cipitation. The seeding of hurricanes might have unexpected, potentially harm-
ful consequences, such as increased flooding of low-lying areas.
HAIL CONTROL
Hail is responsible for great damage to crops and personal property each year.
As early as about 1900, farmers, in conjunction with scientists, were attempting
to control or alter the ways in which thunderstorms produce hail. Because most
hail damage is caused by large hail stones, the most common method of hail con-
trol has been aimed at getting the hail stones to fall to the surface before they can
grow large enough to be destructive.
One early hail control scheme was an attempt to break up the stones, or to get
them to fall before they became large. Blanks were fired from cannons to gen-
erate shock waves. This method did not prove effective. Later, rockets were shot
into storm clouds, with the idea that the disturbance would send the hail earth-
ward while the stones were still small. This did not work either.
More recently, cloud seeding was tried in an effort to interfere with the hail-
formation process. Several different countries have used this method, and some
believe that it works well enough to be cost effective. The Russians claimed a
high rate of hail-control success using artillery to fire shells filled with silver
iodide crystals into cumulonimbus clouds.
FOG CONTROL
Clouds form when the air cools to, or below, the dewpoint temperature.
Normally this happens at a certain altitude above the surface, but not at the sur-
face itself. Occasionally, however, conditions are such that the base of a cloud is
at the surface. This is known as fog. It has contributed to many accidents in avi-
ation and highway travel. Fog has plagued mariners ever since the first boat
floated on the sea.
Can we get rid of fog, at least on a local scale? Would it be worth the effort
and expense? Some airline executives believe that dispersing or burning off of
fog would save more money than it would cost by reducing the number of can-
celed or delayed flights. In harbors, fog dispersal might also prove cost effective.
The two primary methods of dealing with fog have been to warm the air above
the dewpoint temperature, and to attempt to blow the fog away with aircraft, hel-
icopters, or gigantic fans. Both methods have met with some success. If the fog
CHAPTER 9 The Past and Future Climate
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