system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What does the weather forecaster mean by dewpoint? How does it relate to humidity?
gpt
There is pretty much always water in the air. Humidity tells us how much water is in the air, relative to how much *could be* in the air.

The amount of water that *could be* in the air depends upon the temperature of the air.

So, let's say it's 75 degrees out and the humidity is 80%. If the temperature dropped, the humidity percentage would go up, even if the actual amount of water in the air did not, since the maximum amount of water that *could be* in the air has decreased.

So, what happens when the air cools down so much that it hits 100% humidity? If it cooled down any more, there would be more water than the air can hold. At this point, the water in the air drops out as a liquid, forming fog or dew or rain, so that the humidity never rises above 100%. The temperature where this happens is the dew point.

For almost any amount of water in the air, there is a temperature where it will reach 100% humidity and liquify. So, for any humidity/temperature pair, there is a dew point where the water in the air will start to drop out.