In Situ Balloon‐Borne Observations of the Vertical Structure of Relative Humidity in Asian Summer Monsoon Cirrus Clouds

Abstract The vertical structure of relative humidity within cirrus clouds in the upper troposphere remained unexplored, owing to measurement challenges. Twelve years of balloon‐borne simultaneous measurements of upper tropospheric humidity and optical particle backscatter during the Asian summer monsoon are used to consolidate our understanding of cirrus. Thin cirrus are typically supersaturated with respect to ice (median RHi ≳ 100%) in their upper parts, have a median RHi ≈ 100% toward their center, and RHi < 100% below. Most RHi remain below 120%, suggesting that these clouds form predominantly by heterogeneous ice nucleation. In rare cases, RHi > 140% enables homogeneous nucleation. Thicker cirrus show higher backscatter, indicating numerous particles that likely nucleated homogeneously in deep convection. These clouds slowly evaporate, as evidenced by slight subsaturation (RHi ≲ 100%) throughout their vertical extent. Cloud‐base RHi ≈ 70% suggest rapid crystal settling and evaporation. This data set may serve as reliable microphysical reference.