Relative humidity is the water vapor in the air as a percentage of the maximum the air could hold at that temperature. Why it swings with temperature, how it differs from dew point, and when it reaches 100%.
Relative humidity is the amount of water vapor in the air expressed as a percentage of the most the air could hold at that temperature. 50% means the air is holding half of its maximum capacity; 100% means it is saturated, and you get fog, dew, or rain. Warm air can hold far more moisture than cold air, so the same amount of water vapor gives a very different percentage as the temperature changes.
Because it is relative to temperature, humidity swings through the day even when the actual moisture does not change — it is often 90%+ at dawn and 40% by afternoon on the very same air. That is why dew point is the better comfort gauge.
Dew point is an absolute measure of moisture; relative humidity is a ratio that depends on temperature. A 75°F dew point is muggy whether it is morning or noon, but its relative humidity reading will look dramatically different at each time.
When the air cools to its dew point, relative humidity hits 100% and water vapor condenses — morning dew, fog, and cloud bases all form at that saturation point.
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