Geoscience Reference
In-Depth Information
winter night roughly equals the amount of water evaporated during the subse-
quent daylight hours. The form of the water deposited, whether dew or frost,
depends on temperature, but the process of deposition is much the same in each
case. Nighttime loss of longwave radiation lowers the temperature of the ground
and/or vegetation and the saturated vapor pressure of the air in contact with
these; at the dew point, water or ice is deposited on the surface. The movement of
water vapor toward the surface is by turbulent diffusion. Consequently, ambient
wind speed plays a role in controlling the amount of water or ice deposited. In
light winds (0.5 to 1 m s −1 ), the near-surface atmosphere becomes strongly stable
and turbulent transport of vapor toward the surface and hence deposition dew or
frost is restricted. At higher wind speeds (above 3 m s −1 , for example) turbulent
transport is more effective and, providing the ground is cold enough, this gives
greater deposition.
Mist and fog can also give an input of water to the surface which is sometimes
called fog drip because it may be observed as water dripping from solid surfaces
(particularly forest vegetation) that is projecting into warm, moist orographic
cloud. This phenomenon can result in a significant hydrological input, and it is not
uncommon in the mid-latitude coastal regions of western North America, Europe,
and New Zealand. Substantial precipitation also arrives as fog drip in mountain-
ous regions on eastern coasts in the tropics, indeed in some regions of Queensland
in Australia it has been reported that as much as 40% of precipitation arrives in
this way. Fog drip is particularly important from an ecological perspective in
mountainous coastal regions with little rain in Namibia and Chile, for example,
and on isolated mountains in Brazil where it provides the primary source of water
to sustain isolated ecosystems that would otherwise perish.
Important points in this chapter
Cloud particle growth : water droplets and ice particles grow large enough
to fall from clouds as precipitation either by collision processes (called coa-
lescence between droplets, aggregation between ice particles, and accretion
between particles and droplets), or via an intermediate vapor phase when
ice particles grow at the expense of droplets in the Bergeron-Findeisen
process.
Wa r m c l o u d s : growth in warm clouds (> 0°C) is simplest to understand
because it can only occur by coalescence, but it illustrates features common
to collision growth in mixed and cold clouds, including:
— particle collisions do not always result in particle merger (see Fig. 11.1b)
— collision with merger is most efficient for larger particle collisions; and
— when large particles fall, collision growth occurs at an accelerating rate.
Cold and mixed clouds : in cold clouds growth is only by aggregation and
particles are small and fall slowly, hence the opportunity to produce precipi-
tation is low, but in mixed clouds not only can all collision processes occur,
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