Dewpoint Spread

Dewpoint spread, calculated as air temperature minus dew point temperature, is the meteorological parameter that most accurately predicts fog formation and atmospheric visibility for photographers. While humidity and dew point provide valuable information, dewpoint spread directly quantifies how close air is to saturation. A spread of 0°C means air is fully saturated and fog, dew, or frost is actively forming.

Spreads of 1-3°C indicate air approaching saturation, where any cooling through overnight radiation, advection of moist air, or lifting will trigger condensation and fog. Fog photographers specifically seek dewpoint spreads under 2°C combined with light winds under 3 m/s and clear skies allowing radiational cooling, creating the valley fog and ground mist that defines atmospheric landscape photography. Spreads of 3-5°C suggest moderately humid air where fog might form with sufficient cooling, while spreads above 10°C indicate dry conditions where fog is extremely unlikely regardless of temperature changes.

PhotoWeather monitors dewpoint spread trends throughout the day and night, alerting photographers when narrowing spreads forecast fog development hours before it becomes visible. Understanding dewpoint spread evolution helps distinguish between stable conditions where spread remains constant, moistening conditions where increasing humidity narrows the spread, and drying conditions where spreading increases and fog dissipates. For frost photography, monitoring for dewpoint spreads that narrow as both temperature and dew point drop below freezing predicts hoar frost formation on vegetation. Astrophotographers prefer large spreads above 15°C indicating exceptionally dry, transparent air, while fog chasers target spreads under 2°C. This single field essentially combines temperature, humidity, and moisture content into one actionable metric that tells photographers exactly how close they are to fog formation.