Geoscience Reference
In-Depth Information
When the droplets are small and numerous, visibility is
poor, perhaps as little as 5 m. If pollution adds suitable
nuclei, condensation of water vapour is favoured. The
actual formation of radiation fog represents a delicate
balance between radiational cooling, air movement and
condensation. It forms only when cooling occurs faster
than the rate at which latent heat is added by condensa-
tion. Because vapour is converted into water droplets, the
moisture content and saturation temperature fall, so
further cooling is necessary to give saturation. Because
of this, fog is more frequent during the long nights of
autumn, winter and spring than in summer. If winds are
strong, the saturated air near the ground will mix with
drier air above and prevent fog forming.
By reducing visibility, fog can be a major environ-
mental hazard. Fortunately the elimination of coal smoke
emissions in many parts of the industrial world means
that dense fogs (smogs) are now rare. However fogs
derived from car engine pollution are increasing.
as fair-weather cumulus, as it never grows sufficiently to
upward growth the cloud may build up as far as the
tropopause and give heavy rain.
For rain to fall, we need clouds, but many clouds
survive for hours without giving rain. What special cir-
cumstances enable some clouds to produce rain whereas
others give none?
PRECIPITATION
Formation of precipitation
Near the summit of Mount Waiaeale, Kauai, Hawaii,
average annual precipitation is 11,684 mm. In terms of the
amount of water, this is equivalent to 100,000 t ha
-1
yr
-1
.
Without doubt the processes producing precipitation can
be very effective when conditions are favourable. But how
enough to fall as rain within as little as twenty minutes of
the moist air reaching saturation?
To answer this question, we must delve inside a cloud
and see what is happening there. In a cloud made up
entirely of water droplets there will be a variety of droplet
sizes with an average diameter of about 10-15 μm. The
air will be rising within the cloud, perhaps at the rate of
10-20 cm per second, though much more rapidly in
cumulonimbus clouds. As it rises so the drops get larger
through collision and coalescence, slowly at first but
increasing in size rapidly after about forty minutes; some
will reach drizzle size. When the uplift is stronger, say 50
cm per second, the downward movement of the drops will
be reduced, so there will be more time for them to grow.
If the cloud is about 1 km deep, small raindrops of 700
μm diameter may be formed.
When temperatures fall below 0
Clouds
Clouds and fog are the result of similar processes which
vary in intensity and duration. Clouds are composed of a
mass of water droplets or ice crystals almost microscopic
in size. The number of droplets per unit volume of cloud
varies considerably, depending upon its origins; smaller
concentrations of larger droplets occur in clouds formed
in the middle of large oceans, while large concentrations
of smaller droplets are found in continental regions.
Clearly this is a consequence of the greater availability of
nuclei over the dusty continental interiors, but polluted
industrial areas may have a similar effect. Studies of such
condensation nuclei have shown that there are two broad
classes: those with an affinity for water, called hygroscopic
particles, like salt; and non-hygroscopic particles, which
require relative humidities above 100 per cent before they
can act as centres of condensation.
We can find out much about what is happening in the
atmosphere by looking at the type of clouds and especially
their shape (see box, p. 68). The low- and medium-level
stratiform types are the main rain-bearing clouds of
temperate latitudes often associated with depressions.
Around their centre we often see a characteristic sequence
of clouds as the warm air associated with the depression
approaches (
Figure 7.7
).
There are many different types of cumulus cloud,
subdivided on the basis of their appearance. If an
inversion of temperature exists, as often happens with
anticyclones, the bubbles will rise to the inversion and
then start to level out or descend, evaporating the
cloud droplets as they do so. This type of cloud is known
C, because of their
small size the droplets do not freeze immediately but may
remain unfrozen in what is said to be a supercooled state.
With further cooling to -10
C, ice crystals may start to
develop among the water droplets, even forming directly
from water vapour. This mixture of water and ice would
not be particularly important but for a peculiar property
of water. The saturation vapour pressure curve of ice
(
Figure 4.2
)
is slightly different from that of water. The
air can be saturated for ice when it is not saturated for
water. Thus at -10
C, air saturated with respect to liquid
water is super-saturated relative to ice by 10 per cent and
at -20
by 21 per cent. As a result the ice crystals in the
cloud tend to grow and become heavier at the expense of
the water droplets.
