It is the peak of summer and the common complaint around town is that it is uncomfortably HOT! What is comfort? And what causes it?
We should define a couple of terms here. Sensible heat is the heat a thermostat senses. When the temperature goes up, there has been an increase in the sensible heat.
Latent heat is hidden. It involves adding or removing heat without changing the temperature. How does this happen? When we change a liquid to a gas (boiling or evaporation) we have to add heat, but we don’t need to change the temperature. Boiling water produces steam of the same temperature, for example. Similarly, we can remove heat by condensing gases to liquids, without lowering the temperature. Air contains latent heat in the water vapor that is in the air. Removing the vapor removes heat but does not lower the temperature.
Let’s think about comfort. It is easy to understand that most people are more comfortable at 70oF than 100oF. But there is more to it. Most people have experienced how much more uncomfortable it is on a hot, humid day than on a hot, dry day. It helps to understand why that is.
The human body cools itself by sweating. When moisture evaporates off the surface of the skin, there is a great deal of cooling that takes place. Dogs accomplish much the same thing by panting.
There is something called the latent heat of vaporization. The key is that it takes lots of energy to convert liquid to a gas. When a liquid changes to a gas, a tremendous amount of energy is absorbed. That’s why having sweat evaporate off our skin is so helpful in keeping us cool.
People are comfortable when humidity is lower because it is easier for the moisture on their skin to evaporate. The process of evaporation removes heat. It is easy to understand how fans keep people comfortable. As water is also evaporated off the sin, the air immediately adjacent to the skin becomes saturated and cannot hold any more moisture. The faster that air moves across your skin, the more quickly the saturated air is carried away and replaced by dry air, which allows more evaporation.
So, if we want to keep people comfortable inside their homes, we need to cool the air and lower the humidity. If we can keep the house about 15oF to 20oF cooler than it is outside, that is usually adequate. In the winter, we set our thermostats around 70oF, and in the summer 75oF is usually fine.