system
Given the following post, respond as if you're commenting on Reddit:
human
What is the difference between 99% humidity and 100%?

Not sure if this is correct but isn’t 100% humidity just water. If so, what makes that one percent go from air to water?
gpt
Humidity is expressed as a fraction of *saturation*, not the fraction of the air that consists of water. 100% humidity is the maximum amount of water that the air can carry before it condenses into droplets - fog or dew. The actual amount of water carried in the air at, say, 99% humidity, will depend on the temperature of the air. What we're looking at is really "relative humidity" - how the current humidity compares with the maximum possible humidity.