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Trewartha climate classification

Glenn Thomas Trewartha created this adaption of the Köppen climate classification system in 1966 in order to address perceived shortfalls of that system – in particular, that it grouped vast swathes of the world under the single “C” (temperate) category. Trewartha’s system also has some formula changes and the system itself is somewhat simplified (as in, the “core” system only has two-letter designations, not three-letter ones). Here is an outline of his system for classifying climate types around the world:

  • A: tropical: all months have a mean temperature above 18°C (same as in Köppen’s system)
    • Ar: places with no more than two low-rainfall months (defined as less than 60mm average rainfall) – i.e. this is the “wet all (or at least most of the) year” group
    • Am: places where the three driest months have less than 60mm rainfall, but more than 100 minus 4% of annual rainfall
    • Aw: places that aren’t Ar or Am with low rainfall in winter (i.e. short-days/low-sun)
    • As: places that aren’t Ar or Am with low rainfall in summer (i.e. long-days/high-sun)
  • B: arid: places with below a certain threshold of annual precipitation, as determined by a complicated formula (but a different formula than Köppen’s formula)
    • BW: desert
    • BS: steppe
  • C: subtropical: places with eight or more months with a mean temperature above 10°C
    • Cs: dry-summer or Mediterranean climate. Requires the winter half of the year to have at least 3x the summer half’s rainfall; less than 3cm of rainfall in the driest summer month; and less than 89cm of rainfall for the whole year.
    • Cf: humid or subtropical climate (includes the “Cw” group from Köppen’s system). Requires there to be no months with less than 3cm of rainfall on average.
  • D: temperate and continental: places with 4–7 months with a mean temperature above 10°C
    • Do: oceanic (even though many of these regions are not near the ocean); the coldest month has a mean temperature above 0°C
    • Dc: continental; the coldest month has a mean temperature below 0°C
  • E: boreal: places where only 1–3 months of the year have a mean temperature above 10°C (equivalent to all the C…c and D…c areas under Köppen’s system)
    • Eo: oceanic: the coldest month has a mean temperature above -10°C
    • Ec: continental subarctic: the coldest month has a mean temperature below -10°C
  • F: polar: places with no months with mean temperatures above 10°C (the same as Köppen’s “E” group)
    • Ft: tundra: at least one month has a mean temperature above 0°C
    • Fi: ice cap: no months have a mean temperature above 0°C

Universal Thermal Scale

Because the categories above are still pretty broad, Trewartha’s system lets you optionally add third and fourth letters to note the mean temperatures of warmest and coldest months. Unfortunately these letters are not in any kind of sane order like “alphabetical”, but instead apparently go:

  • i – a temperature ≥ 35°C
  • h – 28° to 34.9°C
  • a – 22.2° to 27.9°C
  • b – 18° to 22.1°C
  • l – 10 to 17.9°C
  • k – 0.1 to 9.9°C
  • o – -9.9° to 0°C
  • c – -24.9° to -10°C
  • d – -39.9° to -25°C
  • e – -40°C or less

So for example, my hometown of Melbourne’s “extended” classification would be Cfbl – subtropical, humid, warmest month with a mean temp between 18° and 22.1°C (it’s 21.7°C), and coolest month with a mean temp between 10°C and 17.9°C (it’s 11°C).

Criticisms of Trewartha

  • There are places that don’t qualify as either Cs or Cf, either because they have at least one month with <3cm of rainfall but see more than 89cm of rain over a year or because they get less than 89cm of rain a year but every month sees more than 3cm. Wikipedia has called these places “wet Cs” (in the former case) or “dry Cf” (in the latter).
  • While less broad than under Köppen’s system, the “C” (subtropical) category is still extremely broad. There’s quite a big difference between somewhere like São Paulo, which has a mean temperature in its coldest month of 17.9°C, and somewhere with mean temperatures below 10°C for four months of the year – yet these are both put in the same category. And unlike Köppen’s system the subcategories are purely rainfall-related, so you’re forced to use the extra letters if you want to clarify temperature… which is a lot more confusing than Köppen’s broad “Cfa”/“Cfb”-type differences.
  • The extra letters of the “Universal Thermal Scale” are confusing because there are so many of them and they’re not in order, but they’re also not specific enough to be meaningful! e.g. to me there’s a big difference between Melbourne’s July mean temperature of 11°C and Brisbane’s of 16°C, but both are covered by the single letter “l”. There’s also a pretty huge difference between Berlin’s January mean temp of 0.1°C and Hobart’s 9°C-ish in July, but both of those are covered by the letter “k”.
  • The system is considered pretty US-centric, with thresholds chosen in order to put different regions of the US in different categories (e.g. California from the Pacific Northwest, and east of the Mississippi, the North from the South). As far as Australia goes though, it classifies a huge chunk of the country, which we consider to exhibit a vast range of different climates, all as “subtropical humid”.1 Think Brisbane vs Hobart – the idea that you could even describe Hobart as “subtropical” to begin with is a laugh! The Trewartha system just seems much more finely-calibrated for the northern hemisphere and its temperature extremes than the south.

  1. Wikipedia’s global map on its Trewartha page(external link) conceals this a bit, as it wrongly colours southern Victoria and all of Tasmania as “D” (temperate). In reality Melbourne doesn’t even have one month with a mean temperature under 10°C, let alone five… ↩︎