If you live in a regions with a cold winter climate, you may have wondered why you have to water your indoor plants more in winter than summer.  You may also observed that your hands tend to also dry out.  Your throat and nose may also seem dry out.   

This applet simulates what happens inside your house when the
temperature and dew point change outside.  You have control over the
outdoor values by sliding the red colored portion of the two
thermometers (one for temperature and one for dew point).  The
Relative Humidity meter will instantly tell you the value for the
outside.

As you change the values outside, your indoor environment is slower to
change.  If the air is colder outside, your furnace will cycle on and
off, keeping the house at approximately the value you set the
thermostat.

In addition to controlling the outside weather, you also control your indoor environment.  You can set the thermostat on your furnace.  Opening the door of your house (point and click on the box) allows outside air to mix into your home.  Close the door (point and click the box) turns on the furnace and starts to heat your house to your preset temperature.  Activities to the left can increase relative humidity, or can have no impact.  How can you increase the relative humidity without changing the air temperature?

To see how the outside air impacts your indoor environment

1. Set the outside dew point and  air temperature to -20F
   
2. Set the thermostat to 75F
   
3. Open the door, and keep it open until the indoor temperature drops to 60F.
   
4. Close the door.