Although the temperature at the surface can exceed freezing by a few degrees, the
key is that snow does not melt on its journey. Rain starts as rain in tropical clouds
and as snowflakes in middle latitude and polar clouds. Starting as snow, it must fall
through a sizable above-freezing layer and melt to become rain by the time it
reaches the surface. Sleet starts as snow and melts in an above-freezing layer and
then freezes in below-freezing layer so it strikes the surface in pellet form (on the
British Isles and some other places sleet is defined as a mixture of rain and snow).
Perhaps the most pernicious form ofprecipitationis freezing rain. Freezing rain
starts as snow, melts in an above-freezing layer and then comes through a shallow
below-freezing layer so that it is slightly above freezing as it strikes the ground.
Below-freezing ground causes this cold rain to instantly freeze into a layer of ice.
Hail emanates from large thunderstorms and is the result of an ice embryo traveling
throughregionswith supercooled water that freezes on contact with the ice embryo.
How much precipitation has fallen? This question is sometimes not easily
answered because of complications due to reports from nonstandard gages, blockage
by buildings and vegetation, and wind effects. A standard rain gage has a 20-cm
mouth leading down to a narrow funnel that amplifies the catch and allows ease of
measurement. A special measuring stick is placed in the cylinder and rain is mea-
sured to hundredths of inches in the United States and to millimeters elsewhere.
Manual measurements are being replaced by automated gages. The most common264 Precipitation
Weather Modification
Societies have always had dreams of modifying weather to their advantage. Native Americans
of the Southwest had rain dances designed to moisten their dry climates; weather modifica-
tion prayers were common worldwide in early societies around the world. During the U.S.
Civil War and World War I it was claimed that the roar of large cannon caused rainfall. More
cloud-physics-based modification originated after World War II. Since World War II, the
most common type of weather modification is based on the seeding ofcloudswith silver
iodide. This substance mimics the crystalline structure of ice, helping initiate the cold cloud
process of precipitation. Seeding has been accomplished with apparent success in some pla-
ces, but there is always the question of how much precipitation would have fallen without
cloud seeding. In terms of breaking droughts, one must consider that there must be a combi-
nation of adequate low-level moisture and rising air in existence before clouds can be seeded.
Russia and other countries have attempted to suppress hail by seeding thunderstorms. There
are no world standards for these efforts so it is difficult to assess success rates. Various mod-
ifications ofhurricanesand tornadoes have been proposed, but the immense amounts of
energy released in such natural events have made modification impractical. Finally, grand
schemes of surface modification have been proposed to ameliorate regionalclimates(e.g.,
melt the ice of the Arctic Ocean), but these dreams have never gained serious traction.