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

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.