Dew point is the temperature air must cool to for saturation — a direct, absolute measure of how humid the air is. Why it beats relative humidity for comfort, the summer comfort scale, and why it can never exceed the air temperature.
Dew point is the temperature the air would have to cool to — at constant pressure — for it to become fully saturated, at which point water vapor starts condensing into dew, fog, or clouds. It is a direct, absolute measure of how much moisture is actually in the air. The higher the dew point, the more humid it feels, and unlike relative humidity it does not change just because the temperature does.
Dew point is an absolute measure of moisture; relative humidity is a percentage relative to the current temperature. 90% relative humidity at 45°F feels fine, but a 70°F dew point feels oppressive no matter what the thermometer says. That is why meteorologists trust dew point for judging comfort.
The dew point can never exceed the air temperature. When the two are equal, the air is saturated (100% relative humidity) and you get fog, dew, or rain. The closer the numbers, the muggier and more shower-prone the air.
High dew points are what drive the heat index ("feels like" temperature), because muggy air keeps sweat from evaporating. A sharp jump in dew point between nearby weather stations marks a moisture boundary — a dryline or warm front — often where thunderstorms fire.
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