Complaining about the humidity is a go-to conversation starter in the summer, but we should be aiming our ire at the dew point.
Yes, both of these things — humidity and dew point — are related to the moisture in the air, but they refer to different things, and that difference matters when it comes to how comfortable you are outside.
Relative humidity vs. dew point
When we talk about humidity, we're actually talking about the relative humidity, and all this means is how much moisture is in the air relative to the amount that's required for the air to be completely saturated with moisture. However, to determine how much moisture is required for the atmosphere to be completely saturated, you have to incorporate the temperature outside with the moisture. So, by itself, relative humidity doesn't actually tell us how much moisture is present, according to the National Weather Service (NWS).
While there are a few other variables involved, at its simplest, relative humidity is basically telling us how close the temperature of air is to the temperature of the moisture. The closer they are, the higher the humidity; the further apart, the lower the humidity. This is why when temperatures go up, the relative humidity will go down, and vice versa.
Dew point, however, tells us how much moisture, specifically water vapor, is in the air. It's the temperature at which the air must cool for the moisture in the air to reach saturation, or 100% relative humidity. If it's 100%, water is condensing at the same rate it's evaporating. If there's a difference between the dew point temperature and the air temperature, then things shift. So, if the air temperature cools below the dew point temperature, water vapor begins to condense on solid surfaces. It's why grass is dewy in the morning, or why water molecules clump around air particles to form fog.
While this may sound a little abstract, dew point is consistent — and our response to it is similarly consistent. A day with a dew point temperature of 40 degrees Fahrenheit (4 degrees Celsius) is going to feel the same whether the air temperature is 60 F or 100 F. In fact, days with dew points less than 55 F are going to be fairly comfortable, though anything below a dew point temperature of 40 F is likely to feel a bit too dry.
But once the dew point gets between 55 F and 65 F, the NWS says the outdoors will feel "sticky with muggy evenings." Anything above 65 F means there's a lot of moisture in the air, and most people will start to feel uncomfortable. Once that dew point temperature hits 70 F (21 C), things are getting oppressive, if not outright dangerous.
The heat index is real
High dew points are uncomfortable because the air's moisture is slowing down the rate at which our sweat evaporates off our bodies. It's how we cool down. So, if you're in some place with a very high temperature and a low dew point — pick any number of cities in the Southwestern U.S. — your body is going to sweat and that sweat will evaporate. It's also very easy to become dehydrated in this situation.
This is why paying attention to the heat index is so important. The heat index factors in the actual air temperature with either the dew point or relative humidity. This will give you a sense of what it actually feels like outside. NWS's heat index chart uses relative humidity:
Like with relative humidity, a high dew point will also make it feel hotter outside. If you're in a place with a high dew point and a high air temperature, sweat simply can't evaporate fast enough to cool you down. The result is that you can overheat, and this will make you prone to the effects of various heat illnesses, including heat stroke, which is what happens when your body can't cool off through sweating. You can become confused and even lose consciousness because your body is essentially too hot. Other symptoms of heat illnesses can include dizziness, cramps, nausea, headaches and a quickening of the pulse.
So, the real truth isn't as simple as the phrase you're used to hearing. It's not the heat; it's the amount of moisture in the air and whether or not it's evaporating at a rate that allows our sweat to evaporate. (But it doesn't roll of the tongue quite as easily.)