Dew Point

Dew point temperature is the critical threshold that determines when moisture in the air will condense into visible fog, dew, or frost, making it one of the most important meteorological parameters for atmospheric photography. Unlike relative humidity which varies with temperature, dew point represents an absolute measure of atmospheric moisture content. When air temperature cools toward the dew point, relative humidity approaches 100% and condensation occurs.

For fog photography, the magic happens when temperature drops to within 2-3°C of the dew point, especially during overnight cooling or when warm, moist air moves over cooler ground. A dew point close to or above 20°C indicates oppressively humid conditions, while dew points below 0°C suggest very dry air. Frost photographers monitor for nights when temperatures drop below dew points that are themselves below freezing, creating hoar frost on vegetation.

The dewpoint spread (temperature minus dew point) is even more predictive than dew point alone: spreads under 3°C almost guarantee fog formation when combined with light winds and clear skies. PhotoWeather continuously monitors dew point trends to forecast fog valleys, predict frost formation windows, and alert photographers to ideal atmospheric conditions hours before they develop. Understanding dew point patterns helps photographers distinguish between stable dry conditions (large dewpoint spread), building moisture (decreasing spread), and imminent fog formation (spread approaching zero). For landscape photographers chasing ethereal fog scenes, tracking dew point is more valuable than watching temperature alone, as it reveals the moisture content that temperature changes will eventually condense into visible drama.