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 temperature...in 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.
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.