Trewartha climate classification

There was no specific monsoon climate identifier in the original scheme, but Am was added later, with the same parameters as Köppen's (except that at least three months, rather than one, must have less than 60 mm average precipitation).

However, a different formula is used to quantify the aridity threshold: 10(T − 10) + 3P, with T equaling the mean annual temperature in degrees Celsius, and P denoting the percentage of total precipitation received in the six high-sun months (April through September in the Northern Hemisphere, October through March in the Southern).

This represents subarctic and subpolar oceanic climate realms, defined the same as in Köppen's scheme, where 1 to 3 months have an average temperature of 10 °C (50 °F) or above.

There are no separate counterparts to the Köppen Dfd, Dwd, and Dsd climate types in Trewartha's scheme, but a letter can optionally be added to the end of the symbol to indicate the temperature of the coldest month (see below).

An option exists to include information on both the warmest and coldest months for every climate by adding a third and fourth letter respectively.