# Why does higher dew point indicate more water vapour molecules in the air?

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Why does higher dew point indicate more water vapour molecules in the air?

In: Physics

Let’s talk about dew first, for a second. Warmer air can hold more moisture, cooler air holds less, so if you have air with some water vapor in it, and you cool it down, its “vapor capacity” will fall until it hits a point where it can *just barely* hold all the water vapor it has. That’s called the saturation point or the dew point.

If you cool it a tiny bit more than that, water vapor starts to condense and dew droplets form.

If air is really really moist, it starts out close to its limit and you only have to cool it a little bit before it saturates. It hits saturation while still warm, in other words it reaches its dew point at a high temperature; in short, moist air = high dew point.

If air has very little moisture in it, you can cool it a whole bunch and it can still hold that bit of moisture no problem. You have to get it really cold before condensation happens. In other words it reaches its dew point at a really low/cold temperature. Dry air = low dew point.

(Is that what you were asking? I’m not totally sure I covered the part that was bugging you.)

It’s a little bit the other way around. More water vapor in the air means the dew point is higher. This happens because warm air can hold more water vapor than cold air, when the air cools off, the excess water “overflows” and forms dew (or frost if it is below freezing).

You’ll also see this with the relative humidity percentage. 100% humidity means the air can’t hold anymore water, so even a tiny bit colder and it will form dew. Think of it like the container shrinking but the water has to go somewhere. At 50% humidity the air can hold twice as much water as is currently there, but if the air cools off it increases the relative humidity. In this analogy the container is shrinking and so it is more full with the same amount of water. It can shrink as it cools off further until you hit 100% humidity and it overflows, that temperature where it overflows is the dew point.

Think of dew point as a weird way of saying “water vapor concentration” in the air.

Air can hold water vapor in it just like water can hold salt or sugar. Because of how the atoms work, warm water can hold more salt/sugar than cold water. Same is true for air – hot air can hold more water vapor than cold air.

Making up some numbers, let’s say the air has 10 grams of water vapor per liter of air. If you drop the temperature of the air from 60 degrees to 20 degrees and you measure again, let’s say you find 5 grams of water vapor per liter of air now. At some point, decreasing the temperature pushed the water out, causing it to go from being mixed in the gas to being liquid water on its own. Let’s say this occurred at 50 degrees and starting at 49 degrees you have less than 10 grams of water per liter of air.

This tipping point is called the dew point, because that’s how dew forms: warm air soaks up water and then when it cools down overnight the water gets “pushed” out.

The dew point in that scenario is 50 degrees. If we changed it to starting with 5 grams of water in 1 liter of air (still starting at 60 degrees), the dew point is now 20 degrees. Each dew point corresponds to a water vapor concentration.

The reason we use dew point instead of “water vapor concentration” is that water concentration numbers are not intuitive. But dew point means “the most humid it can be at X temperature”. You usually only feel humidity in the summer when it’s the hottest. Generally, a dew point over 60 degrees F is considered a pretty unpleasant level of vapor, while over 70 is considered really gross. These even numbers are easy to use and understand once you learn what they mean.