Temperature & rain gauges

21 March, 2010

Brod Street checking rain gauge and roof catchment area from the top of of his shed.
Brod Street checking rain gauge and roof catchment area from the top of of his shed.
Image: Rodney Start
Source: Museum Victoria

Question: I saw the temperature gauge when I visited Scienceworks recently and would like to set one up at home – where is the best place to put it for the most accurate reading and where would be the best place to locate a rain gauge? Does the Bureau of Meteorology take readings from a gauge in direct sunlight or in the shade? Where can I buy good quality scientific equipment for these activities?

Answer: The temperature recorded by the Bureau of Meteorology (BoM) is the air temperature. It does not include effects of direct sunlight or any wind. To do this reliably, the Bureau (and other such organisations) put their thermometers in an enclosed ventilated box called a Stevenson screen (it was invented by an engineer who was the father of the novelist Robert Louis Stevenson).

If you want to measure air temperatures at home, the most important factors are to ensure that the thermometer is never exposed to direct sunlight, and is not close to any surface that heats up, such as a brick wall that catches the afternoon sun. If you want to be quite precise, you could build your own form of Stevenson screen.

However you don’t necessarily need to go to all this trouble. If you locate a suitable spot at home, you could take readings at half hourly intervals for a number of days and then compare the readings to the results from your local BoM station. If there is a relatively constant offset between your readings and the BoM readings, then you can just use this offset to calibrate your readings in the future.

Rain gauges can be located more flexibly but there needs to be nothing above or around them. Since rain doesn’t always fall straight down, you want to be clear of tall objects – the rule of thumb is to ideally be twice as far away from any object as the objects height. On the other hand, strong winds can interfere with the rain gauge, so you don’t want the site to be completely exposed.

As far as the best places to buy, specialty scientific stores will be better than general scientific stores, which will be better than general department or consumer electronics stores. However, even cheap electronic thermometers are quite good.

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Stevenson Thermometer Screen, Melbourne Observatory, circa 1879.

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