Geoscience Reference
In-Depth Information
The other source of a return supply (estimated
to be smaller in its effect over the earth as a
whole than point discharges) is the instantaneous
upward transfer of positive charge by lightning
strokes, leaving the earth negatively charged.
The joint operation of these supply currents, in
approximately 1800 thunderstorms over the globe
at any one instance, is thought to be sufficient to
balance the air-earth leakage, and this number
matches reasonably well with observations.
Globally, thunderstorms are most frequent
between 12:00 and 21:00 local time, with a
minimum around 03:00. An analysis of lightning
on visible satellite imagery at local midnight shows
a predominance of flashes over tropical land areas
between 15
the Congo, South Africa, Brazil, Indonesia and
northern Australia, with activity along cyclone
paths in the Northern Hemisphere. In the
boreal summer, activity is concentrated in central
and northern South America, West Africa - the
Congo, northern India and Southeast Asia and
the southeastern United States. The North
American Lightning Detection Network recorded
28-31 million flashes per year for 1998-2000.
Severe storms in the USA may have peak cloud-
to-ground lightning flash rates exceeding 9000
flashes per hour.. In Florida and along the Gulf
Coast the mean flash density is 9 flashes/km 2 . The
median peak current is about16kA. Lightning is a
significant environmental hazard. In the United
States alone there are 100-150 deaths per year, on
average, as a result of lightning accidents.
S ( Plate 5.17 ). In the austral
summer, lightning signatures are along the
equatorial trough and south to about 30
°
N and 30
°
°
S over
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 vapor 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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