# Frequently Asked Questions: Climate Data

## Rainfall

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.

Does the Bureau of Meteorology measure snow depth?

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

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

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

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

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.

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).

Why is some data red or missing?

Each data table page has one or more 'About' links (near the top of the page) that provide detailed information about the data. We recommend reading these pages.

Data that is displayed in a red colour and italics has not yet been quality controlled. There is always a lag between when data is received and when data is quality controlled due to the volume of data we manage.

Empty cells in data tables exist because that data is either not available, or has been flagged during the quality control process.

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## Air temperature

At what height is air temperature measured?

Air temperature is measured at a height of approximately 1.2 metres above the ground. Significant variation in temperature can occur in the layer of air close to the ground, as shown in the diagrams below. This is particularly evident on cold nights and hot days when there is little or no wind to mix the air.

Examples of the variation in air temperature close to the ground

How does the temperature change with altitude?

The change in air temperature with altitude depends on a number of factors - for example, the amount of moisture in the air. In the lower parts of the atmosphere an increase of 1000 metres typically results in a decrease in the temperature of the air of about 6 to 7 degrees Celsius.

What is the difference between air temperature and apparent temperature?

We often use the air temperature as an indicator of how comfortable we will feel when involved in sports or other physical activities. However, the air temperature is only one factor in the assessment of thermal stress. In climates where other important factors, principally humidity, can vary widely from day to day, we need more than just the temperature for a more realistic assessment of comfort. However it is useful to be able to condense all the extra effects into a single number and use it in a similar way to the way we use the air temperature. The Wet Bulb Globe Temperature (WBGT) and the Apparent Temperature are indices which attempt to do this. more »

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## Solar Exposure

Why is there no data for my site?

Solar radiation estimates prior to mid-November 2010 were not made for all locations close to the coast. Since then, an improved algorithm has been used. We hope to run the new algorithm over the old data, but it may be some time before this can be done.

How can I use the solar data to work out my solar energy?

You will need to search for a suitable solar energy calculator, then enter the relevant data into the calculator.

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Page updated: 15 December 2010