Humidity can have a huge effect on how hot it really feels outside. Thankfully, weather apps typically include humidity numbers, but you’re probably looking at the wrong ones. What you should be checking is the dew point.
The Problem with Humidity
Pretty much every weather app has a percentage listed for “humidity.” The full name for this metric is “relative humidity.” It tells us how much the air is saturated with water. Most people assume that a high relative humidity percentage is what determines how humid it feels, but that’s not entirely true.
Here’s the problem: Warm air can hold more water than cold air. Imagine that there are cups of varying sizes in front of you filled halfway with water. They’re all 50% full, but the biggest cup clearly has more total water than the smallest. The big cup is like warm air. It has a higher capacity to hold water.
Now, apply this to the real world. 100% humidity on a 55°F day won’t feel as “muggy” as 50% humidity on a hot 95°F day. The hot day has a higher capacity for water in the air. This is why simply looking at the relative humidity percentages isn’t a great way to get an idea of how it will actually feel outside.
What Is Dew Point?
Let me introduce you to “dew point.” This is a metric that you might also find in weather apps, but most people don’t know what it means. If you want to really understand how humid it will feel, it’s a much more useful thing to know.
In technical terms, the dew point is the temperature at which the air must be cooled to have a relative humidity of 100%. Anything below that temperature is when the airborne water vapor becomes liquid water, and that’s when you see dew on the ground, hence the name.
So what does this have to do with humidity? Let’s go back to the example of the cups. We know that relative humidity gives us the percentage of which the cups are filled with water. Dew point is a way to understand the actual quantity of water in the cups.
This is why dew point is better for understanding how it will feel outside. If you have to pick one cup to be dumped on your head, do you want to know how full the cup is or how much water is in the cup? The latter is a better indication of how wet you’ll get. In the graphic shown above, you can see how more total water translates to a higher dew point.
How to Read Dew Point
OK, that’s enough talk about cups. How do you actually read the dew point to find out how humid it will feel? There’s a simple way to read dew point, and you don’t even have to understand the technicalities of how it works. Here’s a handy chart:
Checking the dew point is as easy as checking the temperature. You just need to find a weather app that provides it. Unfortunately, Apple’s weather app on the iPhone and iPad does not show the dew point—not as of iOS 14 in June of 2021, at least.
The Google Search app has a built-in weather web app that includes a dew point. Simply open the Google app on your iPhone, iPad, or Android device and search for “weather.” You can expand the big weather card at the top to see the dew point.
Now, the next time you open your weather app of choice, skip the humidity number and look at the dew point. You’ll get a much better idea of how it will feel outside. While you’re at it, check the UV Index, too.
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