By my reading of the wikipedia page on the Trewartha climate classification, climate classifications are not mutually exclusive.

Let's take a look at the climate classification of Iowa City, IA.

The Trewartha precipitation limit is defined as 10(T-10)+3P, where T is the mean annual temperature and P is the percentage of the annual precipitation occurring in the six high-sun months. In the northern hemisphere, this is Apr—Sep.

Using the data publicly available on weatherbase.com, the Trewartha precipitation limit is 608.23 mm. Twice that is 1216.46 mm.

BW (desert) climates are defined by mean annual precipitation below the original threshold (608.23 mm for Iowa City), BS (steppe) climates are defined by mean annual precipitation above the original threshold but less than twice the threshold (1216.46 mm for Iowa City), and if mean annual precipitation exceeds twice the threshold then the climate is not classified as B (dry). Iowa City's mean annual precipitation is 922.2 mm, earning it a BS (steppe) classification.

However, Iowa City also satisfies the definition of a Dca (hot-summer continental) climate under the Trewartha climate classification:

  • 4-7 months with mean monthly temperature above 50 degrees (F). Iowa City has 7 (all of Apr—Oct)
  • Coldest monthly mean temperature below 32 degrees (F). Iowa City's is 22.1 degrees (F) in Jan.
  • Warmest monthly mean temperature above 72 degrees (F). Iowa City's is 74.8 degrees (F) in Jul.

The question is...under the Trewartha climate classification, is Iowa City, IA classified as a BS (steppe) climate, a Dca (hot-summer continental) climate, or both?

UPDATE: It's come to my attention that I incorrectly used Fahrenheit instead of Celsius for the mean annual temperature, and that Iowa City, IA therefore does not actually lie within a BS (steppe) climate zone. I'm going to leave the original question, though: what happens when a climate meets the criteria for more than one zone?

  • $\begingroup$ What values of T and P did you use? $\endgroup$
    – justCal
    Feb 11, 2017 at 1:46
  • 1
    $\begingroup$ @user2448131, I incorrectly used T=50 degrees F (blast, my country's units) instead of the correct number, T=10 degrees C. With P=69.410106, the actual Trewartha precipitation limit for Iowa City, IA is 208.23, and twice that limit is 416.46. Therefore Iowa City, IA does not qualify for BS (steppe) classification and belongs to just one climate zone: Dca (hot-summer continental). $\endgroup$
    – The Baron
    Feb 12, 2017 at 12:55


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