Geoscience Reference
In-Depth Information
nuclei to condense around or by facilitating the
formation of large drops through the merging of
small droplets.
the observed banded hailstone structure. Modern
thunderstorm models successfully account for
this; the growing hailstones are recycled by a
traveling storm (see Chapter 9I). On occasions,
hailstones may reach giant size, weighing up to
0.76kg each (recorded in September 1970 at
Coffeyville, Kansas). In view of their rapid fall
speeds, hailstones may fall considerable distances
with little melting. Hailstorms are a cause of severe
damage to crops and property when large hail
falls.
It is usual to identify three main types
of precipitation - convective, cyclonic and oro-
graphic - according to the primary mode of uplift
of the air. Essential to this analysis is some
knowledge of storm systems. These are treated in
later chapters, and the newcomer to the subject
may prefer to read the following in conjunction
with them (Chapter 9).
3 Solid precipitation
Rain has been discussed at length because it is the
most common form of precipitation. Snow occurs
when the freezing level is so near the surface that
aggregations of ice crystals do not have time to
melt before reaching the ground. Generally, this
means that the freezing level must be below 300m.
Mixed snow and rain ('sleet' in British usage) is
especially likely when the air temperature at the
surface is about 1.5
C. Snowfall rarely occurs with
a surface air temperature exceeding 4
°
C.
Soft hail pellets (roughly spherical, opaque
grains of ice with much enclosed air) occur when
the Bergeron process operates in a cloud with a
small liquid water content and ice particles grow
mainly by deposition of water vapor. Limited
accretion of small, supercooled droplets forms an
aggregate of soft, opaque ice particles 1mm or
so in radius. Showers of such pellets are quite
common in winter and spring from cumu-
lonimbus clouds.
Ice pellets may develop if the soft hail falls
through a region of large liquid water content
above the freezing level. Accretion forms a casing
of clear ice around the pellet. Alternatively, an ice
pellet consisting entirely of transparent ice may
result from the freezing of a raindrop or the
refreezing of a melted snowflake.
True hailstones are roughly concentric accre-
tions of clear and opaque ice. The embryo is a
raindrop carried aloft in an updraft and frozen.
Successive accretions of opaque ice (rime) occur
due to impact of supercooled droplets, which
freeze instantaneously. The clear ice (glaze)
represents a wet surface layer, developed as the
result of very rapid collection of supercooled
drops in parts of the cloud with large liquid water
content which has subsequently frozen. A major
difficulty in early theories was the necessity to
postulate violently fluctuating up-currents to give
°
F
PRECIPITATION TYPES
1 'Convective type' precipitation
This is associated with towering cumulus
(cumulus congestus) and cumulonimbus clouds.
Three subcategories may be distinguished
according to their degree of spatial organization.
1
Scattered convective cells develop through
strong heating of the land surface in summer,
especially when low temperatures in the upper
troposphere facilitate the release of conditional
or convective instability (see B, this chapter).
Precipitation, often including hail, is of the
thunderstorm type, although thunder and
lightning do not necessarily occur. Small areas
(20 to 50km
2
) are affected by individual heavy
downpours, which generally last for about 30
minutes to an hour.
2
Showers of rain, snow or soft hail pellets may
form in cold, moist, unstable air passing over
a warmer surface. Convective cells moving
with the wind can produce a streaky distribu-
tion of precipitation parallel to the wind
