 |
Rainfall is an important natural resource which
provides us with the water we drink and wash with,
and which is widely used in industry and for leisure
activities. It is important that the rain which
falls is measured so that water supplies can be
managed and conserved.
Observations of rainfall amount, for instance,
are easy to make. This explains why they are
carried out at so many locations in the UK, by
both amateur and professional observers. The
Met Office knows of over 15,000 locations at
which these observations have been made, with
about 5,000 still active and measuring routinely.
This explains the popularity with schools of
the simple rainfall measurements which satisfy
the National Curriculum requirements for the
study of weather, climate and the environment.
Mathematics and Information Technology can be
used to manipulate the data for a variety of
applications.
|
| What aspects of rain do we measure? |
A particular place can experience long or short periods
of heavy or light rain, or no rain at all. The basic
measurement of rainfall is that of how much rain falls
in a specified period of time. By how much, we mean
the depth of rain accumulating on a level surface without
soaking in, running away or evaporating. The depth
is usually measured in millimetres. The standard periods
are the hour, the day, the month and the year. Measurements
of rainfall depth over a period of an hour or less
are usually only required for specialised purposes,
and have to be made using automatic instruments.
The most common measurement of rainfall is the amount
falling in a day. Meteorologists in the UK use a standardised
day which runs from 9 o'clock GMT each morning to 9
o'clock GMT the next morning (10 o'clock BST). This
period originates from the 19th century when it was
fashionable for amateurs, particularly doctors and
vicars, to make rainfall measurements at a time which
conveniently fitted in with their professional activities.
Daily rainfall amounts measured routinely can be added
together to provide monthly and annual totals.
Since the required observation is that of the amount
of rainfall, then the simplest way of obtaining this
is to accumulate the rainfall in a container and measure
the amount at the end of each day.
The combination of a storage container and a device
for measuring the amount of rain collected is usually
called a storage gauge. Most storage gauges for professional
use are intended for measuring daily rainfall amounts,
although larger versions are made for measuring monthly
totals at inaccessible locations.
In answering this question we can partly explain why
storage gauges are designed and operated the way they
are. Suppose on a particular day some rain falls on
the ground; why do we not just go out and measure the
depth of the rain somewhere with, say, a ruler?
Consider what happens to rainwater when it falls on
the ground. It collects in puddles, drains into gutters
and streams, soaks into the ground and evaporates.
It is quite possible for a puddle in a car park, which
collects water from a large surrounding area, to have
a depth ten times the depth of rain that fell. The
area which drains into the puddle may be dry soon after
the rain stops falling. Rain soaks into porous ground,
such as fields, very quickly and leaves no accumulation
to measure. Daily rainfall amounts in the UK range
from zero to (very rarely) 100 mm, but are most frequently
in the range 0 to 10 mm. If it is warm with bright
sunshine, dry air and a strong wind, it is possible
to evaporate away 10 mm of water in a day.
Obviously then, it is difficult to find a 'natural'
place where an accurate representative measurement
of depth of accumulated rainfall can be made with confidence.
We use a gauge to overcome these problems.
All observing stations run by the Met Office, and
by many of its co-operating observers, use the Met
Office copper 5" standard gauge. It consists of
a 5" diameter funnel with a sharp rim, the spout
of the funnel being inserted into a glass collecting
jar. The jar is in an inner copper can and the two
are contained in the main body of the gauge, the lower
part of which is sunk into the ground. The diagram
shows the whole gauge arrangement.
The main features of the gauge are:
i) the sharp inner edge of the funnel which
allows it to have an accurate 5" internal
diameter;
ii) the whole gauge is set into the ground to
keep it secure and upright with the rim 12"
above the surrounding short grass or gravel,
this height being chosen so that no rain splashes
from the surroundings into the funnel;
iii) the gauge is set vertical so that the rim
of the funnel is horizontal;
iv) the inner can is provided so that the glass
jar can be lowered gently into the gauge and
can also hold the water if the jar overflows
or cracks in cold weather;
v) the funnel has a narrow spout so that there
is little exposure of the water in the jar to
the air, to reduce evaporation.
|
|
|
