Vapor Pressure Deficit

Vapor Pressure Deficit (VPD) is an advanced atmospheric moisture metric that quantifies how far air is from saturation by measuring the difference between saturation vapor pressure and actual vapor pressure in kilopascals. Unlike relative humidity which changes with temperature, VPD combines temperature and humidity into a single measure of atmospheric drying power and moisture stress.

For photographers, VPD provides insights into both atmospheric clarity and fog formation potential. Low VPD values below 0.5 kPa indicate near-saturated air where fog is likely to form, creating the atmospheric conditions prized for moody landscape photography. Moderate VPD of 0.8-1.5 kPa suggests comfortable humidity with good clarity, ideal for general outdoor photography. High VPD above 2.5 kPa indicates very dry air common in deserts, at altitude, and during continental high-pressure systems, offering exceptional long-distance visibility perfect for telephoto landscape work but potentially introducing heat shimmer and dust haze at ground level. Extremely high VPD above 4 kPa creates harsh, low-moisture conditions that challenge both photographers and equipment, though such conditions offer unparalleled atmospheric transparency for astrophotography.

VPD also affects plant behavior: high VPD causes plants to close stomata and wilt, affecting wildlife photography of pollinators and herbivores. PhotoWeather monitors VPD trends to predict fog formation windows when VPD drops toward zero, forecast optimal clarity days for distant subjects when VPD sits in the moderate range, and alert photographers to extreme dry conditions that offer unique creative opportunities but require equipment protection. Understanding VPD helps photographers distinguish between healthy atmospheric moisture and saturation, between crisp clarity and moisture haze, providing a more nuanced atmospheric forecast than humidity alone. Coastal regions typically show VPD of 0.5-1.5 kPa, continental interiors 1.5-3 kPa, and high deserts 3-5 kPa.