Frequently Asked Questions: Climate Data

Rainfall

1. How much water can I collect from my roof when it rains?

For every square metre of horizontal surface area, 1 millimetre of rainfall will produce 1 litre of water.

ie. volume = horizontal area x depth of rainfall

Thus, to estimate the amount of water collected from a roof you will need the horizontal area of the roof (not the actual roof surface area) and some rainfall data.

The resulting volume is approximate only. For example, it is based on the assumption of uniform rainfall over the whole area, and may be inadequate for some roof geometries where rainfall collection is dependent upon prevailing winds during the rainfall. When calculating the water collected from the roof of your house you will need to take into account houses generally have several downpipes, so the effective collecting area for a rainwater tank will be less than the total roof area.

2. Does the Bureau of Meteorology measure snow depth?

A history of snow depth is not available from the Bureau.

3. Is it possible to get an estimate of how much snow fell?

Since the density of snow can vary significantly, it is difficult to determine the amount of snow fall using rainfall data. However, to a first approximation the following relationship can be used:

1cm of fresh snow ≅ 1 mm of rain

4. Where can I get weather and climate information for international locations?

Official Climatological Information for cities around the world (Sponsored by World Meteorological Organisation)

5. What is a rain day?

A rain day is recorded when there has been a daily rainfall total of at least 0.2 mm. For some recent stations the minimum is 0.1 mm. A rain day does not occur when there is only a trace of moisture in the rain gauge, or when the precipitation was observed to be solely from frost, dew or fog.

A rainfall total of 0.2 mm is quite a small amount of rain, and unlikely to have much impact on many activities. Therefore, days of rain greater than or equal to 1 mm, 10 mm, and 25 mm are often used as indicators of the number of "wet" days.

6. When did Australia change from points to millimetres for measuring rainfall?

In January 1974 the Bureau of Meteorology made the conversion from recording rainfall and evaporation in points to millimetres (1 point = 0.254 mm, and 1 mm = 3.94 points).

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