THE possibility of detecting flying locusts by radar was examined theoretically some years ago, using the deduction of Ryde1 that the intensity of a centimetric radar echo from raindrops is proportional to ∑ND 6, where N is the number of drops of diameter D per unit volume. Raindrops do not exceed about 6 mm. in diameter; and recorded rainfall intensities indicate that there can rarely be more than ten such drops per cubic metre in the heaviest precipitation. More than half the weight of a locust is water; from considerations of water-content alone, a swarm averaging ten flying locusts per cubic metre, as recorded in the Kenya Highlands in 19452, with each locust containing about 1 c.c. of water, might be expected to give an echo of intensity at least an order of magnitude greater than that given by the very heavy precipitation considered; and it was concluded that recognizable echoes might well be given by much more attenuated swarms. Subsequent field-work3,4 has suggested that considerably lower volume densities, of the order of 0.1 locust per cubic metre, are in fact more representative of even the denser parts of flying swarms; densities of this order, however, might still be expected to give echoes comparable with those given by heavy precipitation.
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RAINEY, R. Observation of Desert Locust Swarms by Radar. Nature 175, 77 (1955). https://doi.org/10.1038/175077a0