Third-order climatological station
From AMS Glossary
third-order climatological station
As defined by the World Meteorological Organization in 1956, a station, other than a precipitation station, at which the observations are of the same kind as at a second-order climatological station, but are 1) not so comprehensive; or 2) made once a day only; or 3) made at other than the specified hours.
This designation is not used officially in the United States, but types of stations that would fit under this category include climatological substations and certain aeronautical weather reporting stations.