Dew point, explained

Dew point is the temperature air would have to cool to before its water vapor starts condensing into liquid — dew on the grass, fog in the low spots, droplets on a cold glass. That's the textbook definition. The useful part is what it tells you about right now.

It measures actual moisture

Dew point is an absolute measure. If the dew point is 70°, there is a fixed amount of water in that air, whether the afternoon climbs to 95° or cools to 75°. The number moves when the air mass changes — not when the thermometer does.

Relative humidity is a ratio: moisture compared to the maximum that air could hold at its current temperature. And warm air holds far more. So the percentage swings wildly through a day in which the actual moisture never budged.

How it feels

Dew point maps to comfort almost directly, because your body cools by evaporating sweat — and evaporation depends on how much room the air has left for moisture. High dew point means full air, which means sweat just sits there.

It sets tonight's floor

Overnight temperatures struggle to fall much below the dew point. As air cools toward it, condensation begins — and condensing water releases heat, which pumps the brakes on further cooling. If you want to know how cool tonight can realistically get, look at this afternoon's dew point.

It's the fuel gauge for storms

Rain needs moisture, and dew point measures exactly how much is loaded. Below 55°, there's little to squeeze out. At 65° and above, thunderstorms become sustainable. At 70°+, storms rain hard and can regenerate. Fuel isn't fire — you still need something to lift the air, like a sea breeze, a front, or plain daytime heating — but no fuel means no storm, no matter what else lines up.

There's one more trick worth knowing: the spread between temperature and dew point. When they're within a few degrees of each other, the air is nearly saturated and fog or low cloud becomes likely. Pilots read that gap instinctively — it's sitting in every airport observation.