Dew point is the comfort number

Dew point above 20 °C feels oppressive to most people; below 10 °C feels dry.

Why it works

Dew point measures absolute moisture, which governs how well sweat evaporates. Relative humidity alone misleads — 90% RH on a cold morning is comfortable.

Do it exactly

Estimate with the rule, then check it against the calculator that models it properly.

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More in Environmental

• Lightning distanceCount the seconds from flash to thunder, divide by 3 for kilometres (by 5 for miles).
• Air cools as you climbTemperature drops about 6.5 °C for every 1,000 m of altitude.
• Pressure halves every ~5.5 kmAtmospheric pressure falls by half roughly every 5,500 m you climb.
• Ten metres of water = one atmosphereEvery 10 m of seawater adds about 1 atmosphere of pressure.

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