- Why are records PROVISIONAL?
- About whole and rounded temperatures
- What records are presented
- How the records are presented
- Quality control issues
Why are records PROVISIONAL?
The system producing the report is run automatically in real time with updates through the day as the readings come in. The records are shown as PROVISIONAL records, as they still have to pass Bureau of Meteorology quality control tests to check the accuracy of the reading. Other problems can arise, too, but see below. Obvious errors will be corrected or deleted by AWN within a few days when full data is available, and the letter "C" for "Checked" will appear in the comments column with additional notes if needed.
About whole and rounded temperatures
Only whole (rounded) temperature figures are available until AWN receives detailed data from the Bureau of Meteorology early each afternoon. Minimum and grass minimum temperatures will show a decimal as ".0" until then, and maximum temperatures, which officially are taken between 0900 on one day and 0900 on the next will also show as ".0" until the afternoon of the following day. The initial maximum temperatures shown are for the period 0900 to 1500, and may miss a late-occurring high temperature.
While provisional records are shown even if a temperature is rounded, it's not until the full temperature figures to one decimal place are received that it's really known if a record was broken or equalled. 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 seems to be 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.
What records are presented
AWN shows in each day's Daily Weather Report the provisional records set by any Australian rainfall and weather station, depending on what it measures:
- Highest and lowest air maximum, air minimum and grass minimum temperature
- Highest daily rainfall, wind gust, and average daily wind speed
Highest and lowest air maximum and minimum temperatures, along with real-time temperatures, are measured in a Stevenson screen 1.2m above the ground in a well-exposed area. The screen shelters thermometers from rain and snow, as well as shielding them from radiation from the sun or other objects by day. It is painted white and has louvres which allow air to pass through, so that the thermometers are measuring the temperature of the air and can be compared to measurements internationally. Thermometers in the screen are 1.5m above the ground. The BoM has a good explainer here and short video here.
Highest and lowest "grass minimum" temperatures, also known as the ground or terrestrial minimum, is recorded by an ordinary minimum thermometer left overnight with its bulb at grass-tip height, fully exposed to the sky. Although it is only 1.2m lower than the thermometer in the screen, it can be 5° or more colder on still winter nights. It is mostly used to determine the temperature experienced by crops and the presence of frost, and is only found at relatively few stations.
Both types of minimum temperature are for the 24 hours TO 0900 local time while the maximum temperature is for the 24 hours FROM 0900. All are measured to tenths of a degree Celcius.
Highest Daily rainfall is for the 24 hours to 0900 local time, and is measured to a tenth of a millimetre. All stations measure rainfall.
Highest wind gusts are measured in kilometres per hour to the nearest whole figure and are given for the calendar day, midnight to midnight. Highest average daily wind speeds are measured to the nearest tenth of a kilometre per hour to 0900 local time. Both are measured on a mast about 10m above ground level. Almost all Bureau automatic weather stations (AWS) measure wind gusts and average daily wind.
How the records are presented
The columns in the Records report show:
- State and rainfall district, site number and station name
- The reading that produced the record on this day.
- 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). "All-time" means for all months that the station has been operating.
- 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.
- The comments column usually starts with a number and letter(s). The number should be "1", which the observer or AWS should enter to indicate that the observation covers only the one day's observation. A figure missing or greater than 1 should never occur and leaves the observation period open to some doubt. The letters indicates the following: Y means data has been quality controlled by the Bureau, N or P mean it has not, and C means it has been checked by AWN for obvious errors but should not be regarded yet as an official record.
Some quality control (QC) issues
Unfortunately, programs designed to pick out extremes are also very good at finding observer and AWS 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 24-hour wind run (the difference between today's 9am reading and yesterday's). This can give a daily windrun figure such as 5678, which translates 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 (AWSs)
- Rainfall in AWSs is measured by rain entering the gauge via a funnel into a tipping bucket system. As AWSs operate unattended, a blocked funnel or tipping bucket may go unnoticed for a while so that rainfall is lost. More important for record rainfall errors is that the tipping bucket mechanism may be hindered so that it begins to tip rapidly, counting huge amounts of non-existent rain (AWSs)
- 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 (AWSs).
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).
- I regularly scan for obvious errors. They will be corrected or deleted within a few days when full data is available, and the letter "C" for "Checked" will appear in the comments column with additional notes if needed. .
- The Bureau of Meteorology's Quality Control System is an ongoing process. Some errors are picked up within days, however it may take months or over a year for the QC to reach all stations. Even then, with millions of pieces of weather data in the BoM database, some errors will remain. The most recent data on the BoM Climate Data Online pages offer the best quality controlled version available. I receive data each day set back for the last 120 days which includes some of the BoM QC corrections, and this is available on the AWN Daily Climatic Data pages and archives. The Daily Weather Summary, however, relies only on my extraction system and alertness to keep it clean.
Last updated: 10/8/2018