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Stereo photogrammetry reveals substantial drag on cloud thermals

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Stereo photogrammetry, which uses two synchronized cameras to measure three-dimensional positions, is applied here to ascertain whether drag plays a role in the ascent of cloud thermals. In particular, stereo cameras are used to measure the sizes and speeds of cloud thermals in Florida. Using the vertical momentum equation, it is found that a substantial amount of drag (a drag coefficient on the order of 1) is needed to match both the stereo-photogrammetric data and the known buoyancy of clouds from previous in situ measurements and large-eddy simulations. Empirical data on form drag and theoretical calculations of wave drag reveal that, for the observed Froude numbers of cloud thermals, a drag coefficient of about one is to be expected.

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