Geoscience Reference
In-Depth Information
Sleet
Refers in the UK to a rain-snow mixture;
in North America, to small translucent ice
pellets (frozen raindrops) or snowflakes that
have melted and refrozen.
The density of gauge networks limits the accuracy of
areal precipitation estimates. The number of gauges per
10,000-km
2
area ranges from 245 gauges in Britain to
ten in the United States and only three in Canada and
Asia. The coverage is particularly sparse in mountain
and polar regions. In many land areas, weather radar
provides unique information on storm systems and
quantitative estimates of area-averaged precipitation
(see Box 4.1). Ocean data come from island stations and
ship observations of precipitation frequency and relative
intensity. Satellite remote sensing, using infra-red and
passive microwave data, provides independent esti-
mates of large-scale ocean rainfall.
Dew
Condensation droplets on the ground surface
or grass, deposited when the surface tempera-
ture is below the air's dew-point temperature.
Hoar-frost
is the frozen form, when ice
crystals are deposited on a surface.
Rime
Clear crystalline or granular ice deposited
when supercooled fog or cloud droplets
encounter a vertical structure, trees or sus-
pended cable. The rime deposit grows into
the wind in a triangular form related to the
wind speed. It is common in cold, maritime
climates and on mid-latitude mountains in
winter.
2 Precipitation characteristics
The climatological characteristics of precipitation may
be described in terms of mean annual precipitation,
annual variability and year-to-year trends. However,
hydrologists are interested in the properties of individual
rainstorms. Weather observations usually indicate the
amount, duration and frequency of precipitation, and
these enable other derived characteristics to be deter-
mined. Three of these are discussed below.
In general, only rain and snow make significant
contributions to precipitation totals. In many parts of
the world, the term
rainfall
may be used interchangeably
with
precipitation
. Precipitation is measured in a
rain gauge, a cylindrical container capped by a funnel
to reduce evaporative losses, which most commonly
stands on the ground. Its height is about 60 cm and its
diameter about 20 cm. More than fifty types of rain
gauge are in use by national meteorological services
around the world. In windy and snowy regions they are
often equipped with a windshield to increase the catch
efficiency. It must be emphasized that precipitation
records are only
estimates
. Factors of gauge location,
its height above ground, large- and small-scale turbu-
lence in the airflow, splash-in and evaporation all
introduce errors in the catch. Gauge design differences
affect the airflow over the gauge aperture and the
evaporation losses from the container. Falling snow is
particularly subject to wind effects, which can result in
under-representation of the true amount by 50 per cent
or more. It has been shown that a double snow fence
erected around the gauge installation greatly improves
the measured catch. Corrections to gauge data need
to take account of the proportion of precipitation falling
in liquid and solid form, wind speeds during precip-
itation events, and precipitation intensity. Studies in
Switzerland suggest that observed totals underestimate
the true amounts by 7 per cent in summer and 11 per
cent in winter below 2000 m, but by as much as 15
per cent in summer and 35 per cent in winter in the Alps
between 2000 and 3000 m.
a Rainfall intensity
The intensity (= amount/duration) of rainfall during an
individual storm, or a still shorter period, is of vital
interest to hydrologists and water engineers concerned
with flood forecasting and prevention, as well as to
conservationists dealing with soil erosion. Chart records
of the rate of rainfall (
hyetograms
) are necessary to
assess intensity, which varies markedly with the time
interval selected. Average intensities are greatest for
short periods (thunderstorm-type downpours) as Figure
4.10 illustrates for Milwaukee, USA.
In the case of extreme rates at different points over
the earth (Figure 4.11), the record intensity over ten
minutes is approximately three times that for 100 min-
utes, and the latter exceeds by as much again the record
intensity over 1000 minutes (i.e. 16.5 hours). Note that
many of the records for events with a duration greater
than a day are from the tropics. High-intensity rain is
associated with increased drop size rather than an
increased number of drops. For example, with precipi-
tation intensities of 1, 13 and 100 mm/hr (or 0.05, 0.5
and 4.0 in/hr), the most frequent raindrop diameters
are 1, 2 and 3 mm, respectively. Figure 4.12 shows
