapor in the air. It is the temperature that the air must be cooled to in order for air to reach saturation. Dewpoint can vary from the mid-60s all the way up to the high 80s depending upon location and the time of year. It’s also important to remember that humidity levels will affect everyone differently. However, the dew point will remain the same.