Geoscience Reference
In-Depth Information
shells of opaque and clear ice. The core of a
hailstone is a frozen water drop (ice pellet) or
an ice particle (graupel).
result in underrepresentation of the true amount
by 50 percent or more. It has been shown that a
double snow fence around the gauge installation
greatly improves the measured catch. Corrections
to gauge data need to take account of the propor-
tion of precipitation falling in liquid and solid
form, wind speeds during precipitation events,
and precipitation intensity. Studies in Switzerland
suggest that observed totals underestimate the
true amounts by 7 percent in summer and 11
percent in winter below 2000m, but by as much
as 15 percent in summer and 35 percent in winter
in the Alps between 2000 and 3000m.
The density of gauge networks limits the
accuracy of areal precipitation estimates. The
number of gauges per 10,000km
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 quantitaive estimates of area-averaged
precipitation (see
Box 4.1
). Ocean data come from
island stations and ship observations of precipi-
tation frequency and relative intensity. Satellite
remote sensing, using infrared and passive
microwave data, provides independent estimates
of large-scale ocean rainfall.
•
Graupel
- snow pellets, opaque conical or
rounded ice particles 2-5mm in diameter
formed by aggregation ofice crystals.
•
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.
•
Dew
- condensation droplets on the ground
surface or grass, deposited when the surface
temperature 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
suspended 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.
In general, only rain and snow make significant
contributions to precipitation totals. In many
parts of the world, the term
rainfall
can 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 60cm and its diameter about 20cm.
More than 50 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 wind shield to increase the
catch efficiency. It must be emphasized that
precipitation records are only
estimates
. Factors
of gauge location, its height above ground, 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, retention by wetting, and the evapora-
tion losses from the container. Falling snow is
particularly subject to wind effects, which can
2 Precipitation characteristics
The climatological characteristics of precipitation
may be described in terms of mean annual
precipitation, the annual cycle, annual variability
and decadal trends. However, hydrologists are
interested in the properties of individual rain-
storms. Weather observations usually indicate the
amount, duration and frequency of precipitation,
and these enable other derived characteristics to
be determined. Three of these are discussed below.
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
