Environmental Geography

The previous GeoCurrents post harshly criticized several climate maps for extending the subtropical zones too far toward the poles. But after doing a little casual research, I was chagrined to discover that these maps largely fit the formal, or “geographical,” definition of the term. The Wikipedia article on the subtropics states that “they cover the middle latitudes from 23°26′09.3 to approximately 35° to 40° north and south.” Both ChatGPT and Grok give the same figures. But in the United States, the subtropics are seldom if ever imagined in such terms. The idea that Chattanooga, Tennessee (35°N) is subtropical, let alone Columbus, Ohio (40°N), would strike most Americans as absurd. It is also noteworthy that very few maps that purport to depict the subtropics follow this latitudinal definition – and for good reason.

The main reason why the common conception of the subtropics does not match the formal definition is because the former is based on climate and climatic zones do not rigidly follow latitudinal belts. As can be seen below, the Wikipedia map of subtropical climates strays far from the lines of latitude that supposedly bound them. Although the Wikipedia article on the subtropics does not specify that the latitude-based definition of the term is quite different from the climatological definition, both ChatGPT and Grok do so.  As a result, they give a better sense of what the term actually means.

But even if we limit ourselves to the climatological subtropics, conceptual problems remain. To begin with, the definition of the term varies significantly from source to source. Such definitions, moreover, are often too vague to be useful. ChatGPT, for example, tells me that “Some meteorological sources define the subtropics as the region where [the] mean annual temperature is above 18°C (64°F) [and] winters are not cold enough for snow to be reliable,” but as this definition also includes the tropics. ChatGPT further claims that subtropical summers are hot, but that is not the case in many west-coast locations. In Walvis Bay, Namibia, which at 22°57’ is just north of the Tropic of Capricorn, not a single month has a mean daily maximum temperature above 68.5° F (20.3° C). Grok’s climatological definition of the subtropics, in contrast, is precise and seemingly scientific: “Regions where the average temperature of the coldest month is between 0–18 °C (32–64 °F) and the average temperature of the warmest month is above 22 °C (72 °F), following modified Köppen climate classification criteria.” But the “0–18 °C (32–64 °F)” coldest-month range is how the Köppen system defines temperate climates, not subtropical ones.

The Wikipedia article on the subtropics rests on more solid climatological grounds. It follows the Trewartha climate classification system in defining a subtropical climate as one “that has at least eight months with a mean temperature greater than 10 °C (50.0 °F) and at least one month with a mean temperature under 18 °C (64.4 °F).” The main problem with this definition is that it fails to exclude areas with short but chilly winters that have regular bouts of extreme cold – conditions that few people would regard as “subtropical.” Chattanooga, Tennessee, for example, easily slots into Trewartha’s humid subtropical zone, but its mean monthly minimum temperature in January is a frosty 15.9° F (-9.5° C) and its record low is a frigid -10° F (-23° C).

Another way to define the subtropics is by the geographical ranges of certain perennial plants, generally those that require a long period of relatively warm weather but can tolerate cool winters with occasional light frost. The Wikipedia article, for example, tells us that “These [subtropical] climates do not routinely see hard freezes or snow due to winter on average being above freezing, which allows plants such as palms and citrus to flourish.” By this criterion, my own home on the Stanford campus near Palo Alto, California (37° N), is definitely subtropical. But I doubt that anyone who grew up in the area would regard it as such. Most local people whom I queried seemed puzzled that I would even ask such a silly question. Several of them replied that Hawaii is the quintessential subtropical location. Actually, Hawaii is fully tropical (the northernmost point on Kauai is at 22°13′ N latitude).

My own personal definition of the subtropics is keyed to a more comprehensive botanical list found in the same Wikipedia article: “plants such as palms, citrus, mango, pistachio, leechee, and avocado are grown in the subtropics.” Avocado trees cannot be successfully cultivated in Palo Alto, as they succumb to periodic frosts (Palo Alto’s mean annual minimum temperature is 28.1° F [-2.2° C]). When driving south, I can only start to imagine that I have entered the subtropics when I have rounded Point Conception and see the avocado orchards of southern California. Santa Barbara’s mean annual minimum temperature is 36.6° F (2.3° C) and frost-sensitive plants abound. But good luck with mangos and leechees.*

I do think that the “subtropics” is an indispensable geographical category. But it is necessary to clearly distinguish the “latitudinal subtropics” from the “climatological subtropics.” I also think that it is essential to consider regularly occurring extremely low temperatures when defining the subtropics on climatic grounds. These topics will be considered in greater length in coming GeoCurrents posts.

* As the map posted below shows, avocado orchards are found north of Point Conception in San Luis Obispo County, but I have never seen them.

**ChatGPT tells me that “You can grow mango trees in Santa Barbara if you have a warm microclimate and are willing to protect the tree during cold spells. They are not guaranteed producers…” It essentially gives the same answer for leechees.

As earlier GeoCurrents posts in the current series on educational geography have noted, sun angles, which are determined by latitude, play a huge role in shaping the geography of climate. Simply put, the lower the latitude of any given location, the more solar radiation it will receive and the warmer it will be. Historically, latitude has been considered the prime determinant of climate. It often still is.

The actual situation, however, is far more complicated. Climate is not just a matter of average temperatures, nor are average temperatures just a matter of latitude. Even equatorial locations can have cool climates if they sit at high elevations; Quito, Ecuador, at 0° latitude and an altitude of 9,350 ft (2,850 m), has a mild annual daily mean temperature of 60.1° F (15.6° C). And even tropical lowland areas can have seemingly non-tropical climates. Consider, for example, the coastal city of Lima, Peru, which, at 12° S, is well within the tropics. Yet in December, a month with high sun angles, Lima’s mean daily maximum temperature is only 74.9° F (24.4° C), and in August it is only 66.6° F (19.2° C). Unlike most tropical locations, moreover, Lima receives very little rainfall and has nothing that even approaches a rainy season. In August, the city’s “wettest” month, average precipitation is a meager 0.06 inches (1.5 mm). But despite its extreme aridity, with only 0.25 inches (6.4 mm) of rain annually, Lima remains humid throughout the year. In its driest months (December-January), the average relative humidity is 81%.

Lima owes its mild temperatures, lack of precipitation, and humid air to the cold Humboldt Current that courses off its coast. Besides latitude, altitude and proximity to ocean currents are thus key influences on the geography of climate. But they are still not the only ones. Other important considerations include location relative to mountain ranges and prevailing winds, and position within continental landmasses (coastal vs. interior and west coast vs. east coast). These factors will be explored in later posts. For now, I will remain focused on the understandable but simplistic notion that latitude alone determines climate.

Equating climate with latitude with goes back to the ancient Greeks, whose geographical knowledge was largely restricted to the greater Mediterranean world. The term “climate” derives from the Greek term klima, which originally meant “inclination” or “slope.” Greek geographers divided the world into distinct bands called “the climes,” which were strictly defined by latitude, which in turn was defined by the seasonally changing inclination of the sun. In their simpler five-clime model*, used by Aristotle and others, a hot “torrid zone” extended from the Tropic of Cancer to the Tropic of Capricorn. On either side of this tropical (or equatorial) swath lay two temperate belts, which extended to the Arctic and Antarctic circles. Beyond the circles were the frigid, or arctic and Antarctic, zones. Most ancient Greek geographers agreed that only the temperate zone was inhabitable, as the torrid zone was too hot for human habitation and the arctic zone too cold. The existence of the vast torrid zone meant that the ecumene (or oikoumene), the inhabited temperate portion of the world, was isolated from any possible human societies living in the southern temperate belt.

Ancient Greek climatic ideas were highly influential in ancient and medieval Europe, discouraging ventures into far northern and far southern lands. Eventually, however, the weight of empirical evidence crushed the authority of the classical texts. As Margaret Small explains in her 2020 book Framing the World: Classical Influence on Sixteenth-Century Geographical Thought:

[This] chapter first examines how and why the Greeks and Romans developed the concept of uninhabitable frigid and torrid regions denoting the limits of the oikoumene [ecumene]. In the process it demonstrates why these climatic limits became conceptual margins that hindered exploration for nearly two thousand years. It looks at the reevaluation of authority which still saw the Greeks and Romans as the arbiters of knowledge, but enabled non-canonical classical authors to become important in revising attitudes to the climatic zones which had once been thought to define the edges of the oikoumene. In the new geography, the arctic and equatorial regions were believed to play key roles in allowing the different parts of the world to interact with one another. Without this shift in mind-set about the frigid and torrid zones, it would have been impossible for Europeans to begin to think of the world as a single, exploitable, global unit, created by God for human dominion.

The actual contours of global climate were gradually revealed during and after the sixteenth century, and the process was nearly completed with the pioneering research of geographer Alexander von Humboldt (1769-1869). Unfortunately, however, the original latitude-focused model never disappeared in pedagogical circles. In preparing this post, I was surprised to find many educational climate maps that are not merely outdated but are filled with misinformation. Several of these maps will be examined in the next post.

*  Ptolemy used additional lines of latitude to delineate seven climes, based on periods of daylight on the summer solstice. This scheme gained considerable importance in medieval Europe and in the Islamic realm.

People vary greatly in their ability to orient themselves by the cardinal direction. But even those with a good sense of direction tend to get confused in certain places. In my experience, Stanford University is a particularly bad place for direction perception. I never use cardinal directions when describing locations or giving directions on campus, as doing so tends to generate confusion.

Several factors contribute to these difficulties in direction perception on the Stanford campus. The problem begins with the general northwest/southeast orientation of local landforms, which is often perceived to run instead from north to south (see the previous post). As a result, most* people in Stanford are surprised to learn that the east bay city of Berkeley, home of the main rival school, is located to the west of Palo Alto, which is on the west side of the San Francisco Bay. The eastward swing in this topographic orientation as one moves to the south exacerbates the problem. Consider, for example, the two freeways, U.S. Route 101 and Interstate 280, that run between San Francisco and San Jose, providing access to Stanford and the nearby town of Palo Alto. The general southward directions of these roads veer well to the east in several locations. Between Cupertino and San Jose, Interstate 280 essentially runs east/west. But as is difficult to make mental adjustments as directions shifts, many people perceive these highways as retaining their north/south orientations.

This same issue characterizes the Santa Cruz Mountains, located between the southern Bay Area and the Pacific Ocean. Near Stanford, the range is oriented from northwest to southeast. If one could drive directly west from the campus, one would cross the mountains and soon reach the coast. But to the south, both the Santa Cruz Range and the coastline swing to the east. As this change tends to escape perception, the city of Santa Cruz is usually thought to be southwest of Stanford, although it is actually located to the southeast.

Local road angles contribute to the directional confusion. Most visitors to the campus take Palm Drive, which begins at the storied road called El Camino Real (“the King’s Highway”). Palm Drive is often perceived as perpendicular El Camino, but actually intersects it at a 57° degree angle. Because northwest/southeast oriented El Camino is commonly regarded as running north to south, Palm Drive, perceptually perpendicular to it, is often regarded as running from east to west. But as can be seen on the map posted below, when one enters the Stanford campus on this palm-lined street, one is actually heading slightly to the south of south-southwest. Stanford’s central campus maintains this same orientation. Jane Stanford Way, perpendicular to Palm Drive, thus runs in a mostly east/west directions. Builings on it, such as Encina West Hall, are named accordingly. In my experience, however, many people find this name confusing, as they tend to think of Encina West Hall as being located to the north of Encina Hall proper.

* These arguments are based  merely on anecdote and personal experience and would have to be supported by survey data to have solid grounding.