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.