What Is Dew Point? Moisture, Fog, and Comfort

Dew point is the temperature to which air must be cooled — at constant pressure — to become saturated with water vapour. Below that temperature, excess moisture condenses as dew, fog, or cloud droplets. Unlike relative humidity, which changes with temperature, dew point tracks actual moisture content in the air.

Why dew point beats relative humidity for comfort

Relative humidity is percentage of saturation at current temp — 50% RH on a cool morning differs from 50% on a hot afternoon in absolute moisture. Dew point in °F or °C tells you directly: below 50°F (10°C) feels dry; 55–65°F pleasant; above 65°F increasingly sticky; above 70°F oppressive for many people.

Fog and frost formation

• Radiation fog: clear calm night cools ground → air at surface hits dew point
• Advection fog: moist air moves over cool surface (coastal classic)
• Frost: dew point below freezing → deposition as ice crystals
• Condensation on cold drinks: air near glass cooled below dew point

Calculating from temperature and humidity

Given dry-bulb temperature and relative humidity, psychrometric formulas estimate dew point. The Dew Point Calculator handles this conversion for weather stations, HVAC sizing, and photography storage humidity checks.

Frequently asked questions