All imaging experiments were performed with a JEOL JEM-2100F operating at 200 kV and 40 pA beam current. Images and in situ movies were captured in bright field (BF) scanning TEM (STEM) imaging mode with a camera length of 10 cm. STEM imaging was utilized because it produces high contrast, high resolution images at low electron fluxes even for relatively thick sample substrates and water layers. The electron flux was calculated by dividing the beam current (in e -/s) by the STEM image area to yield a value in e -/Å 2 s. Control experiments with NaCl nanoparticles in ~40% RH vapor were performed to establish the optimal imaging magnification range that did not cause electron beam damage to the NaCl (See Supporting Movie 1 for full in situ movie). We found that a magnification of < 100 kX and a dwell time of 5 𝜇𝑠 did not cause observable damage to NaCl particles with sizes of ~500 -1000 nm, which corresponds to electron fluxes of < 0.374 e -/Å 2 s. In situ STEM videos were recorded at a framerate of 10 fps by Camtasia Studios screen capture software and processed in ImageJ and VideoMach. Custom scripts written in MATLAB were used to track the droplet and particle projected area as a function of time and RH in the in situ TEM movies. The droplet and particle edge detection had an error of ± 1 pixel based on visual inspection and comparison of the segmented image and the original image. The script measuring the NaCl surface curvature was adapted from Driscoll et al.