RH @ 850 hPa

Relative Humidity at 850 hectopascals measures atmospheric moisture at approximately 1500 meters (5000 feet) altitude, representing the lower-tropospheric moisture environment that governs fog formation, low cloud development, and boundary layer humidity conditions critical for surface-based weather photography. Unlike surface relative humidity which fluctuates rapidly with diurnal temperature cycles, 850 hPa RH reveals the moisture structure of the lower free atmosphere just above the turbulent boundary layer, providing more stable and forecast-reliable moisture information. RH at 850 hPa interpretation for photography: values below 40% indicate very dry lower troposphere, suppressing fog and low cloud formation while promoting rapid evaporation of surface moisture—ideal for clear morning conditions and crisp landscape photography. RH of 40-70% represents moderately moist lower troposphere where low clouds may form with sufficient lift but fog is unlikely unless very cold surface temperatures create strong inversions. RH of 70-90% indicates moist low-level air favorable for low cloud formation, especially when combined with upward motion or nighttime cooling—typical of cloudy but not foggy conditions. RH exceeding 90% at 850 hPa strongly suggests saturated or near-saturated lower troposphere with high probability of fog, low stratus clouds, or mist formation, particularly during overnight and early morning periods when surface cooling drives saturation from the surface upward to 850 hPa levels.

For fog photographers, 850 hPa RH above 95% combined with light surface winds (< 5 mph) and clear evening skies provides strong indication of morning fog formation—the 850 hPa level often corresponds to typical radiation fog top heights, so near-saturation at 850 hPa with saturation at the surface creates ideal fog depth for photography. Conversely, low 850 hPa RH (< 60%) even with high surface humidity suggests shallow moisture limited to ground level, typically producing ground mist that burns off quickly rather than sustained fog banks. The difference between surface and 850 hPa humidity also matters: when surface humidity is high (>80%) but 850 hPa RH is low (<50%), dry air aloft promotes rapid fog dissipation after sunrise as mixing draws dry air downward. When both surface and 850 hPa RH are high, moisture extends throughout the lower troposphere supporting persistent fog, low stratus, or drizzle conditions that linger well past sunrise.

PhotoWeather's GFS 850 hPa RH forecasts enable photographers to anticipate fog formation 12-36 hours in advance by tracking moisture advection into target locations: watch for increasing 850 hPa RH (from <70% to >90%) combined with light winds and clear skies forecasted for overnight—these conditions favor classic radiation fog ideal for misty landscape photography. For mountain photographers, 850 hPa RH reveals whether summits will be cloud-free or shrouded: since 850 hPa corresponds to approximately 1500m elevation, high RH at this level indicates clouds at or below many mountain peaks, while low RH suggests clear summit conditions.