Page 262 - NS-2 Textbook

METEOROLOGY 257

do above any land: moisture rises from vegetation, meets method. Both methods cause water droplets to form
cooler air aloft, and condenses into clouds. In the tropics, around the foreign substance and then fall as rain.
these clouds often reflect the colors of sandy beaches or Seeding, howevel~ is not successful unless conditions
coral reefs below. Overhanging clouds may also warn sea- are nearly right for natural rainfall. Seeding can make
men of rocks, reefs, or shoals surrounding islands. rain come a bit earlier and may cause more rain to fall
than might have occurred naturally. It might also cause
rain to fall from a cloud that, under natural conditions,
PRECIPITATION
would never have produced raindrops. But seeding can-
Precipitation (rain, snmv, sleet, and hail) emmot occur \Alith- not cause rain to fall from fair skies or from fair-weather
out clouds. The fact that there are clouds, however, does cumulus clouds. Nor is it possible to cause rain to fall
not necessarily mean that the moisture in them will fall as over a large area.
a form of precipitation. Temperature and the presence of
hygroscopic nuclei or ice clystals will determine whether SNOW, SLEET, HAIL, AND FROST
or not there will be precipitation and what form it will take.
Raindrops are formed when moist air is cooled to the Sleet occurs when rain that has formed in relatively warm
point where the moisture condenses into heavy drops. air falls through a layer of freezing air. The air is not qlute
Normally, droplets move about in the cloud somewhat cold enough to cause the falling rain to freeze--until that
like dust blowing. Cloud moisture droplets are velY rain comes into contact with a bit of dust. The dust will
tiny-only 1/2,500 of an inch in diameter-and are too cause the raindrops to freeze, at least partially, into a
light to fall to Earth. Only if the droplet grows to a diam- super-cooled mush, which freezes when it hits the ground,
eter of 1/125 of an inch or larger will it fall from the trees, or telephone wires. Such a sleet or ice storm can
cloud. The average raindrop, then, is a million times cause power lines to collapse, or tree branches to break
larger than a cloud droplet. Cloud droplets grow to a size and fall on power and telephone lines, roofs, and roads.
large enough to fall as rain or snow by combining with Hail usually occurs in the summertime. It begins as
one another-a process called coalescence. frozen raindrops in high levels of cumulonimbus thun-
Coalescence occurs in two known ways. First, bigger derheads. The ice pellets may grow if updrafts of air
droplets move about slowly in the clouds, eventually push them upward one or more times after they are
bumping into other droplets and combining with them. coated with water from lower cloud layers. They will
This is usually the case when rain falls from a nimbo- eventually fall when they are too heavy to be lifted by an
stratus or other low cloud. Second, the more important updraft. They may grow even more during thei.r descent
kind of coalescence occurs when, in higher-altitude by picking up moishlre that then freezes. Most hailstones
clouds (such as the middle layer of cumulonimbus), ice are smaller than marbles, but people and animals have
crystals and water droplets form near each other. The been killed or severely injured by hailstones as large as
droplets evaporate, and the resulting vapor collides with baseballs. Hail can destroy a growing crop in minutes.
the ice crystals and condenses into snow or ice pellets In wintertime, when the upper air is very cold, water
that fall toward Earth, melting into rain as they pass vapor will condense into ice clYstals. What we call snow
through warmer air at lower altitudes. is the result. Water vapor will also clYsta1lize around hy-
groscopic nuclei floating in the air, where the cloud's
temperature ranges from -4 degrees F to +10 degrees F.
RAINMAKING
Dew and frost do not fall from the skies as do rain,
Rainmaking has been a concern of humans since the sleet, and snow. Dew is ,vater vapor that condenses on
most ancient times. Rain dances, sacrifices, drums, can- objects that have cooled below the condensation point of
nons, and smoke have all been used to try to make rain, the air around them. Frost is surrilar to dew, but it forms
especially when the land was parched with drought. at temperatures below freezing. The water vapor
None of these methods worked, of course. But modern changes directly into ice crystals on contact with the ob-
rainmaking technlques, based upon the known facts of ject, without first changing into dew.
coalescence, have been successful in causing rainfall.
In modern rainmaking technlques, an aircraft drops
FOG
dry-ice crystals or silver-iodide crystals into potential
rain clouds. nus process is called seeding the cloud with What we call fog is really a low-lying cloud that is near
artificial nuclei. It has been fotmd that one pound of or touching the surface of the Earth. It is formed when
frozen carbon-dioxide (dry-ice) clYstals spread by air- cool ail" moves in and mixes with warIn air having a high
plane can start a shower from a large cnmulus cloud. Sil- relative humidity. When the temperature falls below the
ver iodide can also, using special generators, be sent up dew point, fog is formed. Each water droplet has a parti-
from the ground in the form of a gas-a less expensive cle of dust or smoke as its central nucleus.