The humidity is often very high in the winter here in Chicago, yet we always hear how dry the air is and that it is desirable to add moisture to the air in our houses. Can you explain this contradiction?
– Billy Kleiman
Averaged through the year, Chicago’s relative humidity is 71 percent but, surprisingly, it runs a little higher during the winter (73 percent) than during the summer (68 percent).
The explanation for higher winter humidity levels is that very cold air contains minimal moisture, even when saturated (100 percent relative humidity). It therefore takes little moisture to elevate the humidity of frigid air. Far more moisture is required to bring hot air to saturation. It takes only 0.001 ounce of water to saturate one cubic foot of air at 0 degrees, but 0.022 ounce (22 times as much) to saturate air at 80 degrees.