Cirrus clouds have large yet uncertain impacts on Earth's climate. Ice supersaturation (ISS) -where the relative humidity with respect to ice (RHi) is greater than 100 % -is the prerequisite condition of ice nucleation. Here we use 1 Hz (∼230 m) in situ, aircraft-based observations from 87 • N to 67 • S to analyze the spatial characteristics of icesupersaturated regions (ISSRs). The median length of 1-D horizontal ISSR segments is found to be very small (∼1 km), which is 2 orders of magnitude

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... aller than previously reported. To understand the conditions of these small-scale IS-SRs, we compare individual ISSRs with their horizontally adjacent subsaturated surroundings and show that 99 % and 73 % of the ISSRs are moister and colder, respectively. When quantifying the contributions of water vapor (H 2 O) and temperature (T ) individually, the magnitudes of the differences between the maximum RHi values inside ISSRs (RHi max ) and the RHi in subsaturated surroundings are largely derived from the H 2 O spatial variabilities (by 88 %) than from those of T (by 9 %). These features hold for both ISSRs with and without ice crystals present. Similar analyses for all RHi horizontal variabilities (including ISS and non-ISS) show strong contributions from H 2 O variabilities at various T , H 2 O, pressure (P ) and various horizontal scales (∼1-100 km). Our results provide a new observational constraint on ISSRs on the microscale (∼100 m) and point to the importance of understanding how these fine-scale features originate and impact cirrus cloud formation and the RHi field in the upper troposphere (UT).