AWN shows in each day's Daily Weather Report any of the following records set that day:
- Highest and lowest maximum, minimum and grass minimum temperatures
- Highest daily rainfall, wind gust, and average daily wind speed
The system producing the report is run in real time with updates through the day as the readings come in soon after 9am and 3pm local clock times.
An important point to note is that only whole (rounded) temperature figures are available early in this process - until about 2.30pm EST the same day for minimum and grass minimum temperatures, and 2.30pm the next day for maximum temperatures. Also, maximum temperatures shown from late today to early tomorrow are those that occurred between 9am and 3pm local times - the full 24 hour maximum temperatures for any day are only available after 9am local time the next day.
When temperature figures to one decimal place are received, stations previously shown as breaking a record may no longer do so. For example, if the record maximum is 41.8 and a rounded maximum of 42 is received it is a record-breaker. But if, when the full figure comes in, it proves to be 41.7, then it no longer breaks the record. Similarly, some stations may fail to break a record with their rounded figure but do when the full figure comes in. Records should therefore only be taken as potential records until the full figure to one decimal place is received.
Some quality control (QC) issues
Unfortunately, programs designed to pick out records and extremes are also very good at finding observer errors. Typical errors include:
- Omitting a decimal point, so 54.2mm of rain becomes 542mm (human observers)
- Misreading a thermometer by 5 or 10 degrees, so a maximum of 35.6 becomes 41.6 or 45.6 (human observers)
- Entering a reading on the wrong day, especially rainfall observations (human observers)
- Entering a rainfall observation as a 24-hour period when in fact it has been for a longer period (human observers)
- Entering the actual reading of an anemometer (which is like a car speedo) instead of the difference between today's 9am reading and yesterday's. This can give a daily windrun figure such as 5678, which tranlates to an average of 236.6km/h! (human observers)
- Power spikes can give wild minimum temperatures (e.g. -41.7) or maximum temperatures (e.g. 58.9), or give a reading of 0.0 (automatic weather stations)
- A technician working on an AWS may reset all values to the time his work was finished, so data recorded will only reflect temperatures, rainfall, wind etc. since the time of reset and not the full 24 hours.
Quality control of the data goes through 3 stages:
- Records produced in real time have no quality control whatsoever. Use the notes above, and compare any suspect figures with nearby locations (the recent weather pages are good for this).
- Soon after each day I will rerun the program and scan for apparent errors. Obvious ones will be deleted, while uncertainties will be noted in the comments column.
- When Bureau quality-controlled data is received (about 3 months after real time), the program will be run again. Observations that the Bureau has flagged as suspicious or wrong are removed. In addition, I again delete any obvious errors remaining in non-QC data.
The comments column carries two pieces of quality control information. The first should normally be the figure "1", which the observer should enter to indicate that the observation covers only the one day's observation. A figure greater than 1 should never occur, as these records are only for single-day observations. However, occasionally the observer omits the figure which leaves the observation period open to some doubt. The second character is normally a Y/y, N/n or P/p. Y/y indicates the data has been quality controlled by the Bureau. A N/n or P/p indicates that it has not yet been quality controlled.
Notes on the statistics
The columns in the Records report show:
- Site number, station name, state and climatic district
- The reading that produced the record. Rain is in millimetres, temperatures in degrees Celcius and wind speeds in km/h.
- Previous monthly record
- The record for this month that was broken or equalled.
- The most recent date on which this occurred.
- The number of years for which the station has records for this month, including the current month. Records are only produced for stations with at least 10 previous years of observations for the element concerned.
- Previous all time record (only given if an all time record for the station has been equalled or set):
- The all time record that was broken or equalled.
- The most recent date on which this occurred.
- The number of years for which the station has records for all or part of a year, including the current year. This may differ from the monthly years column.
- The comments column automatically notes if the record has been equalled as distinct from broken. It also notes if the previous record was set or equalled in the same month. I also manually add notes here if I believe the record is suspect. For information on the first two characters in this column, see the section above.
The "grass minimum", also known as the terrestrial minimum, is recorded by an ordinary minimum thermometer left overnight with its bulb at grass-tip height, fully exposed to the sky. This differs from the normal minimum temperature which is measured by a thermometer in a Stevenson screen about one metre above the ground, and is mostly used to determine the temperature experienced by crops and the presence of frost. It's the temperature you'd experience lying on the ground out in the open, and is often 5 to 7 degrees below the screen minimum on still winter nights.
Maximum average daily wind speeds and maximum wind gusts are provided for stations with anemometers exposed 3m or more above ground level. Almost all Bureau AWSs now measure wind gusts and 3m average daily wind, but have only done so since about 2002. Consequently, they will not show up in records until they have a reasonable period of observations, which I have taken as 10 years. The longer wind gust and 3m average daily wind speed records are limited to some airports, Bureau-staffed stations and agricultural/experimental stations.