Geoscience Reference
In-Depth Information
Table 10.3
Cloud genera based on morphology and height of occurrence.
Type
(abbreviation)
Typical base
temperature (°C)
Water content
(g m
−3
)
Category
Base height (km)
Depth (km)
High
Cirrus (Ci)
5-13
0.6
-20 to
−
60
0.05
Cirrocumulus (Cc)
5-13
-20 to
60
−
Cirrostratus (Cs)
5-13
-20 to
−
60
Medium
Altocumulus (Ac)
2-7
0.6
-30 to +10
0.1
Altostratus (As)
2-7
0.6
-30 to +10
0.1
Low
Stratus (St)
0-0.5
0.5
-5 to +20
0.25
Stratocumulus (Sc)
0.5-2
-5 to +15
Nimbostratus
1-3
2.0
-15 to +10
0.5
Vertical
Cumulus
0.5-2
1.0
-5 to +15
1.0
Cumulonimbus
0.5-2
6.0
-5 to +15
1.5
with these genera. Sometimes an additional word of Latin origin is added as a
qualification of these genera, such as
lenticularis
meaning 'lens-shaped.
Important points in this chapter
Requirements for cloud
: three things are required: (a) sufficient atmospheric
moisture to produce cloud if the air is cooled; (b)
cloud condensation nuclei
(CCN) on which to condense water vapor; and (c) a mechanism to cool air
to generate condensation, this usually being some form of atmospheric
uplift.
●
Cloud generation
: the two main mechanisms that give the required atmos-
pheric uplift to cool air and produce cloud are:
—
buoyant ascent
of parcels of warmed moist air giving
convective cloud
that is tall and has horizontal dimensions of order 100s to 1000s of
meters; and
—
large-scale horizontal movement
in the atmosphere that results in air
moving upward (a warm air mass rising over colder air; a cold air mass
pushing beneath a warmer air mass forcing it up; or air that is forced to
rise over topography) giving shallower
frontal cloud
with horizontal
dimensions of order 1 to 10s of km.
Near-surface mixing of saturated air with different temperatures can also
give small wisps of cloud as in sea frets and above rain-soaked forests.
●
Cloud condensation nuclei
: in air, there are typically
10
12
m
−3
aerosols that
are potential condensation nuclei for cloud droplets, these being mainly of
natural origin, but with local enhancements by human activities. Aerosol
concentration falls rapidly with diameter from Aitken Nuclei (
∼
●
<
0.2
μ
m) to
Giant Aerosols (>2
μ
m)
