RH @ 700 hPa

Relative Humidity at 700 hectopascals measures atmospheric moisture content at the 700 hPa pressure level (approximately 3000 meters or 10,000 feet altitude), expressed as the percentage of water vapor present relative to saturation capacity at that level's temperature. This mid-tropospheric moisture parameter is crucial for cloud photography planning because the 700 hPa level sits within the typical development zone for mid-level stratiform clouds (altocumulus, altostratus) and serves as the moisture source for the upper portions of cumulus and cumulonimbus clouds. RH at 700 hPa reveals whether sufficient moisture exists at mid-levels to support sustained cloud systems: values below 40% indicate very dry mid-levels hostile to cloud development—even strong upward motion will struggle to produce clouds above shallow low-level layers, resulting in limited photographic opportunities. RH of 40-60% represents moderately dry conditions where cloud development requires strong sustained lift; weak lift mechanisms will produce sparse, short-lived clouds. RH of 60-80% indicates ample mid-level moisture supporting robust cloud development when combined with upward motion—this is the typical range for photogenic cumulus fields, developing thunderstorms, and extensive stratiform decks. RH exceeding 80% suggests near-saturation at mid-levels with existing or imminent cloud formation; values of 90-100% almost always correspond to active mid-level cloud coverage including altocumulus, altostratus, or the upper portions of deep convective clouds.

For photographers, 700 hPa RH interpretation combines with vertical motion forecasts: high RH (>70%) combined with upward motion (negative omega) virtually guarantees cloud development and produces the most photogenic cloud-filled skies. High RH with neutral vertical motion suggests stable cloud decks that persist without major development—useful for predicting overcast soft-light conditions. Low RH (<50%) even with upward motion limits cloud development to low levels, often creating blue-sky conditions aloft with only surface-based haze or low clouds—ideal for astrophotography or crisp landscape work needing clear upper-level transparency.

The temporal evolution of 700 hPa RH reveals approaching weather systems: increasing RH ahead of storms signals moisture advection that precedes cloud development, allowing photographers to anticipate sky changes 6-24 hours in advance. Decreasing RH behind systems indicates drying and clearing. PhotoWeather's GFS-derived 700 hPa RH forecasts at 3-hour intervals enable photographers to track moisture patterns and identify optimal periods for cloud photography (increasing RH + lift = developing drama) versus clear-sky photography (low RH = cloud-free conditions).