Observation of rainfall

Observing instruments

The standard Bureau instrument for the manual measurement of rainfall is the 203 mm (8 inch) rain gauge. This is essentially a circular funnel with a diameter of 203 mm which collects the rain into a graduated and calibrated cylinder. The measuring cylinder can record up to 20 mm of precipitation. Any excess precipitation is captured in the outer metal cylinder. The top of the rain gauge is 0.3 m above the ground.

In modern automatic weather stations a Tipping Bucket Rain Gauge (TBRG) is employed, which also has an aperture of 203 mm. Advantages of the TBRG include unattended, automatic operation, and the ability to record the rate at which the rain is falling. An electronic pulse is generated each time the volume of water collected in one of the small brass buckets causes the bucket to tip. This is equivalent to 0.2 mm of precipitation.

A snow gauge is used to measure precipitation at several locations within Australia where snow falls. In one form of snow gauge the snow is melted using an electric element. With another type of snow gauge, snow falls into a tank containing an antifreeze agent, which causes the snow to melt. Measurements are taken by observing the change in fluid level in the tank.

Observations

Daily rainfall is nominally measured each day at 9 am local time. At the majority of rainfall sites observations are taken by volunteers who send in a monthly record of daily precipitation at the end of each month. A subset of observers at strategic locations, as well as automatic weather stations, send observations electronically to the Bureau each day. Very few stations have a complete unbroken record of climate information. Missed observations may be due to observer illness or equipment failure. If, for some reason, an observation is unable to be made, the next observation is recorded as an accumulation, since the rainfall has been accumulating in the rain gauge since the last reading.

Rainfall has traditionally been measured to the nearest 0.2 mm (1 point, or 1/100th of an inch prior to 1974), although in recent years some observations are being reported to 0.1 mm. Any moisture less than this amount is recorded as a trace. The type of precipitation - rain, dew, snow, etc. - is also recorded at some sites when observed, as is the number of days on which rain fell (a rainday). Fog, dew, frost and a trace are not counted as raindays.

Quality control

Where possible an observation station is established following common practice and guidelines (pdf, 600 KB), which enhances the ability to compare observations between stations and over time. Inspections of volunteer rainfall stations are undertaken nominally every five years, and more frequently for Bureau sites, to monitor for potential departures from the observing guidelines.

Once the data arrive at the Bureau they proceed through a number of quality control processes to detect errors, which includes checking for:

• Values that extend beyond what is considered realistic.
• Inconsistent observations (e.g. high rainfall and clear skies).
• Discontinuous or abrupt changes in values over a short period of time.

Measuring your own rainfall

Rain gauges come in various sizes. If a rain gauge with a large diameter is used in an area which often receives large daily rainfall totals, such as in the tropics, then the amount of water collected can be inconveniently large. Gauges with too small an opening may be difficult to read when the rainfall amount is small, are affected more by the wind, and may become blocked with debris.

Installing a rain gauge

Gauges sited near buildings, solid fences and trees can have serious errors in rainfall totals.The distance of the gauge from such objects should be at least twice the height of the obstruction, and preferably four times the height. For instance, the gauge should ideally be more than 10 metres from a house 5 metres high and more than 30 metres from the nearest branches of a tree 15 metres high.

The opening of the rain gauge should be horizontal, and the grass and vegetation around it should not be allowed to grow more than a few centimetres high. The rain gauge should be securely fastened and relatively low to the ground to reduce the influence of wind, which may increase the error associated with the measured rainfall.

Making a rainfall observation

Ideally the rain gauge should be read every day as near to 9 am clock time as possible. During periods of heavy rain it may be necessary to read and empty the gauge at intermediate times to prevent the rain gauge from overflowing. This amount is then added to the amount read at the next 9 am observation.

To read the contents of the rain gauge first ensure that the gauge is vertical. Bring the eye level with the surface of the liquid in the gauge and read from the scale the position of the liquid surface. Make sure you read the bottom of the curved liquid surface (the meniscus), and not the top of the meniscus, which is the slightly higher lip formed where the water surface meets the cylinder wall. If the surface of the water is midway between two divisions, read the higher division. When solid or frozen precipitation is present in the rain gauge it will be necessary to melt the contents. This can be done by one of two methods:

• Place the gauge in warm water
• Add a measured quantity of warm water to the gauge, sufficient to melt the contents. Measure the total contents after it has melted and subtract from it the amount of water added.