Trewartha's modifications to the 1884 Köppen climate system sought to reclass the middle latitudes into three groups, according to how many months have a mean temperature of or higher: •
C (
subtropical)—8 or more months; •
D (
temperate)—4 to 7 months; •
E (
boreal)—1 to 3 months. The tropical climates and polar climates remained the same as in the original Köppen climate classification. The "highland" climate is ambiguously defined. Newer users of KTC generally omit this option.
Group A: Tropical climates This is the
tropical climate realm, defined the same as in Köppen's scheme (i.e., all 12 months average or above). The
A climates are the realm of the winterless frost-free zone. • Climates with no more than two dry months (defined as having less than average precipitation, same as per Köppen) are classified
Ar. • Others are classified
Aw if the
dry season is at the time of low-sun/short days or
As if the dry season is at the time of high-sun/long days. 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).
Group B: Dry (arid and semi-arid) climates BW and
BS mean the same as in the Köppen scheme. However, a different formula is used to quantify the aridity threshold: 10(
T − 10) + 3
P, 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). • If the precipitation for a given location is less than the above formula — that is, if
P < 10(
T − 10) + 3
P — the climate is said to be that of a
desert (
BW). • If it is equal to or greater than the formula but less than twice that amount, the climate is classified as
steppe (
BS). • If the precipitation is more than double the value of the formula, the climate is not in Group
B. Unlike in Köppen's scheme, no thermal subsets exist within this group in Trewartha's, unless the
Universal Thermal Scale (see below) is used.
Group C: Subtropical climates In the Trewartha scheme the
C climate group encompasses
subtropical climates, which have 8 or more months with a mean temperature of or higher. There are only two types within the
C or subtropical climate group: •
Cs, which is a dry-summer or
Mediterranean climate; •
Cf, or
Humid subtropical climate.
Cw types occur within the
Cf group and refer to subtropical monsoon climates (like much of East and South Asia).
Group D: Temperate and continental climates In the Trewartha scheme the
D climate group encompasses
temperate climates that have 4 to 7 months with a mean temperature of or higher.
D climate groups have two types: • Oceanic (
Do), where the coldest month has a mean temperature or higher • Continental (
Dc), where the coldest monthly mean temperature reaches below 0 °C, as in some interior landmasses in North America and Asia. For the continental climates (
Dc), sometimes a third letter (
a or
b) is added to denote a hot or cold summer.
Dca is used where the warmest month has a mean temperature of or higher, and
Dcb is used for cool-summer temperate climates, where the warmest month has a mean temperature below 22.2 °C. Most of Europe north of the 44th
parallel exhibits a
Do or
Dc climate type.
Group E: Boreal climates 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 or above. In this climate zone there is only a short period (normally 50 to 90 days) that is frost free. In the original scheme, this group was not further divided; later, the designations
Eo and
Ec were created: •
Eo (maritime subarctic) signifies that the coldest month averages above . •
Ec (continental subarctic or "boreal") means that at least one month has an average temperature of −10 °C or below. As in Group
D, a third letter can be added to indicate seasonality of precipitation. 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).
Group F: Polar climates This is the
polar climate group, where all months have a monthly mean air temperature below . Polar climates have two subtypes,
Ft (
tundra) and
Fi (
ice cap): • In the
Ft climate type, at least one month has an average temperature above (but not above ), so that there is a brief time when the surface might be free of snow or ice and a
scrub or tundra vegetation cover is possible. • In the
Fi climate type, all months have an average temperature below . This is the region of the vast deserts of perpetually frozen ocean in the
North Pole, and the permanent ice
plateaus of
Antarctica and
Greenland.
Group H: Highland climates Highland climates are those in which
altitude plays a role in determining climate classification. Specifically, this would apply if correcting the average temperature of each month to a
sea-level value—using the formula of adding for each of elevation—would cause the climate to fit into a different thermal group from that into which the actual monthly temperatures place it. Sometimes
G is used instead of
H if the above is true and the altitude is between , but the
G or
H is placed in front of the applicable thermal letter rather than replacing it. The second letter used reflects the
corrected monthly temperatures, not the actual monthly temperatures. ==Universal Thermal Scale==