December 11, 2011

Fog Outside, Desert Humidity Inside

It it wet outside, dripping wet.  Drizzle and fog.  Relative humidity near 100%.  The air is holding every bit of water vapor it can.  And its cold outside  as well...perhaps in the 30s, like most of the last few days.

But the strange thing is that inside the air is as dry as a desert, with relative humidities down into the teens or less.   You walk on a rug and touch a door knob or piece of metal and ZAP...a spark!  Your skin feels rough and uncomfortable from the low humidity--perhaps you get some skin cracks on your feet or hands.   Your throat feels dry.

How can this be?  How can the humidity outside be near 100% while inside its is more like the Mojave Desert? 

Well, it all has to do with the amount of water air can hold as temperature varies.

The amount of water vapor a volume of air can contain (hold is really the wrong word) depends greatly on fact, it increases very, very quickly (exponentially!) with temperature.  Warm air can contain a LOT more water vapor than cold air.  Here is a plot to show you the story:

You are stuck with degrees centigrade...this is science!  The y-axis is the saturation mixing ratio--the number of grams of water vapor per kg of dry air--for saturated air as temperature varies..  At 0C there is only 4 grams of water vapor per kg of dry air, while at 20C (68F) there is 15!.

Lets say the temperature is 0C (32F) outside with a relative humidity of 100%...the air is cold and saturated...EXACTLY what we have been "enjoying" lately.   Your house or apartment has a heating system that warms the air up to say 68F (20C).  The interesting thing is that your house or apartment is hardly air tight and air from the outside is continuously moving inside and heated.  On average there is about one complete air change per hour, more for leaky older homes, less for newer homes built tight.

Anyway, using the above chart, the 100% outside saturated air has about 4 gram of water vapor per kg of dry air.  Now you bring it inside and heat the air.  Inside, the air has the same amount of water vapor that is started with--4 g per kg of dry air.  At 20C the saturation mixing ratio is now roughly 15 g per kg.  The air now can hold MUCH more water vapor than it started with and the air is no longer saturated..not even close!

The relative humidity is defined as 100 times the amount of water vapor in a sample of air divided by the maximum about of water vapor that air can hold at that temperature.
 So in this case we get RH = 100 * 4/15 = 27%  Wow...that air is now very dry in the sense the relative humidity is much lower (remember, it still has the same amount of water vapor)

What happens if you heat your home to a temperature above 68F?  Or if the temperature outside is colder than 32F?  Or if the air outside does not start off completely saturated?  The relative humidity would be much lower...and it is quite easy to get relative humidities down into the teens and single digits.

Supposedly relative humidities of 30-50% are the most healthful. Some people hate the dryness so much they put small water sprayers into their furnace or purchase humidifiers.  Or use lots of creams to create a protective barrier on their skin.


  1. Cliff, all the skiers want to know: When are we going to get back into a La Nina pattern? When is the snow going to start falling in the mtns again?

  2. This weather pattern is so boring. Please make it stop!

  3. Ditto for villager & unknown.... this weather is boring! Where is La Nina?

  4. La Nine = wimpy, lame, lackluster, and pathetic. Am I calling out the Weather Gods to prove me wrong? After last year's lame La Nina and this year not knocking my sox off, YOU BET I AM! ;)

  5. Ha, not last year Joseph! I remember a warmer than normal January and a none-too-chilly February. "Winter" only showed up in April, May, and June. I can take a cruddy Spring, but give us an actual Winter too. :)

  6. Right on Super. I like you Joe, but you can't play the La Nina starts after the new year wild card (which is true lol) when last year was complete opposite.

    2010 La nina only made an appearance between thanksgiving and christmas.

    Then of coarse, mid Feb to July.

    La Nina is only a peg better than El Nino.

    I have never understood why weather lovers blush at the sound of La Nina. I want an equal chance of a pineapple express, windstorm, freak snowstorm, extreme cold or heat wave. Majority all fall on neutral years, latest (December 2008).

  7. I like the resentment that has nothing to do with Cliffs post.

    Your followers are growing weary, lol.

    I'm blaming it on the MJO phase. Strong jet activity in the western Pacific leading to a ridge downstream.

    Two silly things: Its a coin flip. In someways we are so close to a snowy pattern. Just depends on your position of these amplified patterns.

    But these extended outlooks. On November 30th, it was clear a massive eastern pacific ridge was building. Most models agreed through mid Dec.. It was correct. Yet they still have enough "pride" in La Nina to show above chances for precip.

    If you have credence that the first 15 days of Dec will be bone dry and you still have a shade of ESPN would say, come on man.

  8. As a runner, I should be thrilled with this weather, but's beyond boring! At the very least, can't we get a *little* variation? And we have to wonder if this lack of precipitation is going to come back to haunt us ~ just when we're longing for summer, no doubt!

  9. Thank you for the very clear explanation of how relative humidity works, my kid was wondering about this the other day (and presumably the dog was when she sniffed something indoors and got a zap of static electricity to her poor nose!).

  10. You are stuck with degrees centigrade...this is science!

    AWESOME. My heart is a pitter-patter.

  11. "You are stuck with degrees centigrade...this is science!" Science would say "degrees Celsius." It hasn't been centigrade for 60 years.

  12. Not to leave Herr Fahrenheit out of the picture: Here in Oly we got our first killing frost near downtown last night: 25F. The airport reports 21F between 6AM & 7AM. The leaves on the dahlias and cannas have finally turned black, and you can hear the leaves dropping from the deciduous azaleas.

  13. I am more inclined to think that the current anomalies in our weather over these recent winters has more to do with the ice loss in the Arctic than the effects of La Nina/El Nino. After all the Arctic is a big influence on the Northern Jet streams. The temperature difference is a lot more from frozen ice to open water than the 2 or 3 degree water in the Pacific. Closer as well. That is my story and I am sticking to it. At least until someone proves different.

  14. I don't mind this weather... maybe we can get through the winter without the basement flooding.


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