From AMS Glossary
As defined by C. W. Thornthwaite in his 1948 climatic classification, a type of climate that has humidity index values of +100 and above.
This is his wettest type of climate (letter code A) and compares closely to the wet climate that heads his 1931 grouping of humidity provinces.
Thornthwaite, C. W. 1948. An approach toward a rational classification of climate. Geogr. Rev.. 38. 55–94.
Thornthwaite, C. W. 1931. Climates of North America according to a new classification. Geogr. Rev.. 21. 633–655.