A series of articles from USA Today's meteorologist Jack Williams helped me understand this concept better than anyplace else. First, I'll talk about the process by which the air in your house dries out. Then I'll talk about the cold = less humid part.
- When the temperature outside drops, the air in your house cools off too. Plus, if you have lots of drafts or if your house is poorly insulated, even more cold air comes into your house. So now you've got colder air in your house.
- By the laws of nature, cold air is less moist. Just accept this fact for the moment.
- The furnace in your house heats this air that was cold and is also dry. However, the furnace doesn't add moisture to the air, it only heats it. So even though the warmer air now has the potential to hold more moisture, it isn't any more humid because you've done nothing to add moisture. This means that the relative humidity essentially plummets, because the heated air can hold a lot more moisture, but it doesn't.
- The result: furnace-heated air, with no extra moisture added, feels a heck of a lot drier than regular old outside, sun-heated air in the summertime. Because that summer air has also had moisture added to it from water that's evaporated out of big lakes and rivers and the ocean.
- Your skin dries out, itches, and even cracks
- You keep getting shocked every time you touch something metal
- Your nose is dry, but yet you keep getting colds or other respiratory problems
- Drywall and plaster develop cracks
- Wood furniture joints loosen and become wobbly
- Pianos go out of tune
- Wood floors creak and squeak way more than they used to.
- You can buy a humidifier. One that has a filter which absorbs water and then passes in front of a fan that blows the moister air into the house is the most effective. Some types of humidifiers can even be connected to your house's HVAC system.
- You can stop drafts in your house. This can be accomplished by installing more energy-efficient windows, adding insulation, or covering windows with plastic sheeting, or putting draft dampers around door cracks, etc. One of the problems with these sorts of solutions is that if you super-insulate your house, it can build up too much humidity, which can in turn create all kinds of other problems. But that's something that tends to happen with more recently-built, super-energy-efficient homes.
- You can have lots of house plants. They respirate moisture into the air and may help increase the humidity in your home.
- You can do lots of boiling, or dish-washing, or clothes-drying, take lots of showers, or otherwise do household activities that will add water to the air. For most houses, these sorts of activities will add enough moisture to balance things out. For other houses, you may have to take additional measures.
Now, about the cold air = less humidity thing. The short answer to why cold air is drier is that at lower temperatures, evaporation happens less often. This means that at colder temperatures, less water vapor is present in the air.
It is actually incorrect to say "cold air holds less water." Air does not "hold" water, the way a sponge holds water. What is actually happening has nothing to do with some property of air but everything to do with the properties of water (which is a pretty unusual and somewhat perplexing substance, even to scientists who study it). If you want to know more, read on.
Water molecules are very active little things. They're always moving, shifting from one of the three states to another (vapor, liquid, and solid). Say there's a dish of water on the table. Viewed at the molecular level, that isn't just a dish of water. In the dish are a bunch of liquid water molecules and above it are a bunch of water vapor molecules. As the water molecules keep trying to shift from liquid to vapor and back again, a continual process of condensation and evaporation is happening above and in that dish.
If the temperature is warmer, the molecules have more energy. They're moving faster and more of them are moving. With this extra energy, they are better able to escape their liquid form and become vapor. This means that evaporation is happening more often than condensation. The amount of water in the dish is reduced, and the relative humidity in the air increases.
If the temperature is colder, the water molecules have less energy. They can't move as fast. They're sluggish. They don't want to go anyplace. They want to stay in the dish. The water vapor in the air gets cold and slows down too. It says, "I don't feel like being vapor anymore," and it condenses into liquid. Because less liquid is turning into water vapor, but more water vapor is turning into liquid, condensation outweighs evaporation. Eventually, there may be so much condensation that even a cloud, or fog, or dew will form.
Clouds happen higher in the sky because up there, the air is cooler. There's more condensation and less evaporation. If the air cools enough closer to the ground, you get fog instead of clouds. If it's right at the ground, you get dew.
Water condenses when it's cold. It evaporates when it's warm. That's what it comes down to. And that's enough for today.
Jack Williams, "How humidity dries out your house," USA Today, May 20, 2005
Associated Press, "Wrong humidity turns your house into a hassle," USA Today, February 1, 2004
Jack Williams, "Getting a handle on humidity," USA Today, July 18, 2005
Jack Williams, "Understanding humidity," USA Today, December 4, 2005
Alistair B. Fraser, Penn State University, Bad Clouds
USGS, "The water cycle: condensation," October 20, 2005