Relative humidity vs. dew point: Which matters to you?

There are multiple ways to describe how wet the air is (yes, pretty much all air is wet). What I mean by that is there are water molecules tucked in between the gases like nitrogen and oxygen that make up the air around us.

How much water is in the air is important to human comfort and safety, largely because it impacts your body’s natural cooling system: sweat.

But not all methods of communicating how moist the air is are created equal.

The two most commonly used measurements are the relative humidity and the dew point.

While you may be more used to hearing about the relative humidity, the dew point is actually a much more useful number. In fact, the adjective “humid” actually refers to the dew point.

That is because the dew point is a measure of how much water vapor is in the air and relative humidity is a measure of how much water vapor is in the air relative to how much water vapor could be in the air.

That sentence was pretty long and hard to follow, so let’s break it down with a nifty metaphor: a jar of marbles.

Dew point tells you how many marbles are in the jar

A higher dew point directly means that there is more water vapor in the air. This matters for how comfortable the air is outside because it impacts your body’s ability to use sweat as a method of self-cooling.

Sweat cools your body through evaporation. When you sweat, small water droplets collect on your skin. As those droplets evaporate, they take energy from your body, cooling you, and from the air directly above your skin, keeping you more comfortable.

(MORE: The good and the bad about sweat on the skin)

But when there is a higher dew point (meaning there is more water vapor already in the air), that evaporation process slows down. The body can’t cool itself as efficiently anymore, making you hotter and potentially a little more miserable.

You’re also more likely to have more water droplets condense onto your skin. If you stick your hand into the jar of marbles, if there are more marbles, you will have touched more marbles.

And, to beat this metaphor into the dirt, if those marbles have Velcro on them, you’ll come away with more marbles attached to your skin and clothes.

Technically speaking, the dew point is the temperature at which air would be saturated (meaning it would have a 100% relative humidity). When air is saturated, the water vapor condenses into droplets, and you get features like clouds and fog.

In terms of marbles, if you have 16 marbles, the jar of marbles will be full if it’s a 16-marble-sized jar. At that point, you’re going to have marbles spilling all over the place when you try to add more, which is the cloud.

Relative humidity tells you how full the jar is of marbles

Relative humidity is a measure of how much moisture is in the air relative to how much there could be, based on the temperature.

Warm air can hold more moisture than cold air, so as the temperature changes, the relative humidity does as well, even if the actual moisture content in the atmosphere isn’t changing.

Let’s bring back our metaphor one more time before I make you fully lose your marbles.

Warmer air means a bigger jar. That means, even if you have the same number of marbles, the jar is emptier relative to how many marbles could fit in the jar. And even if you change the size of the jar, it doesn’t change how many marbles you’ll touch if you stick your hand in.

This is a very common trend for daily relative humidity cycles. On a typical day (barring any fun weather phenomena like cold fronts and the like), temperatures go from cool in the morning to warm in the afternoon, then back to cool at night.

Relative humidity has an opposite trend (again, making an assumption that the moisture content in the atmosphere is changing very little or not at all). It starts high in the morning, drops in the afternoon and then rises again at night as things cool off.

Even the periods with the lowest relative humidity, as temperatures rise during the day, the relative humidity falls even if the dew point (and the actual moisture content of the air) stays the same.

When it comes to the physiological processes that keep you cool, knowing how much moisture is in the air is much more important than how much moisture the air could have.

Climate snapshot: 

High humidity on a hot day is more than just uncomfortable — it can be deadly. And days like that are increasing in frequency across the country.

Climate Central warns that “high humid heat days” more than doubled in frequency for most of the regions in the country from 1980 to 2020.