You've heard the weatherman talk about how humid it's going to be in the coming days. Perhaps you've heard people complain about high humidity making everything feel hotter. But what is it all about? Is there more to humidity than just hot air? Absolutely. The principle of humidity deals with how much water is in the air, rather than the actual temperature of an air mass. Meteorologists measure humidity in one of three ways. The first is absolute humidity. This is a measure of how much water is in the air based on the temperature of the air mass. It turns out that the hotter an air mass is, the more water it can hold. The principle of absolute humidity explains why powerful thunderstorms can develop in the middle of a summer afternoon, despite the fact that no organized storm is found in the immediate vicinity. Relative humidity is what meteorologists normally refer to during their evening weather forecasts. Relative humidity is the measurement of the amount of water currently in the air as opposed to how much water the atmosphere can actually hold. For example, a relative humidity of 87% means the atmosphere currently holds 87% of the water the air mass is capable of holding, based on air temperature. Finally, meteorologists can measure humidity based solely on the amount of water in a given air mass, separate from any relation to air temperature. This is known as specific humidity. We don't hear much about specific humidity because it's mostly irrelevant in terms of how we perceive the weather. So why do you feel so much warmer on humid days? It all comes down to the fact that your body cools itself through evaporation. In other words, your body perspires in order to carry away some of the excess heat it produces. As perspiration evaporates, you feel cooler. Yet high humidity makes it more difficult for that perspiration to evaporate. When the amount of water in the surrounding air is high, there's no room for your evaporating perspiration. Instead, sweat remains on the skin. And if it can't evaporate, the perspiration can't cool your body. Thus, you feel much warmer on days of high humidity. Now you know what humidity is all about. You can explain to others that it's more than just hot air. Humidity is all about water content and its relation to air temperature.