New York is Subtropical. WHAT? The implication that this great world city, with its notoriously
chill winters, is subtropical, was made in my Secrets of World Climate video on Koppen’s Humid
Subtropical Climate. The comments section of this video makes for some interesting reading.
I had to go into hiding following the death threats. Ok, there were no death threats,
but I still receive comments of protest and complaint about this climate zone being too
much of a one size does NOT fit all when it comes to eastern temperate continental margins.
How is it possible that New York could have the same climate,
and accompanying natural vegetation, as that of New Orleans in the Deep South? How could Milan
in the foggy Po Valley of Northern Italy have the same climate as sunny Sydney, Australia?
And on a related subject how is it that London, with its cool winters,
the mild Azores and even Quito, just miles from the equator, experience the same climate,
since Koppen said they were all Oceanic, right? The sensible answer to all of these questions is
clearly in the negative. And in this video I will pick apart these weaknesses of the
Koppen Climate Classification and provide my own solution to the Subtropical Question.
The Koppen Climate Classification is the oldest and most widely used system of its type,
and is simple enough to be understood by wider audience than academics.
But because of its relative simplicity compared to other systems, limitations can be exposed,
and two of these are the rather too broad definitions of the Cfa and Cfb climate zones – the
Humid Subtropical and Oceanic respectively. These codes are broken down as follows.
C denotes a temperate climate compared to, say, tropical or subarctic, while
f denotes year round rain, and either a or b denoting a hot or warm summer.
In the respect of these code explanations, everything about them is correct. All these
areas would be considered temperate compared to subarctic, continental or tropical zones,
with no extreme winter temperatures, and some relief from heat at some time in the year. All
areas have rain that can occur at any time of the year, and all get either a hot or warm summer. By
hot we mean average summer day/night temperatures being above 22°C and warm being below this.
Let’s focus on Cfa and the most well-known area of this is the South Eastern United States,
with this climate zone taking up a full quarter of the land area and over half
the total population of that country. Now anyone who lives in this region or has
some familiarity with the United States is immediately going to think that this cannot
possibly represent a single type of climate, along with accompanying natural vegetation.
Comparing two iconic cities at opposite ends of this zone, and we find that New York City and New
Orleans have similar average highs in summer of 30 and 33 degrees Celsius respectively.
With high humidity and regular thunderstorms, both these cities fit well the hot and wet summers that
typify the Humid Subtropical Climate. But it’s when we compare winters that we run into trouble.
New York has a January day/night average of just above freezing,
with night low averages of minus -2 and day average highs at a chilly 4 celsius.
So with regular frosts and a thick overcoat required just to get about in the daytime, this
would fit anyone’s description of a proper winter. In New Orleans, however, things are a lot balmier,
with a day time high of 17 celsius, and night time lows significantly above freezing at 8 celsius.
This is a clearly very different winter picture in terms of human experience. And the type of natural
vegetation we find all along the Gulf Coast, with cypress, hickory, oak and pine forest dominating
differs much compared to the north where spruce, fir, maple, beech and birch are most common.
Similar comparisons can be made around the world in the other Cfa regions.
Cool winters are experienced in Northern Italy, Central China and Japan, while mild winters
predominate in the Southern Hemisphere zones around the Rio de la Plata of South America,
the Natal of South Africa, and the Eastern Seaboard of Australia.
So this is clearly a problem of too much generality in this Koppen climate zone.
Well what about other climate classifications? Do they address it? An attempt to fix some of
Koppen’s issues in the temperate band was made in the 1960s by Glenn Thomas Trewartha at the
University of Wisconsin - Madison. To go into detail about the Trewartha system is beyond the
scope of this presentation, but in short, he does address the issue of Koppen Cfa somewhat
by separating out the zone into two regions. The classic Cf Humid Subtropical zone where the
average day/night temperatures for 8 months of the year are above 10 celsius, and Do where they are
below this temperature, but with winter averages still above freezing. But his classification,
in my opinion, introduces other issues, in that this Oceanic zone ends up suggesting that the
Pacific North-West has a similar climate to Kentucky, which is clearly not the case.
Trewartha’s decision to use the 10 celsius average isotherm
over 8 months of the year was driven mostly by predominant vegetation type.
But for me, and probably most non-academics, when we think of climate type, we are thinking
about our own perception and experience of temperature, humidity and patterns of rain
or snow. And this is certainly borne out in the vast majority of comments I receive on my videos.
So to cater for the mob, I have developed my own isotherm to separate out the Cfa zone into
what would be perceived as either a cool or mild winter. I chose a day/night average of 10 celsius
for this, because such an average rules out night frosts in all but exceptional circumstances,
and so will guarantee the thriving of subtropical species of plants, such as palms. Further,
it provides for comfortable winter daytime peaks of 15 celsius or more – that’s t-shirt
weather if you’re actively out and about. For the south-eastern United States, this
line runs fairly close to the Gulf Coast from Texas to along the northern border of Florida
and then following the Atlantic coast up to about Charleston, South Carolina. Bear in mind, like any
divisions on a map, changes are gradual, so the perception shouldn’t be that immediately above
this line it’s a cool winter, or below it, it’s mild. The further north you go, the more likely
you are to encounter winter frosts, and the number of t-shirt days in winter is going to reduce.
In the central south-eastern part of South America, Humid Subtropical mild winters are
experienced in most of the region except for the plains inland from and south-west of Buenos Aires.
All of Europe’s small chunks of Humid Subtropical climate are cool winters, owing to these being the
Cfa regions with the highest latitude. South Africa’s small chunk around the Natal coast
is all mild winter, owing to its latitude much closer to the equator. At a similar latitude,
the same story goes for Australia with all of the eastern seaboard experiencing mild winters.
In Asia, we find the line between cool and mild winter varying in latitude dependent upon
topography. In India and Pakistan, the monsoon variant of the Humid Subtropical climate has
mild winters throughout the subcontinent due to the protection of the Himalayas from icy
winter Siberian winds. In China, lacking such protection, the line is closer to the equator,
and actually runs more or less between the monsoon and year-round rain variants of this climate.
Moderated by the warm South China Sea, Taiwan and the Ryukyu chain of Japanese islands that
includes Okinawa experience mild winters while the southern tip of Korea and all
of Humid Subtropical Japan get cool winters. What about anomalies in Humid Subtropical’s
sibling, the Oceanic climate, where summer temperatures are cooler.
Do we find cool and mild winters? In fact we do, although only in a small number of places
do we find mild winters as Oceanic regions tend to be further toward the poles. For instance,
all of NW Europe, where this climate dominates has cool winters, as do smaller instances in British
Columbia in Canada, and in southern Chile. Where we do find mild winters tends to be in latitudes
closer to the equator, but where dominant ocean winds moderate summer temperatures, and keeping
things cooler. The Azores, in the eastern North Atlantic at the latitude of Portugal have a
climate close to a mild winter Oceanic, but also bordering on Mediterranean and Humid Subtropical.
Away from the coast of the Natal in South Africa and Lesotho, we can find a significant region
that could be described as Mild Winter Oceanic. Although really this is a kind of Subtropical
Highland form of Humid Subtropical, as it’s at the same latitude, but simply raised in altitude.
The same could be said about the region around Curitiba in Southern Brazil, as well as upland
and inland parts of the Australian Eastern Seaboard – it’s a grey area in terms of these
“Oceanic” regions that lie at raised altitudes in Subtropical regions. A clearer case of mild
winter Oceanic in South Africa, however, is the central coast around the city of George.
The largest and most clear-cut example of mild-winter Oceanic,
however, is found, funnily enough, in Oceania, specifically the coast of Victoria, Australia,
including the state capital Melbourne, and the northern tip of North Island, New Zealand,
including that country’s largest city, Auckland. Further inland into Victoria,
and the moderating influence of the Southern Ocean results in cooler winters, while in Tasmania
and the rest of New Zealand, the more polar latitudes lead to cooler temperatures in general.
The last remaining anomaly I’ll cover in this video I’ll briefly touch on,
since I have gone over this in more detail in my other videos, and that is the Koppen Cfb code
shared with the Oceanic and Subtropical Highland climates found close to the equator,
but most notably in Colombia and Ecuador where large cities including both capitals are located.
In these regions there are no seasons in regard to temperature, and so there is no concept of winter
at all. But they share the same code as NW Europe? Well it’s simply that Koppen categorised this zone
as having peak day/night temperatures no higher than 22 celsius and lowest
temperatures being above freezing. There wasn’t really any specificity regarding
variation of temperature beyond that. And of course, we get year-round rainfall in each.
Aware of this from the beginning, I kept these two types of zone apart,
and each was addressed in separate chapters in my Secrets of World Climate series.
In summary, then, any classification system will have its limitations. The more detail you go
into in natural systems such as climate, then the more complex things become. For general patterns,
and ease of understanding, Koppen works, I would say, in 90% of places around the world.
But it’s the 10% that gets the YouTube comments section overflowing.
I hope this video at least addresses some of those comments. But I’m sure you’ll have more, so…
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