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What is the significance of the 18°C isotherm in climate classifications? It seems to be primarily used when defining tropical climates.
Why isn't it used in any subtropical climate classifications (ex. Six months mean average => 18°C)?
I don't think there is any particular reason why the coolest-month threshold for tropical climates should be the same as a threshold for another climate category. Using a minimum number of months above 18C for a subtropical category would raise the problem that in highland equatorial regions tropical climates would border non-even-subtropical climates with little to nothing in between.
Eg place a has a mean of 18.1C in every month and is tropical. Place B is just a little higher up and has a mean of 17.9C in every month and is not even subtropical.
When Koppen developed his climate classification system, 18C or 64.4F marked the lowest average monthly temperature that coconut palms trees could live. This is why in his climate classification system, 64.4F is the minimum threshold for tropical climates.
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in my modified Koppen, which splits the C group up more, I use 18°C for 4 months or more as the minimum for a subtropical climate. Less than that is cool oceanic if cold month is 0°C to 5.9°C or warm oceanic if cold month is 6°C to 17.9°C. And coldest month has to be 6°C to 17.9°C for subtropical in my modified koppen. If coldest month is 0°C to 5.9°C with 4+ months 18°C+, I consider the climate to be temperate transitional
When Koppen developed his climate classification system, 18C or 64.4F marked the lowest average monthly temperature that coconut palms trees could live. This is why in his climate classification system, 64.4F is the minimum threshold for tropical climates.
But did he really? Usually scientists studying climate and vegetation look at broad assemblages of native vegetation (eg tropical rainforests, Mediterranean scrub etc) rather than individual species. Coconut palms get discussed a lot though because they're big, easily recognisable, quite easy to grow and common in suitable climates but I'm not sure Koppen was especially interested in them.
in my modified Koppen, which splits the C group up more, I use 18°C for 4 months or more as the minimum for a subtropical climate. Less than that is cool oceanic if cold month is 0°C to 5.9°C or warm oceanic if cold month is 6°C to 17.9°C. And coldest month has to be 6°C to 17.9°C for subtropical in my modified koppen. If coldest month is 0°C to 5.9°C with 4+ months 18°C+, I consider the climate to be temperate transitional
I can see the logic from a higher latitude point of view, but I'm not sure I like a system that makes this tropical:
But Manizales not even subtropical but warm oceanic.
Between them you only get a tiny sliver of subtropical.
But Manizales doesn't have a single month above 18C, which would exclude it from your subtropical category even though very similar but slightly warmer Pueblo Rico is fully tropical.
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Quote:
Originally Posted by Bisfbath
But Manizales doesn't have a single month above 18C, which would exclude it from your subtropical category even though very similar but slightly warmer Pueblo Rico is fully tropical.
I can't call a climate with all months under 18°C subtropical, it feels just as wrong as calling Erie, PA subtropical which has it's coldest month around -2°C
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