Why does higher dew point indicate more water vapour molecules in the air?
In: Physics
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.
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