Abstract
OCCURRENCES of helm wind have been tantalizingly few; several have coincided with other engagements, or with serious difficulty in crossing the Pennines. It is established, however, that (a) the characteristic phenomena1 are liable to occur whenever a well-developed current of air is flowing from a direction between east and north-north-east, that is, within about 30° of a direction at right angles to the trend of the escarpment. With deviations greater than this, considerable modifications are found to occur, (b) The depth of the surface air current up to the summit level of the clouds should be not greater than about 5,500 ft. The observed phenomena then appear to be analogous to those at a submerged weir with faces inclined gently upstream, and more steeply downstream. The helm wind corresponds to the rapid set up in the air-current flowing over the escarpment; the crest of the downstream standing wave is marked by the helm bar. If the northeasterly current is of greater uninterrupted depth, the standing wave appears to flatten out and the bar, with its underlying reverse current at ground-level, is no longer found.
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References
Meteorological Glossary, p. 101; Marriott, Quart. J. Ray. Met. Soc., 15 (1889).
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MANLEY, G. The Helm Wind of Crossfell. Nature 143, 377 (1939). https://doi.org/10.1038/143377a0
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DOI: https://doi.org/10.1038/143377a0
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