Trewartha climate classification
The Trewartha climate classification is a system for categorizing the world's climates using temperature and precipitation criteria, developed by American geographer Glenn T. Trewartha as a modification of the Köppen classification to better emphasize thermal regimes and their implications for human and ecological adaptations.[1] First introduced in the 1960s and refined in subsequent works, including a 1980 collaboration with Lyle H. Horn, the system addresses perceived shortcomings in the Köppen framework, such as its heavy reliance on vegetation boundaries rather than direct climatic controls on habitability and agriculture.[1] It organizes climates into six main thermal groups based on the number of months with mean temperatures exceeding 10°C (or specific thresholds for tropical and polar types), with an additional dry (B) group defined by precipitation deficits relative to potential evapotranspiration.[1] These groups are: A (tropical humid, all months ≥18°C); B (dry, annual precipitation (cm) < 2.3T – 0.64Pw + 41, where T is annual temperature in °C and Pw is the percentage of annual precipitation occurring in the six winter months); C (subtropical, 8–12 months ≥10°C and coldest month <18°C); D (temperate, 4–7 months ≥10°C); E (boreal, 1–3 months ≥10°C); F (polar, warmest month <10°C); and H (highland, for elevated areas above approximately 1,500 m where climate is influenced by altitudinal temperature lapse).[1][2][3] Subdivisions within each group use lowercase letters to denote seasonal precipitation patterns (e.g., 'w' for winter dry, 's' for summer dry) and temperature characteristics (e.g., 'h' for hot summers), providing a nuanced global mapping that has informed studies on climate change impacts and biome distributions.[1][2]Introduction
Definition and Purpose
The Trewartha climate classification is a modified version of the Köppen-Geiger system, first published in 1966, that refines the categorization of climates by emphasizing thermal and moisture regimes to more closely correspond with natural vegetation distributions and genetic climate patterns, especially in middle latitudes. This adaptation seeks to improve upon the original Köppen framework by adjusting boundaries in temperate and subtropical zones for greater alignment with ecological realities.[4] At its core, the system relies on monthly temperature thresholds of at least 10 °C to distinguish thermal regimes alongside precipitation patterns, assigning locations to one of seven primary groups: A (tropical), B (dry), C (subtropical), D (temperate), E (boreal), F (polar), and H (highland).[5] The classification's purpose is to counter the Köppen system's perceived overextension of polar and tropical categories at the expense of nuanced middle-latitude distinctions, offering enhanced applicability in geography and ecology for analyzing vegetation responses and climate-driven ecosystems.[4]Historical Development
The Trewartha climate classification was developed by American geographer Glenn Thomas Trewartha in 1966 as a refinement of Wladimir Köppen's original system, which was first introduced in 1884.[1][6] Trewartha's motivation stemmed from perceived shortcomings in Köppen's framework, particularly its inadequate zoning of middle latitudes, prompting him to adjust criteria such as requiring more months with mean temperatures of at least 10 °C to better correlate with vegetation patterns and natural landscapes.[1] The system built directly on Köppen's division into thermal (temperature-based) and humidity (precipitation-based) provinces but redefined the boundaries for groups C (subtropical), D (temperate), and E (boreal) to provide a more precise delineation of these zones.[1] In its original 1966 formulation, the classification emphasized temperature thresholds for thermal groups and precipitation effectiveness for moisture regimes, aiming for greater applicability to mid-latitude regions.[1] A key evolution occurred in the 1980 revision, a collaboration with Lyle H. Horn, which consolidated and refined the criteria for thermal groups and precipitation subtypes.[1] No major overhauls followed the 1980 update, though the system has seen continued application in modern contexts, such as analyzing 21st-century climate projections over regions like South America and Northern Asia.[7][8]Classification Criteria
Thermal Criteria
The thermal criteria in the Trewartha climate classification form the foundational basis for categorizing most climate types into thermal groups A, C, D, E, and F, relying on the number of months per year with a mean temperature of at least 10 °C (50 °F). This uniform threshold simplifies the delineation of climate zones by focusing on the duration of thermally effective periods, contrasting with the Köppen system's varied temperature limits that adjust by latitude or season. The 10 °C value is specifically chosen as the approximate minimum required for active plant growth and metabolic processes in vegetation, enabling a direct correlation between thermal regimes and ecological productivity.[9] These criteria apply to monthly mean air temperatures, computed as the average of daily means at a standard height of 2 meters above the ground surface, using long-term observational data to ensure reliability. Climates are assigned to thermal groups based on how many months meet or exceed this benchmark, with the following specific thresholds:| Group | Description | Months ≥ 10 °C |
|---|---|---|
| A | Tropical | ≥ 12 |
| C | Subtropical | 8–12 |
| D | Temperate | 4–7 |
| E | Boreal | 1–3 |
| F | Polar | 0 |
Precipitation Criteria
The precipitation criteria in the Trewartha climate classification primarily serve to identify dry climates and to subdivide other thermal groups based on moisture regimes, using thresholds related to monthly and annual precipitation patterns.[10] These rules build upon the initial thermal group assignments but focus exclusively on aridity and seasonal distribution to refine classifications. For identifying dry climates (Group B), an aridity index determines whether a location qualifies as arid enough to override thermal criteria. The effective precipitation threshold is calculated using the formula: R = 2.3T - 0.64P_w + 41 where T is the annual mean temperature in °C, and P_w is the percentage of annual precipitation occurring in the winter (low-sun) months (October–March in the Northern Hemisphere or April–September in the Southern Hemisphere), with R in cm.[1] If the annual precipitation (P) is less than R/2, the climate is classified as a desert (BW); if R/2 ≤ P < R, it is a steppe (BS). This formula adjusts for temperature and seasonal precipitation concentration to better reflect potential evapotranspiration and aridity impacts. Within non-dry thermal groups, precipitation subtypes are assigned based on the number and distribution of dry months, defined as those with less than 60 mm of average precipitation. For Group A (tropical climates), locations with no more than two dry months are classified as Ar (tropical wet); those with more extensive dry periods are subdivided into Aw or As (tropical wet and dry with winter or summer dry seasons, respectively), while Am (tropical monsoon) applies to regions with a short dry season of at least three months but high overall annual rainfall. In Group C (subtropical climates), Cs (dry summer or Mediterranean) designates areas with a dry summer where the driest summer month receives less than 30 mm precipitation and annual precipitation is less than 890 mm, whereas Cf (humid subtropical) indicates year-round humidity without such pronounced dry periods. Groups D (temperate), E (boreal), and F (polar) are generally considered humid by default, with no aridity formula or precipitation subtypes applied unless they meet Group B criteria.[1] Precipitation data for these criteria are typically derived from 30-year climatological normals at weather stations to ensure reliability and account for long-term variability. This approach emphasizes conceptual moisture availability over exhaustive metrics, allowing the classification to highlight ecological implications of seasonal and annual water balance.[10]Highland Adjustment
The Highland Adjustment in the Trewartha climate classification accounts for the cooling effect of elevation on temperature, enabling more accurate thermal categorization in mountainous regions where observed temperatures deviate substantially from sea-level conditions. Areas above 1,500–2,000 m elevation, where temperatures are significantly cooler than equivalent lowlands, are designated as Group H (highland climates) to reflect this vertical influence on climate zoning.[11] To implement this adjustment, 0.56 °C is added to observed temperatures for every 100 m of elevation (equivalent to 5.6 °C per 1,000 m) prior to applying the standard thermal criteria, simulating sea-level equivalents based on the atmospheric lapse rate. This method reclassifies high-elevation sites accordingly—for instance, elevating tropical lowlands into highland categories when the correction shifts their thermal profile. Some implementations introduce a Group G for intermediate elevations between 500 and 1,500 m to further refine the transition.[11] Unlike precipitation criteria, the highland adjustment focuses exclusively on thermal modifications from elevation and does not incorporate moisture factors. Its limitations include imprecise elevation boundaries, which contribute to ambiguity in application and result in frequent omission from contemporary climate maps lacking strict thresholds. This provision was incorporated after 1966, specifically in the 1968 edition, to better represent vertical climate variations in major mountain systems.[11]Climate Groups
Group A: Tropical Climates
Group A encompasses tropical climates characterized by persistent warmth throughout the year, defined by the thermal criterion that all 12 months have a mean temperature of at least 18 °C (64 °F), ensuring the absence of killing frosts.[1] This threshold aligns with the broader thermal framework in the Trewartha system, where tropical regions maintain at least 12 months above 10 °C, but Group A specifically emphasizes year-round high temperatures to distinguish it from cooler subtropical zones.[1] Within Group A, subtypes are delineated primarily by precipitation patterns, using a dry month threshold of less than 60 mm of precipitation. The Ar subtype, or tropical rainforest, features no more than two dry months, resulting in consistently high rainfall that supports lush, evergreen vegetation.[1] The Aw subtype, known as tropical savanna, experiences a dry season during the low-sun period with more than two dry months, leading to seasonal grasslands interspersed with trees.[1] The As subtype, tropical monsoon with a dry high-sun season, is rare and occurs where dryness aligns with the summer period.[1] The Am subtype, representing tropical monsoon climates with at least three dry months but compensated by intense wet seasons, was incorporated in later refinements to the classification.[12] These climates exhibit high humidity levels due to abundant moisture, coupled with minimal seasonal temperature variations owing to the equatorial position and consistent solar insolation, fostering environments with average annual temperatures often exceeding 25 °C.[1] This stability supports dense vegetation formations, such as rainforests in Ar regions or savanna grasslands in Aw areas, which are adapted to the reliable warmth and variable moisture regimes.[1] Geographically, Group A climates prevail in the equatorial belt between approximately 10° N and 10° S latitudes, encompassing vast lowland areas influenced by the intertropical convergence zone.[1] A prominent example is the Amazon Basin in South America, where the Ar subtype dominates, sustaining one of the world's largest tropical rainforests through year-round precipitation exceeding 2,000 mm annually.[1]Group B: Dry Climates
Group B climates in the Trewartha classification represent arid and semi-arid regions where low precipitation levels dominate, overriding the primary thermal categorization assigned to groups A, C, D, or E if aridity thresholds are exceeded. This moisture-based assignment prioritizes water deficit over temperature, ensuring that even thermally tropical or temperate zones are reclassified as dry when evaporation consistently surpasses available rainfall. The aridity index, referenced in the precipitation criteria, determines the boundary between dry and humid conditions across latitudes.[1] Subtypes within Group B distinguish between extreme aridity and marginal moisture availability. BW designates desert climates, where annual precipitation falls below half the aridity index threshold, resulting in severe water scarcity. BS denotes steppe climates, characterized by precipitation levels at least half but less than the aridity index threshold, allowing for somewhat greater but still limited moisture support. These subtypes are further qualified by thermal modifiers: "h" for hot variants with an annual mean temperature of 18°C or higher, and "k" for cold variants below 18°C, such as BWh for hot deserts or BSk for cold steppes. This structure accommodates diverse thermal regimes within dry environments, from subtropical hot spots to mid-latitude cold expanses.[1][13] Characteristics of Group B climates include persistent water deficits, with potential evapotranspiration far exceeding precipitation throughout the year, fostering environments of sparse vegetation. Deserts (BW) typically feature bare soil, scattered drought-resistant shrubs, or no plant cover, while steppes (BS) support short grasses and hardy perennials adapted to seasonal droughts. These conditions arise from atmospheric dynamics like descending air in subtropical highs or topographic rain shadows, limiting convective uplift and storm formation. Soil profiles often remain dry and nutrient-poor, constraining ecological productivity.[1][14] Group B climates prevail in about 30% of Earth's land surface, predominantly in subtropical and continental interiors influenced by persistent high-pressure systems and orographic barriers. This extensive coverage underscores their global significance in shaping arid biomes and influencing human adaptations to water scarcity.[14][15]Group C: Subtropical Climates
Group C climates are defined by thermal conditions that include at least eight months with mean monthly temperatures of 10 °C or higher, while the coolest month has a mean temperature below 18 °C.[1] This configuration reflects moderate seasonal temperature variation, with sufficient warmth to support extended growing seasons but cooler winters that introduce a degree of seasonality absent in tropical zones. Unlike more polar or boreal regions, these climates rarely experience prolonged cold, though occasional frosts may occur in continental settings.[16] Subtypes within Group C are primarily distinguished by precipitation patterns, as outlined in the overall classification criteria. The Cs subtype, known as the Mediterranean or dry-summer subtropical climate, features a marked summer drought where the driest summer month receives less than 30 mm of precipitation, winter rainfall is at least three times the summer total, and annual precipitation falls below 890 mm.[1] In contrast, the Cf subtype represents humid subtropical conditions with no distinct dry season, where the precipitation difference between the driest and wettest months does not meet the Cs threshold, and all months exceed 30 mm. A rare Cw subtype with dry winters exists but is not emphasized in most applications. These precipitation distinctions align with the dry summer thresholds referenced in the classification's precipitation criteria, emphasizing seasonal aridity in Cs regions. These subtropical climates generally exhibit mild winters with average temperatures rarely dropping below freezing for extended periods and hot summers that can exceed 25 °C in many areas.[16] Vegetation adapted to Group C includes sclerophyllous evergreen woodlands, shrublands, and chaparral in Cs areas, which tolerate summer drought through adaptations like thick leaves and deep roots, while Cf regions support broadleaf deciduous forests and mixed evergreen broadleaved-coniferous formations that thrive on year-round moisture.[17] Compared to the Köppen system, Trewartha's Group C is broader, encompassing some areas formerly classified as continental (D) climates due to the focus on the count of warm months (eight or more above 10 °C) rather than a strict freezing threshold for the coolest month.[1] This adjustment better captures transitional zones with subtropical characteristics in terms of effective growing season length, though it simplifies precipitation subtypes by prioritizing thermal regimes over finer monthly distributions.[16]Group D: Temperate Climates
Group D encompasses mid-latitude temperate climates characterized by 4 to 7 months with mean temperatures of at least 10 °C, distinguishing them from subtropical zones with more extended warm periods. These climates exhibit distinct seasonal variations, including cooler winters compared to Group C subtropical climates, which have at least 8 warm months. The subtypes within Group D are divided based on winter severity and summer warmth. The oceanic subtype (Do) features mild winters where the coldest month has a mean temperature of 0 °C or higher, resulting in relatively even seasonal temperatures influenced by maritime effects. In contrast, the continental subtype (Dc) includes harsher winters with the coldest month below 0 °C; this is further subdivided into Dca, where the warmest month reaches at least 22 °C, supporting hot summers, and Dcb, where the warmest month remains below 22 °C, indicating cooler summers.[2] These climates typically display four pronounced seasons, with greater annual temperature ranges than those in Group C, fostering diverse vegetation such as deciduous forests in humid areas or prairies in regions with transitional moisture levels. Unlike the boreal Group E, which limits growing seasons to 1–3 months and supports sparser taiga, Group D's extended warmth enables denser vegetation cover. Precipitation is generally assumed to be humid unless overridden by Group B dry criteria. Compared to the Köppen system, Trewartha's Group D expands to incorporate certain former subarctic subtypes like Dfc and Dfb into Dc, better aligning climatic boundaries with vegetation patterns such as forest distributions.Group E: Boreal Climates
Group E climates in the Trewartha classification represent boreal or subarctic regions distinguished by a limited thermal regime, where only 1 to 3 months exhibit mean temperatures of 10 °C or higher, defining a marginal growing season that sharply contrasts with the extended cold of the remaining months. This criterion, rooted in the universal thermal index, underscores the transition from temperate conditions to more extreme cold, supporting ecosystems adapted to brief productivity periods. Winters are prolonged and severe, often lasting 6 to 9 months with subfreezing temperatures, while summers remain cool and constrain biological activity.[18] Subtypes within Group E differentiate based on winter severity. The Eo subtype, or maritime subarctic, features milder conditions with the coldest month averaging above -10 °C, influenced by oceanic moderation that reduces temperature extremes. In contrast, the Ec subtype, or continental subarctic, experiences harsher winters where at least one month averages -10 °C or below, reflecting greater distance from moderating marine influences and resulting in amplified continental effects. These climates are typified by coniferous taiga forests of needleleaf evergreens, such as spruce and fir, which dominate the landscape and form dense boreal woodlands in Eo areas, transitioning to more open tayga in Ec regions.[18] In continental Ec zones, permafrost prevails across extensive areas, seasonally thawing to depths of 0.6 to 4 meters and shaping acidic, poorly drained soils that limit root penetration and nutrient cycling.[18] Precipitation remains low, typically under 500 mm annually and mostly falling as snow or rain during the short warm season, yet the overall humidity supports forest persistence due to minimal evaporation amid low temperatures.[18] Unlike the Köppen system, which integrates boreal climates into its Group D (continental) based on similar temperature thresholds but without a dedicated category, Trewartha's Group E explicitly separates them to highlight the critically short growing season—limited to 1–3 thermal months—that fosters distinct subarctic ecosystems rather than temperate ones. This delineation better captures the ecological boundary with polar regions, emphasizing constraints on agriculture and biodiversity unique to boreal environments.Group F: Polar Climates
Group F encompasses the polar climates, the coldest category in the Trewartha classification, where no month has a mean temperature of 10 °C or higher, resulting in the absence of a significant growing season.[19] This thermal criterion directly applies the core temperature threshold from the classification's thermal rules, emphasizing extreme cold without modifications for middle latitudes. These climates are primarily found in high-latitude regions near the poles, covering approximately 7% of Earth's land surface excluding Antarctica, with a noted decline in extent over the 20th century due to warming trends. The group is subdivided into two subtypes based on summer temperatures. The tundra subtype (Ft) features at least one month with a mean temperature above 0 °C (but below 10 °C), allowing limited vegetation growth during brief, cool summers, while winters remain severely cold.[19] In contrast, the ice cap subtype (Fi) has all months below 0 °C, supporting no vegetation and featuring perpetual ice cover.[19] These distinctions mirror the polar categories in the Köppen system, from which Trewartha's framework derives without alteration for this group, focusing solely on thermal extremes. Polar climates exhibit stark environmental characteristics shaped by persistent cold. Biodiversity is notably low, with tundra regions supporting only about 1,700 plant species such as low shrubs, mosses, and lichens, alongside adapted fauna like caribou and arctic foxes, while ice cap areas sustain virtually no life.[20] Permafrost is widespread, forming a permanent frozen subsoil layer that restricts drainage and root growth in tundra zones, and is continuous under ice caps.[19] Precipitation is minimal, typically 150–250 mm annually, often falling as snow and contributing to a dry, desert-like regime despite the cold.[20]Group H: Highland Climates
Group H encompasses highland climates in the Trewartha classification, applied to regions where elevation significantly modifies the thermal regime, typically at altitudes exceeding 1,500 meters.[21] These areas are assigned to Group H when altitude-induced cooling shifts the climate away from its lowland equivalent, using temperature adjustments based on the environmental lapse rate to reclassify the site into an appropriate thermal category.[21] For instance, a tropical lowland climate (Group A) at sufficient height may be reclassified as a highland variant of a subtropical or temperate group after correction.[1] Key characteristics of highland climates include pronounced vertical zonation, where temperatures decrease with elevation, resulting in cooler summers and heightened frost risk even within tropical latitudes.[21] This leads to diverse microclimates shaped by topography, with large diurnal temperature swings due to thinner, drier air at higher altitudes; precipitation is often abundant and humid overall but highly variable, influenced by orographic lift on windward slopes and rain shadows on leeward sides.[21] As noted in the highland adjustment process, these effects stem from the standard lapse rate of approximately 5.6°C per 1,000 meters.[21] No formal subtypes exist within Group H, though informal notations such as Hh for high tropical highlands have been used in some applications to denote elevation-modified tropical zones.[1] Alternatively, the classification may integrate the highland prefix with the base thermal group, for example, CH to indicate subtropical highland climates where adjusted temperatures align with Group C criteria.[21] Criticisms of Group H center on its vague elevation thresholds and the practical challenges of applying lapse rate corrections, which require detailed topographic data often unavailable for large-scale mapping.[1] Consequently, highland climates are frequently omitted or simplified in global Trewartha maps, as the definition lacks sufficient precision for consistent implementation across varied terrains.[1]Universal Thermal Index
Warmth Categories
The warmth categories form an optional component of the Universal Thermal Index (UTI) in the Trewartha climate classification system, represented by the third letter in the climate code, which specifies the mean temperature of the warmest month. This index provides additional detail on peak summer thermal conditions, refining the broader thermal criteria used to define the primary climate groups such as subtropical (C) or temperate (D). By focusing on the hottest month's average temperature, it helps distinguish variations in heat intensity that influence local ecosystems and human activities, beyond the count of months exceeding 10 °C. The 'l' category is particularly relevant for boreal and polar climates with cooler summers.[22] The categories are delineated based on specific temperature thresholds for the warmest month, as follows:| Category | Description | Temperature Range (°C) |
|---|---|---|
| i | Very hot | ≥ 35 |
| h | Hot | 28–34.9 |
| a | Warm | 22.2–27.9 |
| b | Mild | 18–22.1 |
| l | Cool | 10–17.9 |
Cold Categories
The cold categories form part of the Universal Thermal Index in the Trewartha climate classification, where an optional fourth letter denotes the mean temperature of the coldest month to quantify winter severity.[21] This index complements the primary climate groups by providing additional thermal detail, particularly useful for analyzing frost risk and heating requirements in colder regions. These categories, including 'k' for milder winters, are relevant across temperate, boreal, and polar zones.[21] The categories are defined based on the coldest month's average temperature as follows:| Category | Letter | Temperature Range (°C) | Description |
|---|---|---|---|
| Cool winter | k | 0.1 to 9.9 | Mild conditions with rare or no frost. |
| Cold winter | o | -9.9 to 0 | Cold conditions with frequent frost but limited duration. |
| Very cold winter | c | -24.9 to -10 | Pronounced winter cold, typical of continental interiors with significant snowfall potential. |
| Severe cold winter | d | -39.9 to -25 | Harsh winters with prolonged freezing, restricting vegetation and human activity. |
| Extreme winter | e | ≤ -40 | Severely cold, ice-cap-like conditions with minimal biological activity. |