October 29, 2023

The Humidity Inside Our Homes is as Low as the Sahara Desert

Shocking but true.....the relative humidity inside most homes, apartments, and businesses in our region is now as low as the Sahara Desert.

According to a number of references, the average relative humidity near the surface in the Sahara is typically around 25%.

So how close do we come to these dry conditions?

The wonderful PurpleAir network reports inside and outside humidity.  Considering Seattle, here are the current outside relative humidities from this network.  A lot around 60%, which is relatively low for this time of the year.   

But now take a look at the indoor relative humidity from the same network.  Wow.
Many in the 20s, some in the low 20s.    No wonder my skin is cracking!

But let's not be too Seattle-centric.  How about the eastern slopes of the Cascades?

The outside relative humidities vary but are generally around 50%.

But indoor values are generally in the 20s, some in the teens.

So if you had a trip to the Sahara planned to enjoy some nice dry air, you can cancel your trip.    Indoor conditions in the Northwest are just as dry.

But why is the air so dry inside now and during much of the winter?   

It doesn't have much to do with the humidity of the outside air.    Even if the air outside was totally saturated (100% relative humidity), our air would still be dry inside.

But it does have something to do with temperature, and particularly the DIFFERENCE in temperatures between inside and outside air.

I don't think any of you have missed the fact that it is unusually cold outside, particularly during recent mornings.  Take a look at the low temperatures this morning (see below).

A lot of temperatures in the 20s F in the west and teens to the east.  30s near the water.

So heating systems have to bring temperatures up to 30-40F to make us comfortable.

But that has a big impact on relative humidity!  

Relative humidity is the ratio of the amount of water vapor in the air divided by the maximum amount the air can contain at that temperature.

So if a sample of air contains 50% of the maximum amount of water vapor it could hold at that temperature, the relative humidity is 50%.

The big issue is that the amount of water vapor that air can hold goes up very, very quickly with temperature (see the plot below).

Buildings are leaky with cold outside air continuously entering through gaps and cracks, and then being warmed up by our heating systems.

So imagine if the cold outside air is totally saturated (100% relative humidity) and then infiltrates into your house or apartment.   The amount of water vapor in the air (or more exactly, water vapor per unit volume) does not change much.  

But the amount of water vapor the air can hold or contain has gone up a LOT because it is now much warmer.

Thus, relative humidity plummets.   To say it another way, the numerator (the amount of moisture in the air) says the same, but the denominator (the amount of water vapor the air can hold) has gone up a lot.

               Relative humidity= amount of water vapor in a sample of air
                                               max amount of water vapor the air can hold at that temp.

Thus, relative humidity plummets.


  1. This comment has been removed by the author.

  2. North of Ellensburg a little: RH outside is 55, inside 11.
    I have 6 containers of open water in the house, not large and no fans and such.
    A powered humidifier would be needed to produce a significant improvement.
    The downside is the skin-moisture evaporation rate makes for a personal cool
    feeling even with a 70+ inside temperature.

  3. 30% is the lowest my house ever got in years of owning a Davis weather station and on my thermostat, low to mid 20s seems unheard of.

    1. Tim...no...not unheard of...and quite frequent during cold winter periods...cliff

  4. I I need help understanding this reading. Today there was fog at the Olympia airport and the visibility decreased, which I understand. What I do not understand is the value -0.25 at one point for visibility. How can it be less than zero? Can anyone explain this? (Also first snow was reported at the airport today; none where we live.)

  5. There are times of the year where it's damp but not warm enough to run the air conditioning, often in the spring and fall. I sometimes use a dehumidifier during those times to bring down the indoor humidity a bit. With the recent cool and dry, it might be time to store it...

  6. Thanks for your explanation. I monitor the humidity indoors because I have guitars hanging on the walls. I wondered why it plummeted.

  7. Cliff, I know I've asked you before but to what extent can a gardener predict the low temperature based upon dewpoint? For example, if the dewpoint were 33 or 34 degrees F., is it safe to say that the temperature will stop falling when the air reaches saturation? Any gardeners out there who want to weigh in on this?

  8. I wish that TV weathercasters would abandon relative humidity as a concept and just focus on educating the public about dewpoint. Relative humidity has almost no usefulness except as it approaches 100% and causes fog, black ice, etc. Dewpoint is a far more useful metric as it determines whether there's a need to run shower ventilation, whether there's a need to run air conditioning, the list goes on.

    1. But it's not just the dewpoint temperature alone that makes for comfortable or uncomfortable conditions, it's temperature and dewpoint together.

    2. Sure, but relative humidity as a metric doesn't describe anything on its own unless it's above 95% where it causes a clammy feeling. Temperature is of course central to human comfort, but dewpoint is a close second. Most people find dewpoints in the 40s very comfortable, just as they find dewpoints in the 70s and 10s very uncomfortable, irrespective of temperature. Keeping the dewpoint in your home between 35 and 55 is a major focus of HVAC systems, whether through air conditioning or humidification.

  9. Thanks, Cliff. I've been having to slather on the lip stuff. I was wondering if I had been transported to Colorado Springs (once often traveled there for work) where my lips dried up.

  10. Remarkably stable weather for this time of year with impressive daily minimum temperatures in the Bellingham area. BLI tied October 1954 for the most consecutive days with freezing minimum temperatures during October with 6 (10/26-10/31). The minimum temperature of 23F on 10/28 was a new daily record low, the earliest that a temperature that cold or colder has ever been recorded during a season and the 3rd lowest temperature ever measured during October.


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