This episode is brought to you by the Music for Scientists album, now available on all streaming services.

Click the link in the description to start listening. [♪ INTRO]. If you’ve ever paid close attention to the humidity levels on your phone’s weather app, you might have noticed that they seem to make no sense.

Like, in the summer, your app can say there’s 75% humidity, and you’ll be sticky and sweaty. But during winter, 75% might mean that your skin is super dry. So, what’s going on here?

Well, it turns out that the most common definition of humidity is… kind of inconvenient. But there is a better way to think about it. It all comes down to the fact that your phone, and most weather channels, specifically report relative humidity.

This number is measured in percentages, and it’s the amount of moisture in the air compared to how much water the air can hold. And the key is, how much water the air can hold depends a lot on temperature. See, for water vapor to come out of the air, it has to go from a gas to a liquid — usually, by condensing onto something like dust, or your window glass.

And for something to go from a gas to a liquid, it has to lose energy. In other words, the molecules have to physically slow down. Well, when the air is warm, the water molecules in it contain a lot of energy.

They’re moving more than water molecules in cooler air. That means they’re less likely to condense out of the atmosphere — and they end up hanging out in the air and making things feel all sticky. So, temperature plays a big role in relative humidity.

And it’s why this stat usually isn’t that useful in planning your day. If it’s -10 degrees Celsius, the air can’t hold onto as much moisture — so 75% humidity isn’t actually that humid. But if it’s 20 degrees, 75% humidity suddenly means there’s a lot more water in the air to make things feel all muggy.

If you want a number for humidity that isn’t quite so relative… try the dew point temperature. This is the temperature at which water droplets or dew form on things like grass. In other words, it’s the temperature at which the air is completely saturated with moisture.

And the closer the dew point temperature is to the temperature outside, the stickier and more unpleasant it will be. For reference, people react differently to dew points, but most folks are comfortable with a dew point of 10 degrees Celsius. And things get pretty humid and unpleasant around 15 or 21 degrees.

So, it’s still a new scale to learn… but it’s also consistent no matter how cold it is outside. Now, the big question is, if the dew point temperature is a great way to tell you how the outside world feels, why don’t weather apps and weather channels use it? It’s mostly a matter of history.

Instruments that measure relative humidity predate the ones that measure dew points. Like, one of the first mechanical hygrometers, a device that measures humidity, appeared in 1783. And since these devices were widely available to the public and gave reliable measurements of relative humidity, the term, unfortunately, stuck around. ...you know what else is available to the public and gives reliable science inspiration?

Music for Scientists, a tribute album to science inspired by the beauty of science. It was written and recorded by Patrick Olsen, and if you want to check it out, I’d recommend starting off with the song “Aristarchus in the Rain” — which isn’t about humidity, but is about a scientist trying to make sense of this messy and cloudy world. If you want to check it out, look for “Music for Scientists” on all major music streaming services, or click the link below. [♪ OUTRO].