Observation of rainfall

Introduction

Instruments

The standard instrument for the measurement of rainfall is the 203mm (8 inch) rain gauge. This is essentially a circular funnel with a diameter of 203mm which collects the rain into a graduated and calibrated cylinder. The measuring cylinder can record up to 25mm of precipitation. Any excess precipitation is captured in the outer metal cylinder. The top of the rain gauge is 0.3m above the ground.

In modern automatic weather stations a Tipping Bucket Rain Gauge (TBRG) is employed, which also has an aperture of 203mm. There are two advantages of this type of rain gauge. Firstly, it never needs to be emptied, and secondly the amount of rainfall (and even the rate at which the rain is falling) can be read automatically. 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 0.2mm 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. Since the density of snow can vary significantly, it is difficult to derive precipitation amount from snow depth. However, to a first approximation the relationship: 1cm of fresh snow = 1mm of water can be used to estimate long term average precipitation.

Observations

Daily rainfall is nominally measured each day at 9am local time. However there are a number of sites which report 48 or 72 hour totals (or occasionally longer) over weekends if the observer is unable to be present. These are known as accumulated observations. At the vast 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 send their observations electronically to the Bureau each day.

Rainfall has traditionally been measured to the nearest 0.2mm (1 point, or 1/100th of an inch prior to 1970), although in recent years some observations are being reported to 0.1mm. Any moisture less than this is recorded as a trace.

Measuring your own rainfall

The standard Bureau of Meteorology rain gauge has an opening 203mm in diameter. 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 buildings, trees or other objects should be at least twice the height of the obstruction, and preferably four times the height. For instance, the gauge should 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 gauge should also be in a place where it will not be disturbed by people, animals or vehicles.

Having decided on where to locate the rain gauge, the next question is "How high?". As the height of the rain gauge above ground is increased, the influence of the wind becomes more important (because the speed of the wind generally increases with height above the ground). This may increase the error associated with the measured rainfall. The rain gauges installed by the Bureau of Meteorology have the top of the collector 0.3 metre above ground level.

The opening 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 so that it does not blow over in strong storms, when high rainfall totals are of particular interest!

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 liquid surface and not 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.