AUTO

AUTO marks a report produced entirely by an automatic station, with no human intervention. It appears early in the report, after the date-time group, and it is a caveat about everything that follows.

Automatic stations measure some things well and others poorly. Wind, temperature, dew point and pressure are reliable. Present weather is limited: a sensor can detect precipitation and often distinguish rain from snow, but it does not recognise the full range of phenomena a trained observer would report. Cloud is sampled by a ceilometer looking straight up rather than assessed across the whole sky, so amounts can be unrepresentative.

Where an element cannot be measured, its digits are replaced by solidi — /// in a cloud group, or a weather group omitted altogether. NCD, no cloud detected, is an automatic-station code and carries the same caveat: nothing was detected in the sensor beam, which is not quite the same as a clear sky. An AUTO report is good data with known blind spots, and worth reading as such.