If you live in the south, you've probably heard the phrase "It's not the heat, it's the humidity" when describing how hot it gets here during the summer. I've also heard the phrase "it's hot, but it's a dry heat" when describing high temperatures in the southwest. What does this mean, exactly? I always thought 95 degrees is 95 degrees no matter where you live, but the number can be deceiving.
Humidity is defined as the amount of water vapor present in the atmosphere and is measured in terms of a percentage. This is the relative humidity. High humidity coupled with high temperatures gives the air a feeling of being muggy. If you've ever been in Arkansas during the summer you know exactly how that feels. The air is oppressive and it can even be unpleasant to breathe. Your skin feels clammy and everything around you has a damp feeling. A relative humidity of 100% means the air has absorbed as much water vapor as it can hold and rain is likely. The humidity doesn't have to be 100% to rain, but the higher the number the more likely it is to happen.
Let's think about what high humidity means in terms of your body's ability to cool itself. Your body's method for self-cooling is to sweat. Under ideal conditions sweat evaporates from the body taking the heat with it. The removal of the heat makes you feel cooler. If the humidity is high and the air is saturated with water vapor, the sweat won't evaporate into the air. It sits on your skin and clothing making you feel wet, sticky, and miserable. We tend to feel more comfortable when the humidity level is between 30-50%.
Another factor in that muggy feeling is something called the dew point. The easiest way to define dew point is the point at which water droplets form on objects. A higher dew point is going to make you feel uncomfortable at higher temperatures. You've probably heard a weatherman say "It's 95 degrees outside but it feels like 102". This "feels like" temperature (or heat index) is a combination of the actual temperature and the dew point.
Air conditioning definitely makes our lives more comfortable by cooling the air, but is that all it's doing? Along with cooling, one of the primary roles of air conditioning is to remove humidity from the air. To appreciate this you only have to experience an air conditioner that is only doing half its job. I stayed in a hotel once where the air conditioning kept the room cold, but it did nothing to remove the humidity. It was miserable to say the least. The air was cold but everything in the room felt damp. We kept turning the temperature down thinking it needed to be cooler, but that only resulted in a cold room and damp sheets on the bed. I will take hot and damp over cold and damp any day!
So back to the original question - is it the heat or the humidity that makes us feel uncomfortable? We can tolerate higher temperatures when the humidity is lower, and a higher dew point coupled with high temperatures makes it "feel" hotter. Because of this I would definitely say the humidity is the muggy monster behind the uncomfortable Arkansas summer!