Air may be lifted through instability due to surface heating or mechanical turbulence, ascent of air at a frontal zone, or forced ascent over an orographic barrier. Instability is determined by the actual rate of temperature decrease with height in the atmosphere relative to the appropriate adiabatic rate. The dry adiabatic lapse rate is 9.8°C/km; the saturated adiabatic rate is less than the DALR due to latent heat released by condensation. It is least (around 5°C/km) at high temperatures, but approaches the DALR at subzero temperatures.
Condensation requires the presence of hygroscopic nuclei such as salt particles in the air. Otherwise, supersaturation occurs. Similarly, ice crystals only form naturally in clouds containing freezing nuclei (clay mineral particles). Otherwise, water droplets may supercool to -39°C. Both supercooled droplets and ice crystals may be present at cloud temperatures of -10 to -20°C.
Clouds are classified in ten basic types, according to altitude and cloud form. Satellites are providing new information on spatial patterns of cloudiness, revealing cellular (honeycomb) areas and linear cloud streets, as well as large-scale storm patterns.
Precipitation drops do not form directly by growth of cloud droplets through condensation. Two processes may be involved - coalescence of falling drops of differing sizes, and the growth of ice crystals by vapour deposition (the Bergeron-Findeisen process). Low-level cloud may be seeded naturally by ice crystals from upper cloud layers, or by introducing artificial nuclei. There is no single cause of the orographic enhancement of precipitation totals, and at least four contributing processes may be distinguished.
Thunderstorms are generated by convective uplift, which may result from daytime heating, orographic ascent or squall lines. The freezing process appears to be a major element of cloud electrification in thunderstorms. Lightning plays a key role in maintaining the electrical field between the surface and the ionosphere.
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