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Post by grega94 on Feb 2, 2024 23:22:04 GMT -5
Over the years I have thought about why is there a discrepancy between how the Köppen system differentiates hot and cold desert climates, and the subtropical/Mediterranean climates from their oceanic/continental counterparts?
If just looking at temperature, Kennewick, WA, it falls under the subtropical designation as it’s coldest monthly mean is 1.9C and the hottest is 24.9C, but because it only receives 201mm of precipitation and has an annual mean of 12.8C it’s designated as borderline BSk/BWk. If you go a bit further east to Walla Walla, WA precipitation increases to 476mm and is designated as Csa, putting it in the same category as Los Angeles, CA. Coldest month at 2.0C and warmest at 76.3C with an annual mean of 12.4C.
Would it perhaps make sense to differentiate the C climates into hot and cold climates as well, thus putting to rest the whole city x isn’t a true subtropical climate? For example NYC has an annual mean of 13.2C and this would be designated as a cold humid subtropical climate Cfak, while Charleston, SC with an anual mean of 19.2C would be designated as a hot humid subtropical climate Cfah. Likewise Rome with an annual mean of 15.8C would be a Cold Mediterranean climate Csak, while Los Angeles with an annual mean of 18.8C would be a hot Mediterranean climate Csah?
This would also help differentiate highland subtropical climates from their low land oceanic counterparts such as Mexico City with an annual mean of 18.3C making it a Cwbh. Although Bogotá at 14.4C is much cooler, even though clearly a highland subtropical climate.
All D climates are under 18C I’m pretty sure, so the hot and cold designation there is redundant, although I do wonder what is the hottest continental climate? Probably around 14C? Maybe we can lower the threshold between hot and cold at 14C, and all cold temperate climates are designated as D, while hot subtropical climates are C? Thus NYC would become Dfa, Seattle would be Dsb, while Charleston would remain as Cfa and San Francisco as Csb. Opinions?
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Post by grega94 on Feb 2, 2024 23:23:21 GMT -5
Also just realized I mixed up desert with dessert
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Post by Cheeseman on Feb 2, 2024 23:38:26 GMT -5
I've never been a fan of Koppen's system for dry climates; the other zones are based on temperature, not precipitation, so the B zone should too. I think it should be something like Dxa for arid D_a climate and Dta for semiarid D_a climate (yeah I swiped those particular letters from someone else on an old thread about this but you get my point).
As for why 18 C is the cutoff - I dunno, seems just as arbitrary as so many of our favorite drunk Russian's other cutoffs.
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Post by grega94 on Feb 3, 2024 0:21:30 GMT -5
so wouldn't seattle also fall under the subtropical category? Seattle’s hottest month is 19.7C so it doesn’t qualify as a hot summer climate which is required for subtropical climates. It’s annual mean is 12.1C which if it were arid would be classified as “Cold”. So anyway you cut it Seattle is not subtropical. |
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Post by segfault1361 on Feb 3, 2024 13:10:24 GMT -5
Not fully related but the main thing I don't like about arid climate classification is there's no distinction for oceanic deserts. Dry areas near the ocean with low temperature variance for the year. This puts Lima (a climate that's always foggy and mild) in BWh, the same climate classification as Dubai or Riyadh (known for scorching hot summers). It also puts San Diego in BSk, the same climate as Lethbridge, Alberta.
There's been talks about the "n" notification (BWn, BSn) for "foggy" deserts but there's no official definition. My official definition would be annual average temperature within 5C of the 18C threshold (13C-23C) with less than 10C difference from hottest and coldest months of the year. So Lima would be BWn and San Diego would be BSn.
There's also a video that talks about having distinction between 'cold' and 'warm' C-class climates (Cxa and Cxb). IMO the best way would be keep the -3C isotherm for delineation with D, but have a 8C isotherm between 'cold' and 'warm' (Cxxh and Cxxk). That way, we have a clear distinction where:
Dxx = Cold winter with snow that sticks
Cxxk = Cold enough for snowy winter but hard to get it to stick
Cxxh = Mild winter
Also that would allows Cxbh to represent a subtropical highland climate where the winter is mild, while Cxbk would represent the more traditional oceanic climates.
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Post by Ariete on Feb 3, 2024 13:34:55 GMT -5
Also just realized I mixed up desert with dessert
You can change the headline if you press the edit button in the opening post.
For the question, IDK.
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Post by desiccatedi85 on Feb 3, 2024 18:43:26 GMT -5
Köppen's classification system for deserts makes no sense. Just 2 categories, hot and cold, is really weird. I split deserts based on temperature means into equatorial (all months above 75ºF), tropical (all months above 62ºF), subtropical (coldest month 42ºF-62ºF with summers over 75ºF), temperate (coldest month 30ºF-42ºF, summers over 70ºF, or between 42ºF-70ºF year round), hemiboreal (coldest month <30ºF and with summers above 60ºF), subarctic (coldest month <30ºF and summers below 60ºF), and arctic which are just ice caps (all months below 30ºF).
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Post by CRISPR on Feb 3, 2024 21:12:40 GMT -5
Maybe because hot deserts are associated with the subtropical high (~30ºN/S), where easterlies fail to deliver moisture (rain shadow); while the cold water of the westerlies prevents much evaporation. Also, cold deserts are at a higher latitude (~40-50ºN/S), where westerlies fail to deliver moisture (rain shadow); or are far in the continental interior.
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Post by grega94 on Feb 3, 2024 21:40:41 GMT -5
Maybe because hot deserts are associated with the subtropical high (~30ºN/S), where easterlies fail to deliver moisture (rain shadow); while the cold water of the westerlies prevents much evaporation. Also, cold deserts are at a higher latitude (~40-50ºN/S), where westerlies fail to deliver moisture (rain shadow); or are far in the continental interior. I agree, I think hot desert = subtropical desert and cold desert = temperate desert. But my question is why 18C, what is so special about that number? Is that just happens to be the annual average for deserts at ~35N? And if so can that principle be applied to the C climates differentiate the true subtropical climates from the warm temperate climates? |
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Post by Steelernation on Feb 3, 2024 21:54:14 GMT -5
In the US it seems to work reasonably well dividing the more vegetated hot deserts with tons of cactus and the cold deserts with just scrub but that also depends on precipitation and probably doesn’t apply elsewhere
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Post by cawfeefan on Feb 3, 2024 22:43:56 GMT -5
Not fully related but the main thing I don't like about arid climate classification is there's no distinction for oceanic deserts. Dry areas near the ocean with low temperature variance for the year. This puts Lima (a climate that's always foggy and mild) in BWh, the same climate classification as Dubai or Riyadh (known for scorching hot summers). It also puts San Diego in BSk, the same climate as Lethbridge, Alberta. There's been talks about the "n" notification (BWn, BSn) for "foggy" deserts but there's no official definition. My official definition would be annual average temperature within 5C of the 18C threshold (13C-23C) with less than 10C difference from hottest and coldest months of the year. So Lima would be BWn and San Diego would be BSn. There's also a video that talks about having distinction between 'cold' and 'warm' C-class climates (Cxa and Cxb). IMO the best way would be keep the -3C isotherm for delineation with D, but have a 8C isotherm between 'cold' and 'warm' (Cxxh and Cxxk). That way, we have a clear distinction where: Dxx = Cold winter with snow that sticks Cxxk = Cold enough for snowy winter but hard to get it to stick Cxxh = Mild winter Also that would allows Cxbh to represent a subtropical highland climate where the winter is mild, while Cxbk would represent the more traditional oceanic climates. I agree that subtropical highland climates should be separated from more traditional oceanic climates somehow, but the 8C isotherm would leave cities like Melbourne, Auckland, Wellington and Bilbao in the former category. Maybe there should be an elevation requirement, but then it wouldn't be a strictly climatic classification anymore. |
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Post by caspase8 on Feb 3, 2024 22:57:47 GMT -5
The Holdridge life zones system uses a 17C biotemperature (pretty much the same as annual mean temperature for warm climates) to differentiate between subtropical and temperate climates. The logic behind this is that most plant species in climates warmer than this are frost sensitive, while most in climates colder than this are frost tolerant.
18C is close to 17C, so maybe Koppen had a similar idea? But if he did, why did he only apply the 18C annual mean threshold to arid climates? Did Koppen even consider vegetation?
The real answer to your question is probably vodka.
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Post by grega94 on Feb 6, 2024 16:22:07 GMT -5
I asked Bing AI and got the following answer;
The choice of 18°C as the threshold between cold and hot deserts is based on climate classification systems, particularly the Köppen climate classification. This system aims to categorize climates based on temperature and precipitation patterns.
Here’s why 18°C was chosen:
Biological Considerations: Ecologically, this temperature division aligns with the growth and survival of xerophytic vegetation (plants adapted to arid conditions). Hot deserts, with mean annual temperatures above 18°C, support cacti, succulents, and other heat-tolerant flora. Cold deserts, with temperatures below 18°C, host hardy grasses, shrubs, and lichens.
Historical Context: The Köppen system was developed in the early 20th century, and the choice of thresholds was influenced by available climate data and observations at the time. 18°C emerged as a practical boundary that captured the distinct characteristics of desert climates.
Observations of Existing Deserts: When examining existing deserts worldwide, researchers found that this temperature division effectively separated hot deserts (like the Sahara, Arabian Desert, and Mojave Desert) from cold deserts (such as the Gobi Desert and Great Basin Desert).
Generalization: While no single temperature can perfectly represent all desert regions, 18°C strikes a balance between the extremes. It allows for a clear distinction while acknowledging the variability within desert ecosystems.
In summary, 18°C serves as a practical and widely accepted threshold, facilitating the differentiation between cold and hot deserts based on mean annual temperature.
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Post by Cadeau on Feb 7, 2024 19:44:16 GMT -5
Not related to nature, coincidentally 18°C is meaningful when it comes to calculate the HDD and CDD.
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