National Academies Press: OpenBook

The Earth's Electrical Environment (1986)

Chapter: HAIL

Suggested Citation:"HAIL." National Research Council. 1986. The Earth's Electrical Environment. Washington, DC: The National Academies Press. doi: 10.17226/898.
Page 86

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THUNDERSTORM ORIGINS, MORPHOLOGY, AND DYNAMICS 86 ward the storm along the ground. At the Earth's surface, the intersection is called the gust front. It can itself be hazardous to aircraft because of remarkable turbulence and wind shears, and its horizontal winds are sometimes strong enough to do considerable damage to trees and buildings. Figure 7.5 Profile of squall-line thunderstorm (based on series of radar observations, fivefold vertical exaggeration) as it passed Oklahoma City on May 21, 1961. Winds (full barb, 5 m/sec; pennant, 25 m/sec) are plotted relative to squall-line orientation; a shaft pointing upward is parallel to the squall line (azimuth 225°). In sounding behind storm, balloon passed through anvil outflow, and winds shown are not representative of the environment. Arrows indicate main branches of airflow relative to squall line that was moving toward right at 11 m/sec. On the right, dashes outline the supposed air plume; the radar-detected cloud plume at lower elevations consists of small precipitation particles that have partly fallen out of the air plume while drifting downwind from the storm core. Simplified from Barnes and Newton (1985). HAIL At temperatures below the melting point of ice, solid and liquid phases can coexist, but the liquid phase is metastable and starts to freeze with release of latent heat in the presence of a suitable nucleus or on contact with an ice surface. Growth of hail to large sizes occurs when strong and enduring updrafts, with temperature below the melting point of ice, bear liquid cloud particles in addition to some icy motes. The liquid particles start to freeze when contacted by the ice particles, and the latter thereby grow; they descend to the ground when their fall speed exceeds the rising speed of the enveloping air current. The release of latent heat that attends freezing causes a rise of temperature toward 0°C; a growing hailstone thus tends to be somewhat warmer than its environment. (This condition of relative warmth is associated with an important electrification process treated in Chapters 9 and 10, this volume.) In the presence of much liquid water and little supercooling, the hailstone may enter a stage of wet growth, i.e., one wherein all the impacting liquid is not frozen immediately but becomes absorbed into a previously developed porous structure or is somewhat shed. In this growth regime, the frozen material exhibits a clear structure of large crystals, readily distinguishable from each other under polarized light. At sufficiently low temperatures and water contents, liquid cloud and small raindrops impinging on the growing hailstones freeze so quickly that air bubbles remain entrapped within and between the globules. The variable growth process is revealed by the concentric translucent or opaque layers and nearly transparent zones that appear in hailstone sections (Figure 7.7). The magnitude and distribution of vertical and horizontal air currents and the associated content of supercooled liquid water determine the growth and trajectory of hailstones. If the updraft speed has a maximum

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This latest addition to the Studies in Geophysics series explores in scientific detail the phenomenon of lightning, cloud, and thunderstorm electricity, and global and regional electrical processes. Consisting of 16 papers by outstanding experts in a number of fields, this volume compiles and reviews many recent advances in such research areas as meteorology, chemistry, electrical engineering, and physics and projects how new knowledge could be applied to benefit mankind.

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