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Building Science

What Is the Relative Humidity When It’s Raining?

The common assumption is that it’s 100% — but is that correct?

Does rain always bring the relative humidity up to 100%?

Serious question here:  Have you ever thought about what the relative humidity is when it’s raining?  I’ve heard people say it’s 100% because, well… because it has to be.  It’s raining, dammit!  What else could it be?

Not so fast, there.  This is something we can actually look up on a weather app or measure with a hygrometer.  In fact, it was raining at my condo as I wrote this article, so I looked at my weather app and saw this:

There.  The relative humidity was 93% while it was raining.  So it doesn’t have to be 100% when it’s raining after all.  The next day, it happened again and the relative humidity was even lower.  Here are the screenshots:

More rainy weather data

The reason has to do with evaporation and condensation.  When a surface of water (which could be a raindrop) is in contact with open space (which could be filled with air or some other gas or a vacuum), water molecules will be going both ways.  Some will be evaporating from the liquid and becoming vapor.  Others will be going from the vapor phase into the liquid.

The amount of water vapor in the space is determined by the temperature.  If the raindrop is cold, it may actually dehumidify the air it passes through as it sucks up more water molecules from the vapor phase than it releases from the liquid.  If a mass of warm humid air has a lot of cold raindrops passing through, the humidity could go down while it’s raining.

Who knew!?  Well, Bob Dylan sang about it in his song A Cold Rain’s A-Gonna Fall, so he must have known.†

I’m a-goin’ back out ‘fore the rain starts a-fallin’ Where the surfaces o’ raindrops parch the air of its water And…

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One Comment

  1. PioneerBuilders | | #1

    Thank you so much for this article. The back page of the September 2018 "Journal of Light Construction" has a wonderful article, "The Birth of Air Conditioning" that relates the story that you mention above.

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