Geoscience Reference
In-Depth Information
within warm, moist air passing over a cold land surface.
On a clear winter's night, strong radiation will cool the
surface very quickly. This surface cooling extends grad-
ually to the moist lower air, reducing the temperature to
a point where condensation occurs in the form of dew,
fog or frost, depending on the 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
airmass, or of two different airmasses, can also produce
condensation. Figure 4.9 indicates how the horizontal
mixing of two airmasses (A and B), of given temper-
ature and moisture characteristics, may produce an
airmass (C) that is supersaturated 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
between February and April. It develops either through
airmass 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 fall-out of ice crystals
(cf. p. 101). Warm fogs (i.e. having drops above freezing
temperatures) present bigger problems, but attempts at
dissipation have shown some limited success in evap-
orating 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 undoubt-
edly the dynamic process of adiabatic cooling associated
with instability. This is discussed in Chapter 5.
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
0.5 mm and typically 2 mm; droplets of less
than 0.5 mm are termed
drizzle
. Rainfall has
an accumulation rate of ≥1 mm/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 north-
eastern United States and eastern Canada,
some areas received up to 100 mm of freezing
rain.
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, see Plate 10). At low
temperatures (-40°C), crystals may float in
the air, forming 'diamond dust'.
Hail
Hard pellets, balls or irregular lumps of ice,
at least 5 mm across, formed of alternating
shells of opaque and clear ice. The core of a
hailstone is a frozen water drop (ice pellet)
or an ice particle (graupel).
Graupel
Snow pellets, opaque conical or rounded ice
particles 2 to 5 mm in diameter formed by
aggregation.
Figure 4.9
The effect of airmass mixing. The horizontal mixing
of two unsaturated airmasses (A) and (B) will produce one
supersaturated airmass (C). The saturation vapour pressure curve
is shown (cf. Figure 2.14B, which is a semi-logarithmic plot).
Source
: After Petterssen (1969).
