Humidity can have a huge effect on heat really feels outside. Fortunately, weather apps usually include humidity numbers, but you’re probably looking at the wrong ones. What you should check is the dew point.
The moisture problem
Almost all weather apps have a percentage listed for “humidity”. The full name of this metric is “relative humidity”. It tells us how saturated the air is 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 different sizes in front of you half filled with water. They are all 50% full, but the larger cup clearly holds more water than the smaller one. The big cup is like hot air. It has a higher water holding capacity.
Now apply this to the real world. 100% humidity on a 55°F day will not be as “mass” as 50% humidity on a hot 95°F day. The hot day has a greater capacity of water in the air. That’s why just looking at relative humidity percentages isn’t a great way to get a sense of how it really feels outside.
What is dew point?
Let me introduce you to the “dew point”. This is a metric that you can also find in weather apps, but most people don’t know what it means. If you really want to figure out how wet it will be, that’s a much more useful thing to know.
On a technical level, the dew point is the temperature to which the air must be cooled to have a relative humidity of 100%. Anything below that temperature is when suspended 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 cups example. We know that relative humidity gives us the percentage that cups are filled with water. The dew point is a way to understand the real quantity of water in the cups.
This is why the dew point is better for understanding how one feels outside. If you have to choose a cup to throw over 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 will be. In the graph above you can see how more total water translates to a higher dew point.
How to read the dew point
OK, enough about mugs. How do you actually read the dew point to know how wet it will be? There’s a simple way to read the dew point, and you don’t even have to understand the technical details of how it works. Here is 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 iPhone and iPad does not show the dew point, at least as of iOS 14 in June 2021.
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 large weather map at the top to see the dew point.
The majority of dedicated weather apps include dew point. For example, the popular Weather Channel app for iPhone, iPad, and android indicates the dew point on the “Today’s Details” map.
Now the next time you open your weather app of choice, ignore the humidity number and look at the dew point. You will have a much better idea of how you will feel on the outside. While you’re at it, check the UV index as well.