http://en.wikipedia.org/wiki/World_Meteorological_Organization_squares

According to Wikipedia article, It divide World area into areas of 10° latitude by 10° longitude. Each area has a unique, 4-digit numeric identifier (refer chart at NODC World Ocean Database 2005 page) . Each area is allocated a number between 1000 and 7817.
WMO squares are used as the basis for the c-squares spatial indexing system, which further divides 10°x10° WMO squares into smaller units of 5°x5°, 1°x1°, 0.5°x0.5°, 0.1°x0.1°, and so on.