Cold and Humid
As Alaska settles into the nasty part of winter, with the mercury puddling at the bottom of thermometers, we dwellers in the Interior keep reassuring ourselves that it could be worse. Winters in the so-called temperate zone can feel just as bad because nobody's ever dressed properly for the cold, even though there the lowest temperatures are fifty degrees warmer than ours. Cars don't have plug-in heaters. And worst of all, winters Outside are soggy and raw. When it's really cold here, it's really dry. There's no humidity to speak of---the need for humidifiers and constantly steaming kettles to keep the furniture (and our hides) from drying out and cracking testifies to that.
At least that's what I thought until I started hanging out with scientists. According to knowledgeable people, my understanding of the situation was at best only partly right. That's because "relative humidity" doesn't mean the same thing as "total amount of water vapor in the air." It turns out that our frigid midwinter air is quite humid. It's holding about all the water that it can hold.
When air reaches its saturation point---that is, when it can contain no more water vapor---the relative humidity is 100 percent. If the air temperature is above freezing, liquid water droplets will begin to appear as dew on surfaces or as mist in the air itself. If the air is colder when it reaches saturation, often frost appears on surfaces. At much colder temperatures, the precipitating water droplets seem to freeze right out of the air, forming ice fog. (Clean air typically will not support the formation of ice fog until temperatures reach about forty below, a convenient number since it is the same on both Fahrenheit and Celsius scales. Alaska towns see ice fog at much higher temperatures because human activity provides particles of soot, dust, and other contaminants that serve as nucleators, templates on which the fog droplets can freeze.)
The colder the air, the less moisture it can hold. Very cold air attains saturation with very small amounts of water vapor. Thus, perversely, the bitterly cold air we walk into on our way to work is often both very dry and very humid---that is, it is close to its saturation point. At 40 degrees below zero, a cubic meter of air can contain only a tenth of a gram of water. At room temperature, the same volume of air won't be saturated until it holds 20 grams of water vapor---200 times more. Thus, when outside air is brought into a house and heated nearly 120 Fahrenheit degrees, its relative humidity drops precipitously even though the amount of water vapor it holds hasn't changed. It's this thirsty air that seems to pull moisture from our pores and our pianos.
Sometimes when heated air escapes from a building through an opened door, it can offer a double illustration of the relationship between relative humidity and air temperature. Warmed air tends to get away at the top of the opening; as the room-temperature air flows out into the cold outside air, it chills swiftly down to its saturation point or dew point. Then the moisture it contains condenses visibly and almost instantly, producing a plume of mist. At the bottom of the doorway, cold air flows in, causing the room-temperature air it encounters along the floor to cool down to its dew point. The water vapor in the air quickly condenses into billows of low-lying fog surging inward.
Under the right circumstances, this simple experiment can produce impressive results worthy of a science-fiction movie. Under the usual circumstances, it only leads to an uncomfortable draft. Nevertheless, any time a door is opened during a cold snap, it's a reminder that it's both chilly and humid out there.
