Geoscience Reference
In-Depth Information
amount of moisture involved, the thickness of the
cooling air layer and the dew-point value. When
the latter is below 0°C, it is referred to as the hoar
frost point if the air is saturated with respect to ice.
The mixing of contrasting layers within a single
air mass, or of two different air masses, can also
produce condensation.
Figure 4.9
indicates how
the horizontal mixing of two air masses (A and B),
of given temperature and moisture characteristics,
may produce an air mass (C) that is super-
saturated at the intermediate temperature and
consequently forms cloud. Vertical mixing of an
air layer, discussed in Chapter 5 (see
Figure 5.7
),
can have the same effect. Fog, or low stratus, with
drizzle - known as 'crachin' - is common along
the coasts of south China and the Gulf of Tonkin
in February to April. It develops either through air
mass mixing or warm advection over a colder
surface.
The addition of moisture into the air near the
surface by evaporation occurs when cold air
moves out over a warm water surface. This can
produce steam fog, which is common in Arctic
regions. Attempts at fog dispersal are one area
where some progress has been made in local
weather modification. Cold fogs can be dissipated
locally by the use of dry ice (frozen CO
2
) or the
release of propane gas through expansion nozzles
to produce freezing and the subsequent fallout
of ice crystals (cf. p. 127). Warm fogs (i.e., having
drops above freezing temperatures) present bigger
problems, but attempts at dissipation have shown
some limited success in evaporating droplets by
artificial heating, the use of large fans to draw
down dry air from above, the sweeping out of
fog particles by jets of water, and the injection
of electrical charges into the fog to produce
coagulation.
The most effective cause of condensation is
undoubtedly the dynamic process of adiabatic
cooling associated with instability. This is
discussed in the next chapter.
E PRECIPITATION
CHARACTERISTICS AND
MEASUREMENT
1 Forms of precipitation
Strictly,
precipitation
refers to all liquid and frozen
forms of water. The primary ones are:
•
Rain
- falling water drops with a diameter of
at least 0.5mm and typically 2mm; droplets of
less than 0.5mm are termed
drizzle
. Rainfall has
an accumulation rate of
1mm/hour. Rain
(or drizzle) that falls on a surface at subzero
temperature forms a glazed ice layer and is
termed
freezing rain
. During the protracted 'ice
storm' of 5-9 January 1998 in the northeastern
United States and eastern Canada, some areas
received up to 100mm of freezing rain.
Temperature (°F)
-
20
0
20
40
60
80
20
15
B
Saturated
10
C
5
Unsaturated
•
Snow
- ice crystals falling in branched clusters
as snowflakes. Wet snow has crystals bonded
by liquid water in interior pores and crevices.
Individual crystals have a hexagonal form
(needles or platelets). At low temperatures
(-40
A
0
-30
-20
-10 0
Temperature (°C)
10
20
30
Figure 4.9
The effect of air-mass mixing. The
horizontal mixing of two unsaturated air masses A
and B will produce one supersaturated air mass C.
The saturation vapor pressure curve is shown (cf.
Figure 2.13B, which is a semi-logarithmic plot).
Source: After Petterssen (1969).
C), crystals may float in the air, forming
'diamond dust'.
°
•
Hail
- hard pellets, balls or irregular lumps of
ice, at least 5mm across, formed of alternating
