Observations

Rainfall

What is a rain day?

A rain day is recorded when there has been a daily rainfall total of at least 0.2 mm (or 0.1 mm for some more recent sites). 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).

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.

i.e. 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 when it is raining. When calculating the water collected from the roof of your house you will need to take into account that houses generally have several downpipes, so the collecting area for a rainwater tank will only be from the roof area draining to the connected downpipe.

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.

Air temperature profiles
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. The rate at which air temperature decreases with increase in altitude is known as the lapse rate.

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. Learn more about thermal stress and thermal comfort indices.

Where can I find information about the Australian Climate Observations Reference Network - Surface Air Temperature (ACORN-SAT) database?

Information about the Bureau's homogenised temperature dataset (ACORN-SAT) can be found at: http://www.bom.gov.au/climate/change/acorn-sat/. The page includes the data, station details, methods used and details of the peer review that was undertaken.

Solar exposure

Why is there no solar data for my site?

Solar radiation estimates prior to 2007 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.

Where can I find definitions of terms related to solar exposure and terrestrial radiation?
Many solar and terrestrial radiation terms are defined on this page: http://www.bom.gov.au/climate/austmaps/solar-radiation-glossary.shtml.

Wind

How do I obtain wind data?

Wind data can be ordered via the Weather Station Directory. Search for weather stations that provide the data which best meet your needs and send your request.

What is the wind usually like at my specific location?

Wind is particularly sensitive to the local environment; local topography and the proximity to the ocean will both have an effect on the predominant winds. The best way to get an accurate idea of how the wind behaves at your specific location would be to purchase a anemometer, and collect some wind data yourself. Some backyard models are fairly cheap and widely available.

In respect to wind patterns more generally, wind roses are available for a number of locations across Australia from http://www.bom.gov.au/climate/averages/wind/selection_map.shtml. Monthly-averaged data for numerous weather stations is available at http://www.bom.gov.au/climate/data/index.shtml and maps for Australia at http://www.bom.gov.au/jsp/ncc/climate_averages/wind-velocity/index.jsp.