International index numbers
From AMS Glossary
international index numbers
A system of designating meteorological observing stations by number, established and administered by the World Meteorological Organization.
Under this scheme, specified areas of the world are divided into blocks, each bearing a two- number designator; stations within each block have an additional unique three-number designator, the numbers generally increasing from east to west and from south to north. The international language of this system facilitates quick identification of the source of any meteorological report.