OUTLINE OF CLIMATE AND CLIMATE CHANGE As we travel around the Earth, we experience grand changes in climate from lush rainforests of the tropics to stark deserts of the subtropics to frigid ice sheets of the polar regions. Similarly, if we could travel across time we would also experience grand changes in climate, from periods when climate was so warm we have called it Hothouse Earth to periods when climate was so cold that much of our planet was covered by ice and we have called it Snowball Earth. Climate is contrasted with weather. Weather involves the state and events of the atmosphere at a given time – its temperature, pressure, wind, cloudiness, and precipitation, and its storms (thunderstorms, hurricanes, blizzards). Climate is the synthesis of weather – its impact over an extended time. For most climate statistics, a standard averaging period of 30 years is used to smooth erratic year-to-year weather variations. This Presentation illustrates the varieties and patterns of climates around the World. A subsequent Presentation will review some of the great climate changes that have occurred during Earth’s long history, including how we know about the past climates (the evidence) and what we think climate will be like in the future (the theories).
Annual March of Global Temperatures Major features include, 1: T decreases with latitude and altitude, 2: The warmest region moves north and south with the seasons. 3: Seasonal ranges of T increase with latitude and are smaller over the oceans than over the land, 4: T is higher over the oceans in polar regions but higher over the land in the tropics.
Annual March of Global Precipitation, PPT Major features include, 1: PPT decreases with latitude but is particularly high around the equator and dry in the subtropics 2: Rainy and dry regions move north and south with the seasons. 3: East coasts are wetter than West coasts from 0 to 40 lat. but drier from 45 to 60 lat.
Climate Classification and Vegetation Climate is so closely related to the dominant plant life that almost all climate classification systems, such as the Köppen system, rely heavily on botanical categories. There are five main biomes or botanical regimes--forests, grassland, desert, tundra, and icecap (nothing)--and several hybrid regimes such as woodland and savannah (mixtures of forest and grassland). The dominant vegetation depends on the average values and seasonal and extreme variations of temperature and precipitation. Temperature limits plant type and growth rates. Extreme heat or cold kills plants. Many tropical trees are not frost-resistant and cannot survive a cold snap. The growth rate of plants (and many biochemical processes) more than doubles for each increase of 10°C up to about 40°C, above which certain proteins and enzymes become denatured, or literally cooked. The temperature of maximum growth rate depends on the species. Species that thrive in cold climates have maximum growth rates around 20°C, while some hot desert species grow best above 40°C. Almost all trees require at least one month with an average temperature of 10°C, because growth almost ceases below this point, and because it is almost impossible to take root in the shallow layer of mostly frozen soil (permafrost) that turns to mud in the short summer. When average temperature of the warmest month falls below 10°C, low lying tundra vegetation takes over, growing at a snail’s pace in the short season, and clinging to the ground in huddled masses that focus or store the little heat and moisture. In the warmer parts of the tundra, gnarled dwarf trees, up to several hundred years old, cower in depressions between the rocks. Like all wise tundra plants, they never stand up to the arctic winds’ desiccating blast.
Precipitation also limits plants. Forests need the most of any biome. They require at least 50 in (125 cm) per year in the tropics but can survive on 10 in (25 cm) or less in the cool boreal forests of Canada and Siberia, where evaporation rates are lower. But wet and especially dry seasons also limit vegetation. As dry seasons lengthen, open woodland replace forests, and then grasslands replace woodlands. Drought resistant trees may grow in drier climes, but must be widely spaced to avoid the threat of fire. They often have almost fire-proof bark to survive grassland or brush fires (500-900°C). No tree can survive a raging forest fire, which burns hotter (1200°C) and longer. Grasses are not bothered by fire, as they can rapidly reproduce or sprout up again from deep roots. Some seeds are even activated by fire. As precipitation drops still lower, scrubby, thorny desert vegetation takes over. Desert vegetation is designed to retain water (like the cacti), endure severe desiccation in a state of suspended animation, or sprout up rapidly after the rare rains and deposit durable, drought resistant seeds for the next patient generation. Trees can grow in wet spots of deserts such as riverbanks, oases, or foggy coasts such as California and northern Chile, where leaves are designed to capture the fog droplets as the wind blows past them. The graph on the next slide summarizes the relation between biomes and average annual T and precipitation (but unfortunately, not their seasonal variations).
Köppen’s Climate Classification System Köppen’s climate classification system divides the world into 6 primary climate zones as well as second and third order terms. The criteria are given in terms of temperature and precipitation, but are motivated by plant life. Thus, Main Categories A, Tropical -_All months above 18°C, a critical cutoff for many tropical plants. B, Arid and semiarid - Less precipitation than evaporation. C, Temperate - All months above -3°C. Long growing seasons favor deciduous trees. D, Cold winter - 1+ months below -3°C. Short growing seasons favor evergreen trees. E, Tundra and icecap - All months below 10°C and 0°C, too cold for trees, good for ice. H, Highland - Mountainous regions where temperature varies sharply with altitude. Secondary Categories f, wet all year w, dry winter or low sun s, dry summer or high sun
A Brief, World Climate Tour The following slides contain a photographic tour that represents the world’s climate regions starting from the Equator and moving toward the Poles. The rules and latitudes listed below are representative and useful but are not fundamental laws. Af Tropical rainforests all months warm and rainy gird the Equator to about 5° latitude. Aw Savannah marked by a long usually low sun dry season 5 – 20° latitude. B Deserts dominate the centers and west coasts of continents from 20 - 30° latitude. Cs Mediterranean climate with dry summers, mild winters with some rain, brush vegetation, fires and mudslides on West Coasts from 30 – 45° latitude. Cfb West Coasts from 45 – 60° latitude have lush mostly evergreen forests, mild, cloudy winters and cool summers. Cfa East Coasts from 25 - 50° latitude have deciduous forests with cold winters and long, hot summers. Df East Coasts from 50 - 60° latitude have boreal (mostly evergreen) forests with frigid winters and short warm summers. ET Tundra Land bordering the Arctic and Antarctic Oceans 55 - 70° latitude have scrubby vegetation with perhaps isolated or dwarf trees. EI Ice Caps from 70 - 90° latitude.
Tropical Rainforest (Af) The world's great tropical rain forests (jungles) occupy most of the lowlands surrounding the equator. Most of the year is rainy and humid because the ITCZ is never far away. Temperature is an almost constant 80°F (27°C) all year. In this land of perpetual summer, night brings the only cool weather. On rare occasion, cold fronts reach the equator but they are little more than phantoms that enhance cloudiness and precipitation but scarcely drop the temperature. Weather in the rain forest can best be described by one word—repetitive. Day after day is almost identical. Mornings are very humid but clear, with temperatures around 70°F (21°C). By late morning, cumulus clouds begin to form in the deep layer of humid air. Afternoon clouds frequently blossom into cumulonimbus and provide the tropics with much of its rain. These showers are the thermostats of the tropics, for they prevent the temperature from rising too high so that the rainy tropics never have extreme heat waves! Almost every day the temperature reaches around 90°F (32°C), but never reaches 100°F (38°C). Tropical showers are usually brief, but may be incredibly intense with incessant lightning and thunder and strong wind gusts. Descriptions of driving sheets of rain are difficult believe until you experience them. Often, the showers occur at almost the exact time of day for a number of days. Then, as the sun dips down in the western horizon and temperature starts down for the night, the sky often clears again, and peace and tranquility return. In some places, rain persists beyond sunset as thunderstorms knit together to produce stratiform rain through the night. Af climates receive at least 60” (150 cm) of rain annually and usually receive about 80” (200 cm). Along some coastlines and mountain slopes, amounts exceed 200” (500 cm). Almost every month is rainy. The constant high humidity and warmth make the pace of life and death startlingly rapid. As soon as something dies, it rots or is eaten. But wherever the precipitation of the driest month falls below about 2.4” (6 cm), openings begin to form in the rain forest. On an ideal continent the equator gets two peaks in rain each year shortly after the equinoxes as the ITCZ passed overhead. Although a few places such as equatorial Africa do have two rainy peaks, many do not because of irregular geography. For example, Singapore’s wettest time of year is November to January, when the remnants of Siberian cold fronts reach the equator.
The tropical rain forest consists primarily of tall, evergreen trees with broad leaves and giant buttresses to keep anchored in the often flooded ground. Little sunlight reaches the ground (< 1%) so that the forest floor has a dark green hue and is almost bare in many places so that it is possible to move about freely. This contrasts sharply with the phony image of the jungle in some old movies in which men on safari have to hack their way through dense underbrush, which in reality only exists at the edge of the rain forest, in coastal mangrove swamps, savanna woodlands and cloud forest on extremely rainy mountain slopes above the elevation of the true rain forest, where trees are far shorter than in the rain forest. Most action in the jungle takes place in the crown area or canopy, 30 to 60 m (100-200 ft) above the ground. Some inexperienced stragglers have starved on the bare forest floor, while countless tropical flowers and nourishing fruits grow in the canopy above. It is possible to climb to the canopy on lianas (vines often thicker than a man's arm), which hang in profusion from the treetops. Many lianas root in the air until they reach the ground, whereupon they root in the ground and may envelop and choke their host. The lianas and other creeping plants also stretch from treetop to treetop, tying the entire rain forest together and making it possible to travel through the canopy. Rain forest animals tend to be small and live in the canopy. But insects tend to be huge, with beetles up to 6” (15 cm) long. Poisonous insects and animals abound, and colorful tropical birds live and play in the canopy. Indeed, one of the main characteristics of the tropical rain forest is the tremendous diversity of life. Thousands of different plant species live packed together. Many take root in the canopy and either live on the trees (epiphytes) or off them (saprophytes). Some plants in the rain forest have leaves so large that a person can hide under them. Many leaves also have extended points called drip tips, which allow the excess moisture to drip off to prevent rotting. Soils in the rainforest tend to be poor because the heavy and persistent rains plus organic acids dissolve many soluble minerals and nutrients and carry them down the rivers leaving behind insoluble iron and aluminum oxides such as the aluminum ore, bauxite. A significant fraction of the soil develops on the branches of the trees, where it is enriched by dying epiphytes and wastes of tiny animals. When the forest is cleared for agriculture or livestock, the nutrients suffice for only a few years.
Wet and Dry Tropics (Aw) and Monsoons (Am) The wet and dry tropics (Aw) and monsoon climates (Am) are found on the poleward sides of the rainy tropics, between about 5 and 20° latitude. Precipitation usually ranges between 40” and 80” (100 to 200 cm) annually, with one or more dry months less than 2.4” (6 cm) of rain. The climate generally consists of three seasons: 1. The cool, dry season at the time of low sun. 2. The hot dry season when the sun is overhead. 3. The rainy season. The monsoon climate of India is the best known example of Aw or Am. In January, the weather is mostly clear with deep blue skies, because India is situated under the sinking air of the Hadley cell. This is the coolest time of the year, although in the extreme south the average temperature is still 77°F (25°C). Even northern India does not have any extremely cold weather because the Himalaya Mountains block the cold Siberian air to the north. Occasionally, low pressure areas pass around the south side of the Himalayas and bring some rain to northern India. These meager winter rains are desperately needed for the winter wheat harvest to prevent famine. As the sun moves north the land quickly heats. By March it is already brutal in the south. The hottest month is May in the south of India and July in the northwest. The notorious hot season occurs just before the monsoon rains set in. Coastal locations may get some relief because of sea breezes, but even along the coast the warm month averages 86°F (30°C). Inland, the warmest month averages as much as 96°F (36°C). During the day the earth feels like a giant furnace and temperatures regularly approach 110°F (43°C). People suffer and do as little as possible. Plants wither and turn grayish brown. The earth cracks and hardens. Southwest winds start when the ITCZ arrives, around May at India’s southern tip, and by July in the Himalayas. Then, extremely warm, humid air pours into India, often starting with a torrential burst of monsoon rains. The rainy season persists until the ITCZ moves back south in September. By the end of October, it has finally left southern India and dry northerly winds take over again. During the rains the temperature falls several degrees, but the high humidity adds its own discomfort.
Summer monsoon rains are intense but not continuous because much comes from thundershowers. Many of the thunderstorms are organized into weak monsoon lows, which are poorly defined but bring several days of extremely rainy weather. After they pass, there may be several mostly sunny days with little if any rain. The monsoon disturbances pulse to a crude cycle of roughly two weeks. At the height of the rainy phase, thick clouds reflect so much sunlight the atmosphere cools. Rain is suppressed, and openings in the clouds form. The sunlight pours through, heating the atmosphere and producing new rainclouds. Additional rains are produced by potentially deadly tropical cyclones shortly before and after the main rainy season. Monsoon rains vary widely from year to year, particularly in drier, northwest India. During some years incessant rains start early and finish late, flooding the land. In other years weaker rains arrive late and depart early. Drought and famine then hover over the parched earth. The awesome solemnity of Aw’s annual cycle with its pitiless dry season can be seen on nature films of Africa’s Serengeti Plains. A month or two after the rains have stopped, the grasses and leaves begin to wither. Small animals bury themselves in the ground while large ones migrate with the ITCZ in pursuit of food and water. Often the waterholes and smaller streams completely dry out so that only the fittest animals survive. This is a good time only for the vultures and hyenas. Toward the end of the dry season, the gasping trees put out new leaves and delicate flowers, in anticipation of the rains. When the rains arrive, they flood the land and turn it green overnight. Life, kept in suspended animation during the dry season, springs up in profusion or quickly migrates into the area. During the rains everything springs back to life and the earth turns green with amazing rapidity. The ground becomes muddy and impassable. Insect populations rise exponentially, spreading misery and disease. Nevertheless, this is the time to raise young. Although vegetation grades from tropical rainforest at the equatorial edge to semiarid scrub on the poleward sides of the region, the dominant vegetation of Aw and Am regions is savanna, an open type of woodland-grassland combination, with the umbrella shaped acacia as the signature tree. The extended Aw dry season, with its constant threat of fires (either natural or manmade), keeps the trees far apart and has encouraged others, such as the enormously wide baobob, to evolve thick, almost fireproof bark. The longer the dry season, the more isolated the clumps of trees, which appear like islands in seas of grass.
Dry Climates (BS and BW) Poleward of the savanna, even the grasses die out and give way to sparse, thorny vegetation or desert sand and bare rock. Semiarid (BS) climates occur where precipitation is less than potential evaporation while desert climates (BW) are regions in which precipitation is less than half the potential evaporation. In these regions, exposed water rapidly evaporates, leaving soluble minerals to bleach the soil and sow it with salt. There are two main types of deserts. The great subtropical deserts including the Sahara, the Arabian Desert, the Australian Desert, the Kalihari and Namib of South Africa and the Atacama of South America are located between about 20 and 30° latitude. They are the product of persistently sinking air of the Hadley cells and extend from the center to the west coast of the continents (except in the Horn of Africa). The second type of desert occurs in the hearts of the continents surrounded by mountains or downwind from mountain ranges. Deserts do not extend poleward of about 50° latitude because the evaporation rate is so low in the cool air that there is almost always adequate moisture. Only the driest parts of the driest deserts are rainless. Most desert areas receive from 4 to 12” (10 - 30 cm) of precipitation annually, and on the tropical borderline they can receive as much as 16” (40 cm). Most of the scanty precipitation comes from brief and sporadic summer showers. Much of that rain evaporates before reaching the ground, but the rare heavy showers can produce flash floods. On the poleward side of the subtropical deserts and in the midlatitude deserts some winter rain or even snow may fall from rogue lows that have strayed off the normal track. In general, The smaller the annual average precipitation, the more irregular and unreliable it is. For example, Port Sudan in the Sahara averages 4” (10 cm) per year; but they have received as little as 0.8” (2 cm) and as much as 16.5” (41 cm). Many places in the heart of the desert may go several years without any rain and then one large shower may produce several inches in an hour. Then, no more rain may fall for several more years. Therefore, Most years in dry climates have below average precipitation. Below-average rain is not serious in the desert, where no one expects rain, but has dire consequences to people in BS climates. After a series of rainy years in the 1950’s and 1960’s, the population of the Sahel just south of the Sahara swelled to insupportable numbers. Then, beginning in the late 1960’s two decades of drought claimed more than one million lives. Water shortages also recur in the southwest United States,
despite all conservation measures, because the population continues to swell, but the water supply doesn’t. Extreme temperature is another mark of the desert. The hottest month in the subtropical deserts often averages between 86 and 100°F (30 to 38°C). Greenland Ranch in Death Valley, California, has a mean temperature of 100°F during July, just about the warmest month anywhere on Earth. Only late at night is it tolerable. After dawn it rapidly heats up. Summer afternoons can make you gasp for breath. Daytime highs almost always exceed 104°F (40°C), and large areas have daytime highs above 113°F (45°C). Let no one tell you that the desert's “dry heat” is not so bad. I once drove through Baker, California, when it was 120°F and was never hotter in my life. (The car was overheating so we couldn’t run the air conditioner.) When I put my hand out of the window to feel the air go by, it felt like a blast furnace. Remember also that air temperature is taken in the shade, but everything in the desert is exposed to the sun. Despite the high albedo, dry sand can reach 175°F (80°C) on sunny afternoons. Radiation from the broiling ground and sizzling sun will fry you. Midlatitude deserts seldom get quite so hot. Still, summer temperatures average near 86°F (30°C), while daytime highs frequently reach 104°F (40°C). Altitude effects are also quite important, and valley stations such as Lukchun in the heart of Asia and the Dead Sea (both, below sea level) have temperatures that wilt highlanders. The misery of the desert does not end with the scorching temperatures. Relative humidity often drops below 10% in the afternoon. Gusty winds race over the bare ground stirring up dust, simultaneously sandblasting, scorching, and desiccating you. Here, you find the world’s highest evaporation rates over any moist surface (such as human skin). The moisture leaves with every breath and through every pore. An adult must drink 9 quarts a day and cannot survive without water for two days! Such extremes lead to many adaptations such as the traditional, flowing dress of the Bedouin Arabs, which not only protects the body from sun, wind, sand, heat, and cold of night, but also helps retain some moisture. It may seem strange to go about so completely covered up, but in the desert that is wise policy. To preserve water, it is best to travel by night and rest by day. For example, insects come out at night and hide by day when they would dry up in minutes. Some desert insects obtain their daily water by drinking the dew that condenses on their cold-blooded bodies at night. A single dew drop by dawn is enough to keep a beetle going for the whole day.
Another nasty feature of the desert is its nightly cold, particularly in winter. Winter in Mongolia’s Gobi Desert is notorious for its inhospitable, frigidity, and even the subtropical deserts have occasional frost. Thus, the desert traveler who is prepared for the heat must also endure the cold of night, and many claim that it is this feature that they have found to be truly unbearable. The clear, dry air cools so rapidly when the sun goes down that diurnal temperature ranges generally exceed 30°F and sometimes approach 60°F. West coastal subtropical deserts such as the Atacama of Chile and the Namib of Southwest Africa are cooled by persistently upwelled waters. This makes fog common but stabilizes the lower atmosphere and makes precipitation even more unlikely. When the west coasts are lined with mountain ranges, the subtropical high tends to get locked in place. The net result of all these factors is an almost rainless climate. The sparseness of plants is a desert trademark. And the few plants that grow are stingy of water. They tend to be spherical to minimize surface area and are often coated with a waxy layer. Leaves are small or missing, so stems are green. Many desert plants have extensive root systems and underground tubers that store water safely without exposure. Many more, such as the giant saguaro cactus of Mexico and the American Southwest are fluted so that they can expand to store water during the occasional rains. Most plants are covered with thorns or painful little prickles to ward off thirsty animals. Desert plants have distinct biochemical processes. Some do not produce seeds until they near death so that their offspring do not compete for water. Some exude toxins that prevent other plants from growing nearby. Many desert plants even photosynthesize differently, using a process called crassulacean acid metabolism (named after the crassulacea). Most plants open their stomata during the day to obtain CO2 for photosynthesis. But desert plants must keep the stomata closed during day to prevent water loss. Therefore, they respire and accumulate CO2 at night when water loss is not serious, and store it as malic acid. When the sun comes up, the stomata close and the malic acid releases its CO2. Oxygen then accumulates in the plant until night, when the stomata reopen. The desert landscape is unmistakable. Landforms are angular, and colors of the exposed rocks and soil can be striking. Most of the desert consists of rocky plains. Only 25% consists of shifting sand dunes, which may be as much as 1000 feet (300 m) high. Very little of the desert is actually lifeless, and in all but the most barren places the ground is laden with seeds ready to burst into flower at the first shower if the temperature is right. On Earth there are few more joyous and miraculous sights than the desert in bloom.
Maritime West Coast Climates Mediterranean Climate (Cs) Along the west coasts of every continent poleward of about 30° latitude, the desert gives way to dry summer subtropics (Cs) or Mediterranean climate. Here, there is a distinct winter cool and rainy season but summer reverts to an almost rainless state. For example, Jerusalem has not recorded a single drop of rain during July for over 100 years. Many people consider Cs to be the world’s best climate, and the longevity of the animals in the San Diego Zoo attests to this. Even winter has abundant sunshine, while near the water, summer temperatures are rarely excessive. Thus, Cs is a climate of moderation, that prodded the Ancient Greeks into formulating their Golden Rule. During summer, the sinking air of the combined Hadley cell and subtropical highs coupled with offshore, downslope flow keeps skies clear and blue except where pollution is trapped or where fog results from upwelling. Coastal temperatures are much warmer in the Mediterranean, where upwelling is rare. Temperatures are also much higher a few miles inland (about 81°F = 27°C), with a large diurnal range where desert-like conditions prevail. In winter the westerlies bring upslope winds and occasional passing lows. The ocean’s modifying influence keeps winter temperatures attractively mild so that you can walk the streets of San Francisco in January wearing the same sweater you wore in July. Winters tend to average from 45 to 59°F (7 to 15°C), and frost only presents an occasional problem to agriculture inland. Annual precipitation averages from 16 to 32” (40 to 80 cm), with higher totals on exposed slopes. Snow is freakishly rare near sea level, but is more likely than rain above elevations of about 2 km (6600 ft), where it can accumulate to impressive depths. Storms typically last a day or two, with many embedded showers that can be especially intense on hilly areas. However, even in winter there is a lot of sunshine because the subtropical high is never too far away. Even though the world’s four Cs regions are totally separated, all have remarkably similar vegetation. The modest winter rainfall enables drought tolerant, sclerophyllous trees with thick, leathery leaves and impervious trunks such as the cork oak to survive the long, dry summer. In the drier parts of Cs, trees are widely spaced and the dominant vegetation is a scrubby, thorny cover of bushes called chapparal or maquis. Grasses do not dominate because summer is too dry. Where precipitation approaches 32” (80 cm), the natural vegetation is sclerophyllous forest. But once cut down, these delicate forests are not easily
reestablished. Plato referred to the deforestation of Greece, and Greek myths alluded to it with dire warnings. Erysichthon, an ancient land developer, defied the warnings and had his servants slaughter a sacred, ancient oak. The spirit in the dying tree prayed for revenge. Erysichthon was stricken with such an insatiable hunger he finally ate himself. Toward the end of winter the land turns green and flowers bloom everywhere. In the warmest parts of the Cs climate winter is the green season, while in the cooler parts the greenest time of year is spring. Then, as the dry season progresses, the landscape turns tan and gray, and the threat of fire greatly increases. Fire is one of the scourges of the Cs climate and keeps trees far apart. And if the scrubby plant cover has burned, winter’s first rains trip mudslides down the bare hillsides. Cloudy Oceanic Climate (Cfb) From about 40 to 65° latitude along the west coast the rainy winter season is noticeably cooler and longer. At the southern margins of Cfb, the subtropical high still influences summer weather, but no longer dominates, so that rain can fall during the summer. Consider a typical year in Cfb. On New Year’s Day we arrive in London. As the plane drops below the clouds we see the wet, drizzly ground below. People say it has been drizzling for three days, but at first we think little of it. After a few more overcast, drizzly days, the persistence of dank weather begins to dawn on us. In Cfb winters, it rains about two of every three days, the temperature hovers about 5°C, and it is more than 75% cloudy. The air almost always feels damp, and fog is common. The westerlies bring one low pressure area after another. Most of these lows are old and deeply occluded, which accounts for the drizzle. Annual precipitation averages between 20 and 40” (50 - 100 cm) in the lowlands, surprisingly little considering how much of the time it falls. But after all, it is only drizzle. At sea level, winds from the nearby ocean keep winter averages above freezing despite the high latitude. This makes rain more common than snow, but slushy snow is common enough. Cold spells are infrequent because the cold air must come from the land to the east, but in some European winters, easterly winds pour out of the heart of the continent and lock the coast in an icy grip. This happened with greater frequency during the Little Ice Age.
Weather is more severe on the exposed mountain slopes. Here precipitation is more intense and annual totals increase above 100” (250 cm). Above about 1 km (3300 ft) most winter precipitation falls as snow since the cool sea air only has to be lifted a small distance before temperature drops below freezing. The windward slopes thus receive some of the world’s highest snowfall totals. Paradise Ranger Station, at 5000 feet (1.5 km) on Mount Rainier, the world's snowiest place, is just such a spot. During spring the weather warms slowly because the ocean remains cool. Cloudiness and precipitation gradually diminish, but fog is especially common along the coastline. When it finally does clear up people rejoice. For the most part, summer is delightful. Temperature averages from 59 to 68°F (15 - 20°C) so that you need a sweater most evenings, but most days are pleasant. The moist ocean air heats quickly over the land, producing cumulus clouds and occasional showers. Heat waves are rare but do occur. Afternoon temperatures may then rise to 90°F (32°C) or even a bit warmer. This may not seem brutal to most people, but the British have become so acclimatized to cool weather that their puny heat waves increase the mortality rates. No wonder, they were so often the authors of the vivid tales of "unbearable" tropical heat. Because the summer remains cool, even small changes in altitude or climate have disproportionately large effects on the vegetation. At sea level the growing season (T > 10°C = 50°F) is about 180 days a year. But because the average temperature never rises far above 10°C, any small increase in altitude or slight cooling is sufficient to greatly shorten the growing season. During the Little Ice Age the cooling caused famine and expanded mountain glaciers. In autumn, cloudiness returns and fog is common. Autumn is sometimes the rainiest season because the oceans are relatively warm. Once the general cloudiness sets in around October, the sun won't shine much again until the following May. The cool summers and mild, wet winter produce a rich green landscape that is naturally forested, as in Ireland. On mountainous coasts where rain and fog are more abundant, some of the world's densest and grandest rain forests are found. Here the famous coast redwoods, which attain heights over 380 ft (117 m) thrive, as do other giant trees such as the Douglas fir, while inland the giant sequoia reigns. Evergreens dominate, because their leaves are ready to photosynthesize at the first sign of sun during the brief winter clearings.
Continental Climates Climate is quite different on the eastern side of the continents. In summer, warm, humid air ready to gather in thunderstorms or occasional hurricanes arrives from the tropics. In winter, the westerlies bring passing lows with rain and snow but mostly cold, dry air from the hearts of the continents. Therefore, at any given latitude the east coast has a much larger annual temperature range than the west coast, and this increases sharply with latitude. Thus, both Cfa and D climates have similar annual weather sequences, but do not feel the same. The trademark of Cfa and Cw, located roughly between 25 and 40° latitude, is the enervating, hot, humid summer (Figure 16-26), while the trademark of D is its frigid winter. Hot Summer Climate (Cfa) Imagine spending a year in Montgomery, Alabama. January 1 is a sunny, pleasant afternoon with a temperature of 70°F (22°C). The next day, temperature begins to fall after a thunderstorm. Within 24 hours it drops to 23°F (-5°C). Similar cold waves pass through Montgomery several times every winter. Further north, they occur with increasing frequency and severity. Montgomery is far enough south so that there are many warm winter days, and people are rarely confined indoors for long. After the cold wave sets in, we observe the typical weather sequence of an approaching low. A day or so of rain will follow the cirrostratus and altostratus clouds, for snow or freezing precipitation is uncommon so far south. After the rain ends it often gets warm again for several days, but sometimes another cold wave follows immediately. During spring in Montgomery the weather rapidly begins to warm up and plants come to life again. Temperatures at this time are "just right" and the weather can be exhilarating. A few hot days may give a hint of the hot summer to come, but they are not common until late April. Cold waves decrease in frequency and intensity, but are increasingly announced by thunderstorms, which may be severe and have hail and tornadoes. By the end of May, Montgomery is unpleasantly hot, and the long, hot, humid summer lies ahead. Low pressure areas and cold fronts have retreated poleward and only occasionally ooze so far south. When they do, they often stall and provide merely warm, dampness for a day or two before the steam bath returns.
Throughout the long summer, the weather forecast reads hazy, hot, and humid with a chance of showers or thundershowers. The temperature tops out between 90 and 100°F (32 to 38°C), while if you are lucky, nighttime lows get down to 70°F (21°C). The mean temperature of the warmest months approaches 85°F (29°C), making it even hotter and more miserable than on the equator. Even the shore has little relief from the intense heat because ocean temperatures approach 86°F (30°C). Such high water temperatures mean that it must get very hot inland to start a sea breeze, but hurricanes have no trouble in such waters. Several times each summer heat waves send temperatures soaring to 100°F (38°C) or more. These heat waves are associated with some of the world's deadliest weather. The killer heat wave of July 10-16, 1995, claimed more than 700 lives in Chicago alone, as temperatures reached 104°F (40°C) and dew points hovered near 80°F (27°C). During such heat waves it is wise not to overeat or exercise too strenuously. It is even wiser to leave. If you can’t, then get into an air-conditioned room or a cool bath. Heat waves in the United States occur when the Bermuda-Azores subtropical high expands over the land. They typically last a week or two and suppress the normal afternoon showers and thunderstorms of summer. Once every several years a hurricane may strike, and can produce more rain than normally falls in several months. In fall, the nights get longer, and the weather finally begins to cool. Warm air is still pumped up from the tropics, but cools as it passes over the land to produce frequent morning fog. Even so, the weather at this time is particularly lovely. All Cf regions have similar characteristics, with a few significant differences. Precipitation averages from 32 to 64” (80 to 160 cm) and is spread rather evenly over the entire year, with the summer usually being somewhat rainier than the other seasons, and late fall the driest season. The American branch of Cf has the worst winter cold waves, but China’s Cf winter is colder and drier on average because north winds from the Siberian high blow almost continuously. South America and South Africa have the warmest Cf winters because they are separated by an ocean from Antarctica. Thus at 32° latitude, Shanghai has a January average of only 39°F (4°C), Montgomery registers a cool 48°F (9°C), while Port Macquarie, Australia (54°F) and Rosario, Argentina (52°F) are much warmer.
In China and the United States, every part of Cf, down to Miami, has recorded some snow. On the tropical margins, snow may only occur once a century. In warm places like Montgomery, even tiny snowstorms paralyze the unprepared residents. On the poleward margins of Cfa, significant snow falls every winter. Precipitation is more seasonal in China’s Cfa because of the huge Asian landmass. As winter gives way to summer, it is marked by a period of sultry weather called the Baiu (plum) Rains because they come when the plums ripen. China’s abundant summer rains may more than compensate for its dry winter, with the peak monthly total roughly 10” (25 cm). This seasonality of the rains is reflected in the river levels, which are also swollen by melting winter snows and periodically have transformed the land into vast shallow seas of famine and death. Where the river runs through narrow canyons it can rise impressively. Before being dammed, the water level rose more than the height of Niagara Falls in the famous Wind Box Gorge of the Yangtse River during the height of the rainy season! But epic floods remain a threat, as in 1998, when 230 million people were affected. The dominant vegetation largely depends on the rainfall. Near the coast, ample year-round rains keep the land forested wherever it has not been cleared. In China and the United States, the natural forest a mixture of evergreen and deciduous trees. In Australia, eucalyptus trees dominate. Further inland, rains diminish and grassland takes over. In the United States, where summer winds blow from the south, the border between forest and grasslands in the United States is a line that extends straight north from the western edge of the Gulf of Mexico. Further west over the high Plains, the climate turns semiarid. In China, the climate rapidly dries toward the northwest, causing a rapid transition from forest to grassland to desert vegetation. The Great Wall, designed to keep marauding nomads out of China’s productive farmland, closely matches the annual rainfall contour of 15 inches (38 cm) and the border with B climates.
Cold Continental Winter Climate (D) From about 40 or 45° latitude to the fringes of the Arctic Ocean in North America and Asia, winter is the dominant season in the Df and Dw climate regions. Once the average temperature of the coldest month drops below about 27°F (-3°C), snow remains on the ground much of the winter because daytime highs typically remain below freezing. Thus, in just 100 miles from Cape Cod to Concord NH, where January temperature drops from 32 to 20°F (0 to -7°C), snow cover increases from 20 to more than 120 days. Beautiful or sombre, winter through much of D regions is brutally cold. The frigid air completes the accurate picture given in almost every Russian novel of the cruel Dc winter. And, never forget how Russian winters decimated the armies of Napoleon and Hitler. Moscow averages 14°F (-10°C) in January and may only get a few, brief incursions of above-freezing air each winter before the freezer returns. But Moscow is tropical compared to Siberia and northern Canada. In fact, the temperature difference in January between Moscow and Yakutsk, Siberia is greater than between Moscow and Miami! January temperature averages around -15°F (-26°C) in northern Canada and below -22°F (-30°C) in Siberia. Towns located in "protected" river valleys such as Verkhoyansk and Oimyakon are even colder and may be plagued by ice fog for days on end. The extreme cold makes it risky to stay outdoors. Winter is a tough season. Clouds are abundant (typically 50 to 60%) in the southern half of D regions because the storm tracks are near. In early winter the sky is often overcast and snow sometimes seems to continue without end. Moscow gets 120 days of snow a year--25 in January alone. And, in the brutal winter of 1976-77, snow fell on 45 consecutive days in Buffalo, NY. Buffalo typically gets a lot of snow because it is located east of Lake Erie. But most snow totals are not high because frigid cannot hold much vapor. Thus, Moscow averages only 1.5” (3.8 cm) of precipitation in January. But blizzards can never be ruled out. In most D and E regions, the extreme cold makes winter the dry season. A striking example of the dry winter occurs in Port Harrison, Quebec on the east shore of Hudson Bay. Snow begins in September and peaks in November at 33” (84 cm). Then Hudson Bay freezes. With little vapor in the air, February snowfall is only 3” (7.5 cm).
On the polar half of D regions, winter skies are much clearer and precipitation totals much lower. There, cP air is manufactured and exported to lower latitudes as cold waves. During the clear, cold snaps, winter can bring with it an astounding pristine beauty. Snow glistens on the ground and the occasional freezing rain storms make the trees look as though they were covered with dazzling jewels. Sculptures carved in ice are part of the culture of many D cities. After long months of bitter cold, winter gives way to a brief, marshy spring, and life emerges from the muddy, melting snows. One of the most heart wrenching portrayals of this rebirth can be seen in the movie, Dr. Zhivago. But spring’s birthpangs often seem to be a battleground between winter and summer. Snow still falls to the north of the lows at the same time almost tropical weather with severe thunderstorms may occur further south. And the storm tracks vary enough so that any place can experience the full range of weather every few days. Spring rains help melt the snows of winter in a hurry, and can produce disastrous floods. The most monumental floods occur on rivers that flow northward to the Arctic Ocean because they are choked and dammed by their own ice. When the Lena River in Siberia peaks in June, average discharge increases from 4000 to over 80,000 m3/s. In spring, mosquitoes and vicious black flies appear in unimaginable numbers. They even bite the birds that feed on them, sometimes spreading disease in the process. There are times black flies swarm so thickly, they completely mask their victim. They have whipped caribou into such frenzies that the poor beasts run blindly until they drop dead from exhaustion. In Siberia, one form of punishment in days gone by was to strip a man stark naked outdoors and let the insects kill him. The ordeal took a day or two at most. Finally the spring, which always seems to be delayed, phases into a brief summer. D summers are hot at their southern margins, but chilly further north. Heat waves occur, but are less frequent and shorter as you move north. Even so, they have pushed the thermometer above 95°F (35°C) at many stations, which is as hot as it ever gets along the equator. Moscow’s all time high is 99°F (38°C)! But the heat doesn’t last, for even in mid summer, cold fronts regularly bring intervals of pleasant or even chilly weather.