Frequently asked questions - Data
For a more detailed explanation of the headers in our standard data format, see the data glossary.
SYNOP reports are observations that are made at internationally agreed times by all met observers. The regulations and practices are laid down by WMO (World Meteorological Organisation) and adhered to by all NMSs (National Meteorological Services). For example temperature and rainfall are reported at the following times:
- Tmin is reported at 0600 UTC for the previous 12 hours
- Tmax is reported at 1800 UTC for the previous 12 hours
- Rainfall is reported at 0600 and 1800 UTC for the previous 12 hour periods
Climate data are reports of absolute values of temperature, rainfall, sunshine that can be made at any time, and assigned to a calendar day or month. These values are the extreme events for any particular period, as defined locally. The reporting times vary from country to country and there is no internationally agreed standard, for example in the UK values are recorded for the period 0900-0900, in Germany 0730-0730, in France 0600-0600, in Finland 1800-1800 for temperatures and 0600-0600 for rainfall.
All extracted data is automatically quality controlled using the following techniques:
1. Each element is checked to see that it falls within acceptable limits. Where an element falls outside these limits it is removed from the dataset and logged.
| Limits currently applied to each element |
| Pressure |
Temperatures |
Wind Speed |
Humidity |
Rainfall |
Sunshine |
| 920 - 1070mb |
-70 - 70 °C |
0 - 200 kts |
0-100% |
0-1000mm |
0-24 hours |
2. Each element is checked against other reference data to ensure consistency, such checks include identifying where the following occur:
Tmax temperature < hourly temperatures; Tmin temperature > Tmax temperature;
air temperature > 5 AND present weather = snow.
3. A check is made between the synoptically reported extreme values against the intermediate observations to detect any values that have remained the same for a sustained period of time. These errors can indicate faulty sensors or incorrectly submitted reports. These errors are flagged but not removed at this stage.
4. Large differences between 2 near stations are flagged, but not removed at this stage. This check is applied to most elements but is not appropriate for rainfall.
5. The intermediate observations are checked for large step changes between 3 hourly reports, these are flagged, but not removed at this stage. For example, if within 3 hours the air temperature jumps up by 10C and in the subsequent 3 hours the temperature falls by 10C. These checks are also made for large changes in pressure, wind speed and humidity.
6. Checks are made that values do not fall outside the climatological extreme values for a station. There is a limited tolerance in the checks, this avoids the rejection of new extreme values.
7. Any values that are flagged as being suspect in the checks described above are then checked by climate experts within the Met Office who will either correct or reject a value based upon the evidence available. For example a suspect value where the Tmin > Tmax could be a case where Tmin is obviously wrong (looking at hourly temperatures) and the Tmax value is OK. If this is the case there is no need to reject the Tmax as well as the Tmin.
It is only after the above checks have been carried out that the dataset is released by the Met Office to weatherXchange.
There are two main reasons for NIL values appearing in a dataset, these are:
1. The observation of the specific element(s) has not been made in the first instance. There are various reasons for this, events such as a power or communications outages at the observing site, or even strike action at the station. If strike action is the reason then it is likely that other data within the same region will also be missing for that period.
2. weatherXchange SYNOP data is quality controlled by the Met Office to check that it falls within acceptable limits. Where an element falls outside these limits it is removed from the dataset and replaced with a NIL value. Other quality control techniques, such as checking against climate extremes, will flag suspect data. Climate experts within the Met Office will then correct or reject the suspect value(s) based upon the evidence available.
A trace is the smallest amount of rain that can fall without being measurable. A trace indicates that rain fell on a particular day and nothing was recorded on the measuring instruments.
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