- —w indicates dry winters (driest winter month average precipitation less than one-tenth wettest summer month average precipitation. s indicates at least three times as much rain in the wettest month of winter as in the driest month of summer. f means significant precipitation in all seasons (neither above-mentioned set of conditions fulfilled).[1]
The third letter indicates the degree of a indicates warmest month average temperature above 22 °C (71.6 °F) while b indicates warmest month averaging below 22 °C.
- D (continental) - as in C but colder during the year
- E (polar).
Each group and subgroup is represented by a letter. All climates are assigned a main group (the first letter). All climates except for those in the E group are assigned a seasonal precipitation subgroup (the second letter). For example, Af indicates a tropical rainforest climate. The system assigns a temperature subgroup for all groups other than those in the A group, indicated by the third letter for climates in B, C, and D, and the second letter for climates in E. For example, Cfb indicates an oceanic climate with warm summers as indicated by the ending b. Climates are classified based on specific criteria unique to each climate type.[8]
The Thornthwaite system, [4] in use since 1948, incorporates evapotranspiration along with temperature and precipitation information and is used in studying biological diversity and how climate change affects it. The Bergeron and Spatial Synoptic Classification systems focus on the origin of air masses that define the climate of a region.