Visualizing Climate

Objectives:

Climate is the synthesis of weather conditions, both the average of parameters, generally temperature and precipitation, over a period of time and the extremes in weather. For this reason, much of the information on climate is given in statistical terms. For greater ease of interpretation, these statistical values are often shown in graphs, typically as the magnitude of the average value (or extremes) versus the months of the year. One form of display that shows the relationships between temperature and precipitation during the yearly cycle is the climograph.

After completing this investigation, you should be able to:

Portray the statistical climate values of mean monthly temperature and average monthly precipitation in a graphical form called the climograph.

• Compare temperature and precipitation distributions on climographs from different locations noting similarities and differences.
• Explain how certain climograph patterns can be explained by various climate controls.
• Relate certain patterns of temperature and precipitation to particular climate classification types.

Introduction:

Every place on Earth has climate characteristics that distinguish it from other places. It is desirable to systematically describe these characteristics so that the climates of various locations can be compared. This investigation focuses on climate as described by averages. It is important to remember that by using averages, only a generalized picture of the climate is created. A climograph is a commonly used tool to describe the climate of a given place and compare climates in various places. A climograph can be drawn to show monthly mean temperatures and average precipitation totals for a single station through the year on the same graph. Figures 2 - 7 are climographs for six locations in the United States which give examples of major climate types discussed in the Climate Classification, provided at the end of this Investigation 15A.

A climograph can provide at a glance the magnitudes and ranges of monthly mean temperatures and average monthly precipitation throughout the year. These statistics are genetically tied to various climate-controlling factors which vary systematically from place to place. By relating distributions of temperature and precipitation to specific controls it is possible to gain a more comprehensive understanding of the causes of the climate in a specific area. Because these controls and the climates which result have significant impact on other elements of the Earth system (vegetation, soils, weathering rates of rocks, etc.), such an understanding has widespread applications. It is also desirable to have a shorthand classification for the major types of recurrent temperature and precipitation patterns so one may be able to generalize about climates up to the global scale.

By convention, climographs are usually constructed with time of year displayed horizontally across the base of the graph. The mid-letter of the month abbreviation is listed at mid-month along the bottom, with the precipitation scale along the left side and mean temperature scale on the right side. Mean monthly precipitation totals (rain plus melted snow) are presented as a bar graph. The mean temperature values are plotted as points connected by a curve. In the U.S., climate data are prepared with precipitation in inches and temperatures in degrees Fahrenheit. Use the data and grid in Figure 1 below to make a climograph for Boston, MA. Mark a short horizontal line at mid-month to note the position of the mean total precipitation value of that month and fill in the space below to create a bar. (Use Figures 2 - 7 as guides.) Place a dot at mid-month at the level denoting the mean monthly temperature value. When all the months are plotted, connect the dots with line segments to represent the march of average monthly temperature.

Figure 1. Humid Continental Climate (Dfa) – Boston

Month	Jan	Feb	Mar	Apr	May	Jun	Jul	Aug	Sep	Oct	Nov	Dec
Pcp (in.)	3.36	3.25	4.32	3.74	3.48	3.68	3.43	3.29	3.44	3.94	3.99	3.78
Av T (F)	29.3	32.0	38.6	48.4	58.2	68.0	73.7	72.4	65.2	54.3	45.0	35.0

1. Your completed climograph for Boston, MA (Figure 1) shows that the mean monthly temperature rises from below freezing during mid-winter (Dec., Jan., Feb.) to means over 70 °F during July and August and then falls as winter approaches. The observed temperatures from which the means are computed result mainly from the seasonal swing of solar heating, which in turn is largely determined by latitude. As a general rule, the higher the latitude the lower the winter season temperatures. The lowest mean monthly temperature in Boston occurs in [(January)(December)].

2. This minimum monthly temperature [(is)(is not)] within a month or so of the time of minimum solar heating in a mid-latitude, Northern Hemisphere location.

3. Where solar heating varies significantly from the winter to summer solstices, the range of temperatures, indicated by the amplitude of the temperature curve on a climograph, is relatively great. Where the amplitude is relatively small, the seasonal temperature contrast is also small. Examine the temperature curve on the climograph for Kahului, HI (Figure 2). The range of mean monthly temperatures for Kahului is about [(20)(10)(30)] Fahrenheit degrees.

4. From the shape of the curve and range of temperature, it is evident that Kahului experiences relatively [(little)(significant)] variation in solar heating through the course of a year.

5. The highest mean monthly temperature in Kahului occurs in August. This temperature is about [(72)(80)] °F.

6. The lowest mean monthly temperature is about [(72)(80)] °F. in both January and February.

7. These temperatures suggest that Kahului is a [(high)(low)] latitude location.

8. The month of occurrence of the highest mean temperature suggests that Kahului is located in the [(Northern)(Southern)] Hemisphere.

Temperature and temperature range can also be influenced by large bodies of water (ocean or large lake). Generally speaking, a maritime influence will moderate temperatures in places that would normally be colder in winter and warmer in summer based on latitude alone. The seasonal range in temperatures is likely to be less due to a maritime influence; that is, the temperature curve on the climograph will exhibit less amplitude. By comparison, temperatures at continental locations or locations downwind of large land masses tend to be higher in summer and lower in winter. As a result, the seasonal temperature ranges at continental locations will be far greater than for places surrounded by or downwind of a large water body.

9. It is likely that Kahului’s relatively low annual temperature range [(is)(is not)] also moderated by the surrounding Pacific Ocean.

10. Examine the climograph for Fairbanks, AK (Figure 6). The highest mean monthly temperature is about [(62)(82)] °F.

Figure 2. Tropical Wet-Dry Climate (Aw) - Kahului.

Figure 3. Subtropical Desert Climate (BWh) - Yuma.

Figure 4. Subtropical Humid Climate (Cfa) - Columbia.

Figure 5. Marine West Coast Climate (Csb) - Eureka.

Figure 6. Subarctic Climate (Dfc) - Fairbanks.

Figure 7. Polar Tundra Climate (Et) - Barrow.

11. The lowest monthly mean temperature for Fairbanks is about [(-10)(10)] °F.

12. These temperatures suggest that Fairbanks is a [(high)(low)] latitude location.

13. The average monthly temperatures in Fairbanks cover a range of about [(70)(50)(30)] Fahrenheit degrees.

14. This range of temperatures suggests that Fairbanks has a [(continental)(maritime)] climate.

15. Compare the annual temperature range for Boston, on the Atlantic coast, (Figure 1) and Eureka, CA, on the Pacific coast, (Figure 5). Eureka’s annual temperature range is [(greater than)(less than)] that of Boston.

16. These cities are at essentially the same latitude and both are located near the coast. The climate control causing the difference in annual temperature range is the influence of the prevailing westerly wind at both locations. For Eureka, the temperature range is influenced mainly by the [(ocean)(continent)] which is upwind and in Boston by winds blowing from the continent.

17. Climate classification systems allow climate differences and similarities to be expressed in a “shorthand” form. The broad-scale climate boundaries in the Köppen climate classification system (see Climate Classification at the end of this investigation) are based on patterns in annual and monthly mean temperature and precipitation, which closely correspond to the limits of vegetative communities. The major classifications of Tropical Humid (A), Subtropical (C), Snow Forest (D), and Polar (E) are based on temperature; the group Dry (B) is based on precipitation; and the group Highland (H) applies to mountainous regions. Temperatures for both Fairbanks and Boston place them in the [(Tropical Humid (A))(Subtropical (C))(Snow Forest (D))(Polar (E))] classification.

18. The second letter of the Boston (Figure 1) and Fairbanks (Figure 6) classifications corresponds to seasonal precipitation regimes with an “f” signifying year-round precipitation. According to their climographs and climate classifications, Boston and Fairbanks have [(similar)(very different)] month-to-month uniformity in their precipitation regimes.

19. Arid and Semiarid climates can be caused by several climate controls. Locations on the eastern side of planetary-scale, persistent high pressure systems, such as those occurring around 30 degrees N in the Atlantic and Pacific, experience subsiding air, which inhibits cloud formation and precipitation. The west side of such systems, by contrast, tends to be humid. The cause of dryness in Yuma AZ, for example, is due mainly to its position [(east)(west)] of a subtropical high pressure system which persists off the southwest U.S. coast in the Pacific.

20. Columbia, SC at about the same latitude as Yuma, but in the southeastern United States, is humid because it is located [(east)(west)] of such a high pressure system in the Atlantic.

21. Dry or wet conditions can also be caused by a location being upwind or downwind of a mountain range. Areas to the lee of high mountains tend to be dry because of the “wringing out” of moisture on the wet, windward slopes (due to orographic lifting, cooling and condensation) and the compressional warming of air which occurs as the air descends on the leeward slopes. The atmospheric stability caused by cold ocean currents offshore can also prevent precipitation by stabilizing the air and inhibiting convection. Instability, however, can occur if ocean currents are warm. The dryness of Yuma, which is downwind of the Coastal Ranges and the cold California Current, is [(probably)(not likely)] drier because of the influence of mountains and ocean currents.

As directed by your course instructor, complete this investigation by either:

1. Going to the Current Weather Studies link on the course website, or
2. Continuing the Applications section for this investigation that immediately follows.

Investigation 15A: Applications

Figure 8. Köppen Climate classification. (Adapted from UN Food and Agriculture Organization, Sustainable Development website.)

22. The Figure 8 world map showing the Köppen (or Koeppen) climate classification demonstrates the actual application of climatic controls. For example, on this Mercator projection map, horizontal lines (if they were drawn) would represent constant latitudes. Therefore, at similar latitudes across the Eurasian land mass, Europe is shown in purple indicating a temperate climate, while eastern Asia is yellow indicating a cold type climate. The climate control primarily at work in these local climate types is the prevailing wind circulation in relation to [(elevation)(proximity to large bodies of water)(Earth’s surface characteristics)].

23. The region of Tibet in south central Asia is shown in the greenish-brown of a polar type climate although it is surrounded by dry or temperate climates. This classification is most likely the result of Tibet’s [(elevation)(proximity to large bodies of water)(Earth’s surface characteristics)]

Suggestions for further activities: You can make your own climographs with monthly average temperatures and precipitation totals for selected U.S. cities from http://drought.unl.edu/DroughtBasics/WhatisClimatology/ClimographsforSelectedUSCities.aspx () and international cities from http://drought.unl.edu/DroughtBasics/WhatisClimatology/ClimographsforSelectedInternationalCities.aspx (). Monthly and annual values are provided in both English and metric units. By inputting data to spreadsheet software, graphing can be easily accomplished allowing comparisons among stations.

CLIMATE CLASSIFICATION

• Tropical Humid Climates
• Dry Climates
• Subtropical Climates
• Snow Forest Climates
• Polar Climates
• Highland Climates

One of the most widely used climate classification systems was designed by German climatologist and plant geographer Wladimir Köppen (1846-1940) and subsequently modified by his students R. Geiger and W. Pohl. The Köppen system is an empirical approach to organizing Earth’s myriad of climate types. Recognizing that indigenous vegetation is a natural indicator of regional climate, Köppen and his students looked for patterns in annual and monthly mean temperature and precipitation, which closely correspond to the limits of vegetative communities thereby revealing broad-scale climatic boundaries throughout the world. Records of annual and monthly mean temperature and precipitation are sufficiently long and reliable in many parts of the world that they serve as a good first approximation of climate. Since its introduction in the early 1900s, Köppen’s climate classification has undergone numerous and substantial revisions by Köppen himself and by other climatologists and has had a variety of applications.

As shown in the Table below, the Köppen climate classification system identifies six main climate groups; four are based on temperature, one is based on precipitation, and one applies to mountainous regions. Köppen’s scheme uses letters to symbolize major climatic groups: (A) Tropical Humid, (B) Dry, (C) Subtropical (Mesothermal), (D) Snow Forest (Microthermal), (E) Polar, and (H) Highland. Additional letters further differentiate climate types.

Tropical Humid Climates

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Tropical humid climates (A) constitute a discontinuous belt straddling the equator and extending poleward to near the Tropic of Cancer in the Northern Hemisphere and the Tropic of Capricorn in the Southern Hemisphere. Mean monthly temperatures are high and exhibit little variability throughout the year. The mean temperature of the coolest month is no lower than 18 °C (64 °F), and there is no frost. The temperature contrast between the warmest and coolest month is typically less than 10 Celsius degrees (18 Fahrenheit degrees). In fact, the diurnal (day-to-night) temperature range generally exceeds the annual temperature range. This monotonous air temperature regime is the consequence of consistently intense incoming solar radiation associated with a high maximum solar altitude and little variation in the period of daylight throughout the year.

Although tropical humid climate types are not readily distinguishable on the basis of temperature, important differences occur in precipitation regime. Tropical humid climates are subdivided into tropical wet (Af), tropical monsoon (Am), and tropical wet-and-dry (Aw). Although these climate types generally feature abundant annual rainfall, more than 100 cm (40 in.) on average, their rainy seasons differ in length and, in the case of Am and Aw, there is a pronounced dry season and wet season. In tropical wet climates, the yearly average rainfall of 175 to 250 cm (70 to 100 in.) supports the world’s most luxurious vegetation. Tropical rainforests occupy the Amazon Basin of Brazil, the Congo Basin of Africa, and the islands of Micronesia. For the most part, rainfall is distributed uniformly throughout the year, although some areas experience a brief (one or two month) dry season. Rainfall occurs as heavy downpours in frequent thunderstorms triggered by local convection and the intertropical convergence zone (ITCZ). Convection is largely controlled by solar radiation and rainfall typically peaks in midafternoon, the warmest time of day. Because the water vapor concentration is very high, even the slightest cooling at night leads to saturated air and the formation of dew or radiation fog, giving these regions a sultry, steamy appearance.
Tropical monsoon (Am) climates feature a seasonal rainfall regime with extremely heavy rainfall during several months and a lengthy dry season. The principal control for these climates involves seasonal shifts in wind from land to sea, typified by the Asian monsoon. During the low-sun season, high air pressure over the Asian continent causes dry air to flow southward into parts of Southeast Asia and India. During the high-sun season, low air pressure covers the Tibetan Plateau and the winds reverse direction, advecting moisture inland from over the Indian Ocean. Local convection, orographic lifting, and shifts of the ITCZ combine to deluge the land with torrential rains. Am climates also occur in western Africa and northeastern Brazil.

For the most part, tropical wet-and-dry climates (Aw) border tropical wet climates (Af) and are transitional to subtropical dry climates in a poleward direction. Aw climates support the savanna, tropical grasslands with scattered deciduous trees. Summers are wet and winters are dry, with the dry season lengthening poleward. This marked seasonality of rainfall is linked to shifts of the intertropical convergence zone (ITCZ) and semipermanent subtropical anticyclones, which follow the seasonal excursions of the sun. In summer (high-sun season), surges of the ITCZ trigger convective rainfall; in winter (low-sun season), the dry eastern flank of the subtropical anticyclones dominates the weather.

The annual mean temperature in Aw climates is only slightly lower, and the seasonal temperature range is only slightly greater, than in the tropical wet climates (Af). The diurnal temperature range varies seasonally, however. In summer, frequent cloudy skies and high humidity suppress the diurnal temperature range by reducing both solar heating during the day and radiational cooling at night. In winter, on the other hand, persistent fair skies have the opposite effect on radiational heating and cooling and increase the diurnal temperature range. Cloudy, rainy summers plus dry winters also mean that the year’s highest temperatures typically occur toward the close of the dry season in late spring.

Dry Climates

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Dry climates (B) characterize those regions where average annual potential evaporation exceeds average annual precipitation. Potential evaporation is the quantity of water that would vaporize into the atmosphere from a surface of fresh water during long-term average weather conditions. Air temperature largely governs the rate of evaporation so it is not possible to specify some maximum rainfall amount as the criterion for dry climates. Rainfall is not only limited in B climates but also highly variable and unreliable. As a general rule, the lower the mean annual rainfall, the greater is its variability from one year to the next.

Earth’s dry climates encompass a larger land area than any other single climate grouping. Perhaps 30% of the planet’s land surface, stretching from the tropics into midlatitudes, experiences a moisture deficit of varying degree. These are the climates of the world’s deserts and steppes, where vegetation is sparse and equipped with special adaptations that permit survival under conditions of severe moisture stress. Based on the degree of dryness, we distinguish between two dry climate types: steppe or semiarid (BS) and arid or desert (BW). Steppe or semiarid climates are transitional between more humid climates and arid or desert climates. Mean annual temperature is latitude dependent, as is the range in variation of mean monthly temperatures through the year. Hence, a distinction is made between warm dry climates of tropical latitudes (BSh and BWh) and cold, dry climates of higher latitudes (BSk and BWk).

Dryness is the consequence of subtropical anticyclones, cold surface ocean currents, or the rain shadow effect of high mountain ranges. Subsiding stable air on the eastern flanks of subtropical anticyclones gives rise to tropical dry climates (BSh and BWh). These huge semipermanent pressure systems, centered over the ocean basins, dominate the weather year-round near the Tropics of Cancer and Capricorn. Consequently, dry climates characterize North Africa eastward to northwest India, the southwestern United States and northern Mexico, coastal Chile and Peru, southwest Africa, and much of the interior of Australia.

Although persistent and abundant sunshine is generally the rule in dry tropical climates, there are some important exceptions. Where cold ocean waters border a coastal desert, a shallow layer of stable marine air drifts inland. The desert air thus features high relative humidity, persistent low stratus clouds and fog, and considerable dew formation. Examples are the Atacama Desert of Peru and Chile, the Namib Desert of southwest Africa, and portions of the coastal Sonoran Desert of Baja California and stretches of the coastal Sahara Desert of northwest Africa. These anomalous foggy desert climates are designated BWn.

Cold, dry climates of higher latitudes (BWk and BSk) are situated in the rain shadows of great mountain ranges. They occur primarily in the Northern Hemisphere, to the lee of the Sierra Nevada and Cascade ranges in North America and the Himalayan chain in Asia. Because these dry climates are at higher latitudes than their tropical counterparts, mean annual temperatures are lower and the seasonal temperature contrast is greater. Anticyclones dominate winter, bringing cold and dry conditions, whereas summers are hot and generally dry. Scattered convective showers, mostly in summer, produce relatively meager precipitation.

Subtropical Climates

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Subtropical climates are located just poleward of the Tropics of Cancer and Capricorn and are dominated by seasonal shifts of subtropical anticyclones. There are three basic climate types: subtropical dry summer (or Mediterranean) (Cs), subtropical dry winter (Cw), and subtropical humid (Cf), which receive precipitation throughout the year.

Mediterranean climates occur on the western side of continents between about 30 and 45 degrees latitude. In North America, mountain ranges confine this climate to a narrow coastal strip of California. Elsewhere, Cs climates rim the Mediterranean Sea and occur in portions of extreme southern Australia. Summers are dry because at that time of year Cs regions are under the influence of stable subsiding air on the eastern flanks of the semi-permanent subtropical highs. Equatorward shift of subtropical highs in autumn allows extra-tropical cyclones to migrate inland, bringing moderate winter rainfall. Mean annual precipitation varies greatly-ranging from 30 to 300 cm (12 to 80 in.) with the wettest winter month typically receiving at least three times the precipitation of the driest summer month.

Although Mediterranean climates exhibit a pronounced seasonality in precipitation (dry summers and wet winters), the temperature regime is quite variable. In coastal areas, cool onshore breezes prevail, lowering the mean annual temperature and reducing seasonal temperature contrasts. Well inland, however, away from the ocean’s moderating influence, summers are considerably warmer; hence, inland mean annual temperatures are higher and seasonal temperature contrasts are greater than in coastal Cs localities. Climatic records of coastal San Francisco and inland Sacramento, CA illustrate the contrast in temperature regime within Cs regions. Although the two cities are separated by only about 145 km (90 mi.), the climate of Sacramento is much more continental (much warmer summers and somewhat cooler winters) than that of San Francisco. The warm climate subtype is designated Csa and the cooler subtype is Csb.

Subtropical dry winter climates (Cw) are transitional between Aw and BS climates and located in South America and Africa between 20 and 30 degrees S. Cw climates also occur between the Aw and H climates of the Himalayas and Tibetan plateau and between the BS and Cfa climates of Southeast and East Asia. Northward shift of the subtropical high pressure systems is responsible for the dry winter in South America and Africa. The narrowness of the two continents between 20 and 30 degrees S means a relatively strong maritime influence and dictates against extreme dryness. In spring, subtropical highs shift southward and rains return. In Asia, winter dryness is caused by winds radiating outward from the massive cold Siberian high. As the continent warms in spring, the Siberian high weakens and eventually is replaced by low pressure. Moist winds then flow inland bringing summer rains. Mean annual precipitation in Cw climates is in the range of 75 to 150 cm (30 to 60 in.).

Subtropical humid climates (Cf) occur on the eastern side of continents between about 25 and 40 degrees latitude (and even more poleward where the maritime influence is strong). Cfa climates are the most important of the Cf climate subtypes in terms of land area and number of people impacted. Cfa climates are situated primarily in the southeastern United States, a portion of southeastern South America, eastern China, southern Japan, on the extreme southeastern coast of South Africa, and along much of the east coast of Australia. These climates feature abundant precipitation (75 to 200 cm, or 30 to 80 in., on average annually), which is distributed throughout the year. In summer, Cfa regions are dominated by a flow of sultry maritime tropical air on the western flanks of the subtropical anticyclones. Consequently, summers are hot and humid with frequent thunderstorms, which can produce brief periods of substantial rainfall. Hurricanes and tropical storms contribute significant rainfall (up to 15% to 20% of the annual total) to some North American and Asian Cfa regions, especially from summer through autumn. In winter, after the subtropical highs shift toward the equator, Cfa regions come under the influence of migrating extratropical cyclones and anticyclones.

In Cfa localities, summers are hot and winters are mild. Mean temperatures of the warmest month are typically in the range of 24 to 27 ºC (75 to 81 ºF). Average temperatures for the coolest months typically range from 4 to 13 ºC (39 to 55 ºF). Subfreezing temperatures and snowfalls are infrequent.

A strong maritime influence is responsible for the cool summers and mild winters of Cfb climates. These climates occur over much of Northwest Europe, New Zealand, and portions of southeastern South America, southern Africa, and Australia. The coldest subtype, the Cfc, is relegated to coastal areas of southern Alaska, Norway, and the southern half of Iceland. Cfb and Cfc climates are relatively humid with mean annual precipitation ranging between 100 and 200 cm (40 and 80 in.).

Snow Forest Climates

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Snow forest climates (D) occur in the interior and to the leeward sides of large continents. The name emphasizes the link between biogeography and the Köppen climate classification system. These climates feature cold snowy winters (except for the Dw subtype in which the winter is dry) and occur only in the Northern Hemisphere. Snow forest climates are subdivided according to seasonal precipitation regimes with Df climates experiencing year-round precipitation whereas Dw climates have a dry winter. D climates with dry summers (Ds) are rare and small in extent. Additional distinction is made between warmer subtypes (Dwa, Dfa, Dwb, and Dfb) and colder subtypes (Dwc, Dfc, Dwd, Dfd).

The warmer subtypes, sometimes termed temperate continental, have warm summers (mean temperature of the warmest month greater than 22 °C or 71 °F) and cold winters. They are located in Eurasia, the northeastern third of the United States, southern Canada, and extreme eastern Asia. Continentality increases inland with maximum temperature contrasts between the coldest and warmest months as great as 25 to 35 Celsius degrees (45 to 63 Fahrenheit degrees). The southerly Dfa climates have cool winters and warm summers and the more northerly Dfb climates have cold winters and mild summers. The freeze-free period varies in length from 7 months in the south to only 3 months in the north. The weather in these regions is very changeable and dynamic because these areas are swept by extra-tropical cyclones and anticyclones and by surges of contrasting air masses. Polar front cyclones dominate winter, bringing episodes of light to moderate frontal precipitation. These storms are followed by incursions of dry polar and arctic air masses. In summer, cyclones are weak and infrequent as the principal storm track shifts poleward. Summer rainfall is mostly convective, and locally amounts can be very heavy in severe thunderstorms and mesoscale convective complexes (MCCs). Although precipitation is distributed rather uniformly throughout the year, most places experience a summer maximum.

In northern portions, winter snowfall becomes an important factor in the climate. Mean annual snowfall and the persistence of a snow cover increase northward. Because of its high albedo for solar radiation and its efficient emission of infrared, a snow cover chills and stabilizes the overlying air. For these reasons, a snow cover tends to be self-sustaining; once established in early winter, an extensive snow cover tends to persist.

Moving poleward, summers get colder and winters are bitterly cold. These so-called boreal climates (Dfc, Dfd, Dwc, Dwd) occur only in the Northern Hemisphere as an east-west band between 50 to 55 degrees N and 65 degrees N. It is a region of extreme continentality and very low mean annual temperature. Summers are short and cool, and winters are long and bitterly cold. Because midsummer freezes are possible, the growing season is precariously short. Both continental polar (cP) and arctic (A) air masses originate here, and this area is the site of an extensive coniferous (boreal) forest. In summer, the mean position of the leading edge of arctic air (the arctic front) is located along the northern border of the boreal forest. In winter, the mean position of the arctic front is situated along the southern border of the boreal forest.

Weak cyclonic activity occurs throughout the year and yields meager annual precipitation (typically less than 50 cm, or 20 in.). Convective activity is rare. A summer precipitation maximum is due to the winter dominance of cold, dry air masses. Snow cover persists throughout the winter and the range in mean temperature between winter and summer is among the greatest in the world.

Polar Climates

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Polar climates (E) occur poleward of the Arctic and Antarctic circles. These boundaries correspond roughly to localities where the mean temperature for the warmest month is 10 ºC (50 ºF). These limits also approximate the tree line, the poleward limit of tree growth. Poleward are tundra and the Greenland and Antarctic ice sheets. A distinction is made between tundra (Et) and ice cap (Ef) climates, with the dividing criterion being 0 ºC (32 ºF) for the mean temperature of the warmest month. Vegetation is sparse in Et regions and almost nonexistent in Ef areas.

Polar climates are characterized by extreme cold and slight precipitation, which falls mostly in the form of snow (less than 25 cm, or 10 in., melted, per year). Greenland and Antarctica could be considered deserts for lack of significant precipitation, despite the presence of large ice sheets. Although summers are cold, the winters are so extremely cold that polar climates feature a marked seasonal temperature contrast. Mean annual temperatures are the lowest of any place in the world.

Highland Climates

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Highland Climates (H) encompass a wide variety of climate types that characterize mountainous terrain. Altitude, latitude, and exposure are among the factors that shape a complexity of climate types. For example, temperature decreases rapidly with increasing altitude and windward slopes tend to be wetter than leeward slopes. Climate-ecological zones are telescoped in mountainous terrain. That is, in ascending several thousand meters of altitude, we encounter the same bioclimatic zones that we would experience in traveling several thousand kilometers of latitude. As a general rule, every 300 m (980 ft) of elevation corresponds roughly to a northward advance of 500 km (310 mi).