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
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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