Geoscience Reference
In-Depth Information
Locations receiving hail do not coincide with areas
of maximum thunderstorm or tornado development.
Figure 4.5 illustrates the annual frequency of hail-
storms in the United States. Areas with the highest
incidence of hailstorms on this map do not correspond
well to those areas of maximum thunderstorm activity
shown on Figure 4.1. The middle of the Great Plains,
underlying the zone of seasonal migration of the jet
stream, is the most frequently affected region. Most
hailstorms in the United States occur in late spring and
early summer as the jet stream moves north. A similar
seasonal tendency occurs in Australia.
The size of hail is a direct function of the severity
and size of the thunderstorm. General physics indi-
cates that a hailstone 2-3 cm in diameter requires
updraft velocities in excess of 96 km hr
-1
to keep it in
suspension. An 8 cm hailstone would require wind
speeds in excess of 200 km hr
-1
, and the largest stones
measured (>13 cm) require wind velocities greater
than 375 km hr
-l
. If graupel is present, hailstones can
grow quickly to large sizes as pieces freeze together
(Figure 4.6a). Multiple-layer hailstones indicate that
the stone has been repetitively lifted up into the thun-
derstorm (Figure 4.6b). Ice accumulates less when the
stone is being uplifted because it often passes through
air cooler than the freezing point. The ice in this
process can be recognized from its milky appearance.
When the stone descends again, water freezes rapidly
to the surface forming a clear layer.
Because hail can do tremendous damage to crops,
there has been substantial research on hail suppression
in wheat- and corn-growing areas. Supercooled water
(water cooled below the freezing point but still liquid)
is very conducive to hail formation, and attempts have
been made to precipitate this water out of the atmo-
sphere before large hailstones have a chance to form.
This process is accomplished by seeding clouds con-
taining supercooled water with silver iodide nuclei,
under the assumption that the more nuclei present in
a cloud, the greater will be the competition for water
and the smaller the resulting hailstones. In the United
States seeding is carried out using airplanes, while in
Russia and Italy the silver iodide is injected into clouds
using rockets. In the former Soviet Union, seeding
reduced hail damage in protected areas by a factor of
3-5 at a cost of 2-3 per cent of the value of the crops
saved. In South Africa, crop loss reductions of about
20 per cent have been achieved by cloud seeding. In
1
1
4
4
2
1
0
1000 km
< 2
days
4-5
days
≥
8
days
2-3
days
6-7
days
Annual frequency of hailstorms in the United States (from Eagleman, 1983).
Fig. 4.5
