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.

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.