Cloud Due to Largescale Uplift

Most mid-latitude cloud and rain are due to ascent on a synoptic scale (Table 1.1), caused either by upper-level 'divergence' near a jet stream (Chapter 12) or low-level 'convergence' in low-pressure regions (Chapter 13) or both. The rate of such large-scale ascent is typically small, e.g. 10 mm/s or about 1 km/day, which is too small to measure amongst the turbulence of the horizontal winds. Nevertheless, the small vertical movements are important in forming cloud. Unsaturated air ascending at 10 mm/s cools at almost 10 K per day, and such a rate of

Large Scale Ascents
Plate 8.2 Wave clouds downwind of Macquarie Island (at 55°S), which is near the top left of the picture. The ribs of cloud are about 8 km apart. The photograph was taken from an orbiting satellite at 0512 GMT on 17 October 1985.

cooling soon lowers the air's temperature to dewpoint, with consequent cloud formation. So one of the least measurable variables in meteorology—large-scale vertical motion—is one of the most important. This makes weather forecasting more difficult.

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