Environmental Engineering Reference
In-Depth Information
of the limited areal extent of the storms, and their relative infrequence, their contribution
to water inputs is generally small.
Hailstones vary considerably in size, but are usually less than 1 cm in diameter. Stones
of this size can cause some damage, but it is the larger stones, possessing considerable
kinetic energy, that produce spectacular effects, such as damage to cars, greenhouses and
vegetation.
Data on the frequency of hailstorms are not entirely reliable. Standard statistics
probably underestimate the true frequency of hail because many storms pass between
observing stations. For example, in South Africa, where hailstorms are prevalent, the
standard network of recording stations gave an average of five storms per year. When the
network was increased to one observer per 10 km, eighty days with hail were recorded.
DEW, FOG-DRIP AND RIME
Walking through grassland on a cold autumn morning after a clear night, we would
almost certainly be conscious that the vegetation was moist from dew and guttation
drops. Similarly, in a dense, tropical forest with lots of mist or cloud we might see, hear
or feel water dripping from the leaves as fog-drip. In these two cases we are dealing with
some of the smallest contributors to the precipitation input, although locally they may
have some importance. Dew forms on cold surfaces at night when the air is close to
saturation. Under these conditions, of course, the air can hold little moisture and, as the
atmosphere loses moisture to the ground in the form of dew, it dries out further.
Consequently the total amount of dewfall that can occur in a single night in temperate
latitudes is normally limited, rarely more than 0ยท6 mm. The moisture released through the
plant stomata at night will also condense on the leaves as guttation drops and is often
more important than dewfall. In tropical forests the amounts can be greater but they still
form an insignificant proportion of the annual total. As evaporation rates are high once
the sun rises, such small quantities of moisture are soon returned to the atmosphere, so
the contribution of dew to the local water budget is likely to be negligible.
Where cloud droplets are blowing continuously across a rough surface such as a forest
we get fog-drip. The process results from the deposition of small water droplets moving
horizontally by contact with the vegetative surface. Eventually the droplets combine to
form larger drops which fall to the ground. The effect is accentuated if the trees increase
the turbulent motion of the air as it moves over the canopy. The vertical motion brings
the cloud droplets downwards and impacts them on to the leaves.
Fog-drip is most important in areas where forested mountain ridges extend into
persistent cloud sheets which produce little precipitation by normal methods. Examples
include California, Hawaii, Tenerife and Japan. Studies in all these areas indicate that
there is a significant increase in moisture input at the ground which would not otherwise
occur. At Berkeley, California, as much as 200-300 kg of water per square metre of
surface is found during the summer, when little rain falls. On Hawaii trees planted at 800
m altitude catch trade-wind cloud droplets at the rate of about 4 mm per day; over the
year this represents an input of about 750 mm to the island's water budget. Without this
additional input it is unlikely that the forests would be able to survive. Coastal deserts
such as in Namibia and Peru also have this feature where fog droplets can be caught by
shrubs or low vegetation.
