Bureau of Meteorology enclosure and Westpac barometer

1 Pluviographs. No longer used but 4 The instrument shelter or Stevenson screen they measured the amount of rainfall over a allows air to flow freely over the thermometers in certain period of time. Rain was collected through order to permit accurate readings. The door must a funnel into a cylindrical container with a float. A open due south so that sunlight never strikes the pen was attached to the float to record changes in thermometers directly. The shelter contains water level on graph paper. As the level rose, so several thermometers. did the pen stroke. The graph paper showed the (i) A dry bulb thermometer measures the shade timescale along the horizontal axis and vertical temperature in degrees Celsius. scale showed millimetres of rain collected. A litre of water collected was the equivalent of 10 mm of (ii) A second thermometer is kept damp by a rainfall; 100 mL equalled 1 mm. muslin wick soaking in distilled water to record the wet bulb temperature. It gives a 2 Traditional 203 mm (or 8 inch) rain gauges lower reading than the dry bulb because of used as backups. Rain falls into a funnel and the cooling effect of the water evaporating collects in a graduated measuring cylinder. from the muslin wick. Typically, minimum temperatures occur just 3 Tipping Bucket Rain Gauge (TBRG). As rain before dawn and maximum temperatures (or precipitation) falls into the funnel, it drips into typically before 3.00 pm. one of two very carefully calibrated ‘buckets’ balanced on a pivot (like a seesaw). The top bucket is held in place by a magnet until it has filled to the calibrated amount (0.2 mm of rain). When the bucket has filled to this amount, the weight of the bucket is enough to break free of the magnet, causing the bucket to tip. The water then empties down a drainage hole and raises the other to sit underneath the funnel. Whenever the bucket tips, it triggers an electronic switch. A computer counts the number of tips and calculates the rainfall in millimetres. The minimum amount of rain to be recorded is therefore 0.2 mm. Tipping Bucket Rain Gauge (TBRG). 5 The Westpac barometer. The eight horizontal lights indicate the barometric pressure from 990 hPa to 1025 hPa in 5 hPa increments.

If only the bottom light is illuminated the pressure is 990 hPa or lower. If all lights are illuminated the pressure is above 1025 hPa or higher.

As the pressure increases, the lights ripple upwards; as the pressure falls, the lights ripple downwards. If the pressure remains static, so will the lights.

As a general ‘rule of thumb’, higher pressures bring stable weather as air from the upper atmosphere pushes downwards. Lower pressures bring usettled weather as warm moist ground level air rises and condenses to form clouds.

Don’t forget your students will need to watch the Westpac barometer to answer the question on page 9 of the Observing the weather booklet.

Useful links • www.sydneyobservatory.com • www.bom.gov.au • www.anmm.gov.au

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