The rain gauges installed by the Bureau of Meteorology have the top of the collector 0.3 metre above ground level. This may increase the error associated with the measured rainfall. 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). Having decided on where to locate the rain gauge, the next question is "How high?". The gauge should also be in a place where it will not be disturbed by people, animals or vehicles.
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. 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. 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. 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. The standard Bureau of Meteorology rain gauge has an opening 203mm in diameter. Any moisture less than this is recorded as a trace. 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. A subset of observers at strategic locations send their observations electronically to the Bureau each day. These are known as accumulated observations.Īt 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. 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. Observationsĭaily rainfall is nominally measured each day at 9am local time. However, to a first approximation the relationship:Ĭan be used to estimate long term average precipitation. Since the density of snow can vary significantly, it is difficult to derive precipitation amount from snow depth. Measurements are taken by observing the change in fluid level in the tank. With another type of snow gauge, snow falls into a tank containing an antifreeze agent, which causes the snow to melt. In one form of snow gauge the snow is melted using an electric element. This is equivalent to 0.2mm of precipitation.Ī snow gauge is used to measure precipitation at several locations within Australia where snow falls.
An electronic pulse is generated each time the volume of water collected in one of the small brass buckets causes the bucket to tip. 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. There are two advantages of this type of rain gauge. In modern automatic weather stations a Tipping Bucket Rain Gauge (TBRG) is employed, which also has an aperture of 203mm. The top of the rain gauge is 0.3m above the ground. Any excess precipitation is captured in the outer metal cylinder. The measuring cylinder can record up to 25mm of precipitation. This is essentially a circular funnel with a diameter of 203mm which collects the rain into a graduated and calibrated cylinder. The standard instrument for the measurement of rainfall is the 203mm (8 inch) rain gauge.
0 kommentar(er)
