Samantha S., a 6th grade student in Naples, Florida, recently noticed that “everything” she touched was shocking her. While such a predicament is not common in South Florida, Hawaii or along the immediate West Coast, it is in just about every other place in the U.S. Mountain regions and the desert Southwest may suffer the condition the most or for the greatest amount of time. But those who heat their home in winter anywhere are quite vulnerable, as well.
Samantha’s shocking situation is linked to static electricity. Many of us know about static electricity from our heated clothes dryer. When we take the clothes out of the dryer immediately after the drying process ends, we find the clothes have stuck together. And while dryer sheets lessen the effect, so-called “static cling” follows us around the house and elsewhere. Women’s clothing and hosiery, because of their fabrics, are often the worst offenders.
We also experience the discharge of static electricity when we scruff sock-footed across a carpet and then touch a doorknob. Kids like to get under the covers in a darkened bedroom, rub their clothes together and watch the sparks fly!
Static electricity is a problem for our pets (especially cats), computers and copiers. The dryness is also not good for wood furniture and cabinets and indoor plants. And our bodies (sinuses, lungs, lips, eyes and skin) don’t like it much either.
Getting A Handle on Atmospheric Moisture
To help Samantha, my wife, Barbara (a middle school science-math teacher) had to delve into an understanding of relative humidity (RH). RH is one common measure of atmospheric moisture. She did not address static electricity (the process of electron transfer from one material to another and the build-up of electric potential) to Samantha, and neither will I. But I promise to return to it in another AC feature.
We meteorologists have several ways of describing the amount of atmospheric moisture (specifically water vapor, the gaseous form of water) that is in the air. You may have heard TV meteorologists say the air is saturated or full of water vapor and that its relative humidity (RH) is 100%. This term compares how much of the air’s capacity to hold moisture is being used. Temperature determines how much moisture the air can hold; warmer air can hold significantly more moisture than cold air. The trouble with this measure is that it is relative. A 50% RH in summer is a lot different than a 50% reading in winter!
But, we can also say the air contains a certain amount of water vapor, without comparing it to anything. This is known as dew point or absolute humidity. The dew point is the temperature to which you would have to cool the air in order for dew or condensation to form. I prefer this measure because I think it is easier to understand.
So, please take a look at the two PyrexTM cups or glasses shown in Figure 1. For visualization, imagine that the liquid water represents the water vapor in the air.
Each of these PyrexTM cups contains 1 cup of water. The cup on the left is basically full of water (i.e., it is holding all of the water it can). The cup on the right is only half full. Although both contain the same “absolute” amount of water, one has a 100% relative fullness; the only is only 50% full.
Computing Relative Humidity
Now, let’s put this into an atmospheric context. The dew point (or absolute humidity) would tell about the total amount of water vapor actually in the air. The temperature would determine the air’s capacity to hold the moisture. The ratio of the two (times 100%) would help define the percentage of water vapor actually present. This translates into “relative” humidity since the amount of water vapor has to be compared to the air’s capacity to hold it. Figure 2 provides the description in the form of a mathematical formula. Figure 3 is a table you can refer to whenever you want to compute RH. Don’t panic because of the metric units. They cancel out in the equation.
Now, let’s work through an example (Figure 4).
Suppose the air temperature is 70oF and the dew point is 30oF. Then the air is actually holding 3.52 grams of water vapor per kilogram of dry air. But because of its temperature, the air could hold 15.96 grams of water per kilogram of dry air. This yields a relative humidity of (3.52/15.96)*100% or 22%. In other words, the air is only holding 22% of the water vapor that it could hold at 70oF.
This scenario is typical of a snowy day in the Mid-Atlantic region, for example. The outside temperature and dew point are both around 30oF and the air is brought inside where it is warmed to 70oF. With the outside temperature and dew point identical, the RH would be 100%.
Now let’s get rid of the clouds and precipitation and let an arctic high-pressure system move in. Daytime solar heating lets the outside air temperature warm to 30oF, but the dew point is near 0oF. If that dry air is brought indoors, the ratio becomes (0.94/15.96)*100% or 6%. Welcome to your version of the Sahara Desert!
The table here is given in 10-degree increments, so you’ll have to interpolate (or determine values in between adjacent table values) to get a more exact RH computation. For example, the water vapor value for a temperature of 22oF is 2/10 of the way between 20oF and 30oF. 2/10 of the difference (3.52-2.32) is 0.24. Add that to the value for 20oF to get a water vapor value of 2.32 + 0.24 = 2.56.
But, you can still get a reasonable estimate by just using the values shown in the table for proximate temperature and dew point values.
Now that you at least understand RH computations, you are permitted to visit Professor Horn’s interactive humidity web site calculator. Just click on and drag the red dials or columns to manipulate the atmospheric conditions. You can also turn on a humidifier and/or your furnace. Allow the applet to run for about a minute to come to equilibrium.
Managing Indoor RH
With RH values this low (and they usually are whenever the outdoor dew point is below 40oF) it becomes imperative to raise them to the 30%-40% comfort range. And there are several ways to do this.
For short period and/or focused humidification needs (e.g., a baby with a respiratory ailment), a small vaporizer can do the trick. I prefer cool vapor systems because they are less likely to allow mineral deposits (e.g., limestone) to collect on any heating elements. Several years ago, while staying in a Colorado ski resort, we were able to humidify our hotel room because the resort provided small cool spray humidifiers.
Freestanding, portable floor humidifiers (Figure 5 – left side) work better for somewhat larger rooms. These units often come with replaceable filters. They also require frequent work to control algae, bacteria and lime deposits. These types of humidifiers must be manually refilled frequently.
Even with maintenance, both systems can disperse airborne bacteria and mineral dust.
A better choice is a furnace-based humidifier (Figure 5 – right side). This will add moisture to the air, as needed, based upon the actual heating that occurs. You control the amount of humidification via humidifier settings.
The most common (and least expensive) furnace system involves a reservoir through which a wheel, covered by a moistened evaporation pad, rotates. As part of the pad moves out of the water, it is heated, allowing water to evaporate. Airborne bacteria are not dispersed with this system.
Unfortunately, dissolved minerals in the water are left behind and these eventually coat the entire reservoir-wheel system. This process replicates how the Great Salt Lake became so salty.
A system that uses a waterfall approach (and one highly recommended by many home air conditioning service providers) provides maximum safety and efficiency, while only “wasting” a small amount of water. When the furnace turns on, a cascade of water flows down and wets a replaceable filter (changeable annually). Excess water runs off into a drain, eliminating the collection of mineralized water and associated deposits and also the risk of airborne bacteria. Heated air blowing across the filter evaporates moisture to the air.
In a pinch, you can also humidify your bedroom by turning on the shower (high temperature) and/or filling a bathtub with hot water. Since this wastes lots of water, I use this approach sparingly.
So, keep a wary eye on the indoor humidity and make your life more comfortable. Your sinuses, skin, furniture, computer and pets will say “thanks.”