My Science School

          Large volcanic eruptions can have an almost immediate effect on the world’s weather. The dust that is thrown into the atmosphere creates a kind of screen, which reflects more of the Sun’s energy back into space. As a result, temperatures around the world can drop slightly and weather patterns may be affected for several years.

          When Mount Pinatubo erupted in the Philippines June 15, 1991, an estimated 20 million tons of sulfur dioxide and ash particles blasted more than 12 miles (20 km) high into the atmosphere. The eruption caused widespread destruction and loss of human life. Gases and solids injected into the stratosphere circled the globe for three weeks. Volcanic eruptions of this magnitude can impact global climate, reducing the amount of solar radiation reaching the Earth's surface, lowering temperatures in the troposphere, and changing atmospheric circulation patterns. The  extent to which this occurs is an ongoing debate.

          Large-scale volcanic activity may last only a few days, but the massive outpouring of gases and ash can influence climate patterns for years. Sulfuric gases convert to sulfate aerosols, sub-micron droplets containing about 75 percent sulfuric acid. Following eruptions, these aerosol particles can linger as long as three to four years in the stratosphere.

         Major eruptions alter the Earth's radiative balance because volcanic aerosol clouds absorb terrestrial radiation, and scatter a significant amount of the incoming solar radiation, an effect known as "radiative forcing" that can last from two to three years following a volcanic eruption.

          "Volcanic eruptions cause short-term climate changes and contribute to natural climate variability," says Georgiy Stenchikov, a research professor with the Department of Environmental Sciences at Rutgers University. "Exploring effects of volcanic eruption allows us to better understand important physical mechanisms in the climate system that are initiated by volcanic forcing."

          By comparing the climate simulations from the Pinatubo eruption, with and without aerosols, the researchers found that the climate model calculated a general cooling of the global troposphere, but yielded a clear winter warming pattern of surface air temperature over Northern Hemisphere continents. The temperature of the tropical lower stratosphere increased by 4 Kelvin (4°C) because of aerosol absorption of terrestrial longwave and solar near-infrared radiation. The model demonstrated that the direct radiative effect of volcanic aerosols causes general stratospheric heating and tropospheric cooling, with a tropospheric warming pattern in the winter.

        "The modeled temperature change is consistent with the temperature anomalies observed after the eruption," Stenchikov says. "The pattern of winter warming following the volcanic eruption is practically identical to a pattern of winter surface temperature change caused by global warming. It shows that volcanic aerosols force fundamental climate mechanisms that play an important role in the global change process."

        This temperature pattern is consistent with the existence of a strong phase of the Arctic Oscillation, a natural pattern of circulation in which atmospheric pressure at polar and middle latitudes fluctuates, bringing higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45 degrees north latitude. It is forced by the aerosol radiative effect, and circulation in winter is stronger than the aerosol radiative cooling that dominates in summer.

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          Fossils contained in layers of rock can reveal details about the climate millions of years ago. Rock that contains a large variety of fossils was formed during a time when the climate was warm; fewer fossils indicate a cooler climate. Rocks that show signs of glacial erosion were part of the Earth’s surface during an Ice Age. Geologists can work out the age of the layers, which tells us when the changes took place.

            Many of the techniques used for palaeoclimatic reconstruction discussed in the preceding sections have only a limited time scale open to their period of study. Most ice cores are restricted to the last million years, whilst tree ring analysis can only provide proxy climate information for at best the last 10,000 years. Ocean sediments provide some of the longest proxy records available, and offer a window on palaeoclimates dating back to the age of the dinosaurs, 100 million years ago. Most older sediments, however, will have been subducted beneath overriding tectonic plates as the continents continue to drift about the Earth. To reconstruct climates older than this, therefore, one needs to look elsewhere for the evidence.

           Sediments laid down on the ocean floor become progressively buried by subsequent debris transported from continental interiors. Deeply buried sediments are subjected to considerable pressures from the overlying layers, and after tens to hundreds of millions of years, the sediments are gradually lithified, forming sedimentary rocks. If, through tectonic movements, these sedimentary rocks are uplifted and exposed, scientists may study them, as they do other forms of evidence, to reconstruct past climates.

          Numerous techniques of analysing sedimentary rocks are used for palaeoclimate reconstruction. Principally, rock type provides valuable insights into past climates, for rock composition reveals evidence of the climate at the time of sediment deposition. However, depositional climatic regimes vary not only due to actual climatic changes but also due to continental movements. The Carboniferous limestones and coals (evidence of warm, humid climates) of Northern England (300Ma), for example, were laid down at a time when Britain was located near the equator, whilst large scale glaciation was occurring in the high latitudes of the Southern Hemisphere.

        The study of rock type is geologically known as facies analysis. Facies analysis investigates how the rock type changes over time, and therefore provides a potential tool for investigating past climatic change. A sedimentary formation consisting of a shale layer (fine-grained mudstone) interbedded between two sandstone layers (coarse-grained), for example, provides evidence of a changing sea level, potentially linked to climatic change (caused either by epeirogeny or ice formation). Sandstones are deposited in coastal zones where the water is shallow, whilst mudstones (shales) are deposited in deeper water of the continental shelf region. A change in the rock type in the vertical cross section must therefore reflect a change in sea level and associated coastline movements.

          Other principal marker rock types include evaporites (lithified salt deposits and evidence of dry arid climates), coals (lithified organic matter and evidence of warm, humid climates), phosphates and cherts (lithified siliceous and phosphate material and evidence of ocean upwelling due to active surface trade winds) and reef limestone (lithified coral reef and evidence of warm surface ocean conditions).

         As well as facies analysis, other techniques, including analysis of sedimentation rates, sediment grain morphology and chemical composition provide information on the climatic conditions prevailing at the time of parent rock weathering. In addition, some of the methods used to reconstruct past climate discussed in earlier sections may be equally applied to sedimentary rocks. For example, the type and distribution of marine and continental fossils within fossil-bearing rocks (principally limestones and mudstones, but occasionally sandstones) are valuable palaeoclimate indicators. Microfossil type, abundance and morphology may also be studied, and palaeotemperatures derived from their oxygen isotope analysis.

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          By studying the growth rings in ancient trees, scientists can gather information about climates of the past. This science is called dendroclimatology. In each year of a tree’s growth, new layers are added to the centre of its trunk, producing a growth ring. Warm, wet growing seasons produce several layers, creating a wide growth ring. In a cold, dry period, fewer layers are produced, and the ring will be narrower.

          The characteristics of the rings inside a tree can tell scientists how old a tree is and what the weather conditions were like during each year of that tree’s life. Very old trees can offer clues about what the climate in an area was like long before measurements were recorded.

         But to understand what the trees tell us, we first have to understand the difference between weather and climate.

          Weather is a specific event—like a rain storm or hot day—that happens over a short period of time. Weather can be tracked within hours or days. Climate is the average weather conditions in a place over a long period of time (30 years or more).

          Scientists at the National Weather Service have been keeping track of weather in the United States since 1891. But trees can keep a much longer record of Earth’s climate. In fact, trees can live for hundreds—and sometimes even thousands—of years!

         One way that scientists use trees to learn about past climate is by studying a tree’s rings. If you’ve ever seen a tree stump, you probably noticed that the top of the stump had a series of rings. It looks a bit like a bullseye.

          These rings can tell us how old the tree is, and what the weather was like during each year of the tree’s life. The light-colored rings represent wood that grew in the spring and early summer, while the dark rings represent wood that grew in the late summer and fall. One light ring plus one dark ring equals one year of the tree’s life.

        Because trees are sensitive to local climate conditions, such as rain and temperature, they give scientists some information about that area’s local climate in the past. For example, tree rings usually grow wider in warm, wet years and they are thinner in years when it is cold and dry. If the tree has experienced stressful conditions, such as a drought, the tree might hardly grow at all in those years.

          Scientists can compare modern trees with local measurements of temperature and precipitation from the nearest weather station. However, very old trees can offer clues about what the climate was like long before measurements were recorded.

          In most places, daily weather records have only been kept for the past 100 to 150 years. So, to learn about the climate hundreds to thousands of years ago, scientists need to use other sources, such as trees, corals, and ice cores (layers of ice drilled out of a glacier).

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            The causes of an Ice Age are not clear. One theory is that the Earth’s tilt and its orbit of the Sun have changed. An orbit that took our planet further from the Sun would result in a cooler climate.

          An ice age is a time where a significant amount of the Earth's water is locked up on land in continental glaciers.

          During the last ice age, which finished about 12,000 years ago, enormous ice masses covered huge swathes of land now inhabited by millions of people.

          Canada and the northern USA were completely covered in ice, as was the whole of northern Europe and northern Asia.

          At the moment the Earth is in an interglacial period - a short warmer period between glacial (or ice age) periods.

          The Earth has been alternating between long ice ages and shorter interglacial periods for around 2.6 million years.

          For the last million years or so these have been happening roughly every 100,000 years - around 90,000 years of ice age followed by a roughly 10,000 year interglacial warm period.

Causes:

Ice ages don't just come out of nowhere - it takes thousands of years for an ice age to begin.

          An ice age is triggered when summer temperatures in the northern hemisphere fail to rise above freezing for years. This means that winter snowfall doesn't melt, but instead builds up, compresses and over time starts to compact, or glaciate, into ice sheets.

          Over thousands of years these ice sheets start to build up - it seems to be in northern Canada when that first happens - and then they spread out across the northern hemisphere.

          "It's a long term trend over thousands of years to colder summers," Dr Steven Phipps, an ice sheet modeller, said.

          Dr Phipps is also a climate system modeller and palaeoclimatologist with the University of Tasmania.

          The onset of an ice age is related to the Milankovitch cycles - where regular changes in the Earth's tilt and orbit combine to affect which areas on Earth get more or less solar radiation.

          When all these factors align so the northern hemisphere gets less solar radiation in summer, an ice age can be started.

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          It is thought that Ice Ages occur roughly every 100,000 years. The last one ended around 10,000 years ago, so we may experience another in 90,000 years time. Scientists call the time between Ice Ages an interglacial period.

         An interglacial period (or alternatively interglacial, interglaciation) is a geological interval of warmer global average temperature lasting thousands of years that separates consecutive glacial periods within an ice age. The current Holocene interglacial began at the end of the Pleistocene, about 11,700 years ago.

          During the 2.5 million years of the Pleistocene, numerous glacials, or significant advances of continental ice sheets, in North America and Europe, occurred at intervals of approximately 40,000 to 100,000 years. The long glacial periods were separated by more temperate and shorter interglacials.

          During interglacials, such as the present one, the climate warms and the tundra recedes polewards following the ice sheets. Forests return to areas that once supported tundra vegetation. Interglacials are identified on land or in shallow epicontinental seas by their paleontology. Floral and faunal remains of species pointing to temperate climate and indicating a specific age are used to identify particular interglacials. Commonly used are mammalian and molluscan species, pollen and plant macro-remains (seeds and fruits). However, many other fossil remains may be helpful: insects, ostracods, foraminifera, diatoms, etc. Recently, ice cores and ocean sediment cores provide more quantitative and accurately-dated evidence for temperatures and total ice volumes.

          Interglacials and glacials coincide with cyclic changes in the Earth's orbit. Three orbital variations contribute to interglacials. The first is a change in the Earth's orbit around the sun, or eccentricity. The second is a shift in the tilt of the Earth's axis, or obliquity. The third is the wobbling motion of Earth's axis, or precession.

          Warm summers in the Southern Hemisphere occur when it is tilted toward the sun and the Earth is nearest the sun in its elliptical orbit. Cool summers occur when the Earth is farthest from the sun during the summer. Such effects are more pronounced when the eccentricity of the orbit is large. When the obliquity is large, seasonal changes are more extreme.

          Interglacials are a useful tool for geological mapping and for anthropologists, as they can be used as a dating method for hominid fossils.

          Brief periods of milder climate that occurred during the last glacial are called interstadials. Most but not all interstadials are shorter than interglacials. Interstadial climate may have been relatively warm but not necessarily. Because the colder periods (stadials) have often been very dry, wetter (not necessarily warmer) periods have been registered in the sedimentary record as interstadials as well.

          The oxygen isotope ratio obtained from seabed sediment core samples, a proxy for the average global temperature, is an important source of information about changes in the climate of the earth.

          An interglacial optimum, or climatic optimum of an interglacial, is the period within an interglacial that experienced the most 'favourable' climate and often occurs during the middle of that interglacial. The climatic optimum of an interglacial both follows and is followed by phases within the same interglacial that experienced a less favourable climate (but still a 'better' climate than the one during the preceding/succeeding glacials). During an interglacial optimum, sea levels rise to their highest values but not necessarily exactly at the same time as the climatic optimum.

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            The world’s climates have been through many changes since the planet was formed over 4000 million years ago. The Earth has been both hotter and colder than it is now. In the age of the dinosaurs, there were no polar ice caps, and tropical and desert climates were predominant. Since that time, there have been several Ice Ages, when the polar ice sheets expanded to cover up to one-third of the planet. The planet will continue to experience such dramatic changes, as well as minor fluctuations in the weather. Many people are concerned that the activities of mankind will have a catastrophic effect on our planet’s weather patterns.

            The Earth’s climate has changed throughout history. Just in the last 650,000 years there have been seven cycles of glacial advance and retreat, with the abrupt end of the last ice age about 7,000 years ago marking the beginning of the modern climate era — and of human civilization. Most of these climate changes are attributed to very small variations in Earth’s orbit that change the amount of solar energy our planet receives.

            The current warming trend is of particular significance because most of it is extremely likely (greater than 95 percent probability) to be the result of human activity since the mid-20th century and proceeding at a rate that is unprecedented over decades to millennia.

            Earth-orbiting satellites and other technological advances have enabled scientists to see the big picture, collecting many different types of information about our planet and its climate on a global scale. This body of data, collected over many years, reveals the signals of a changing climate.

            The heat-trapping nature of carbon dioxide and other gases was demonstrated in the mid-19th century. Their ability to affect the transfer of infrared energy through the atmosphere is the scientific basis of many instruments flown by NASA. There is no question that increased levels of greenhouse gases must cause the Earth to warm in response.

            Ice cores drawn from Greenland, Antarctica, and tropical mountain glaciers show that the Earth’s climate responds to changes in greenhouse gas levels. Ancient evidence can also be found in tree rings, ocean sediments, coral reefs, and layers of sedimentary rocks. This ancient, or paleoclimate, evidence reveals that current warming is occurring roughly ten times faster than the average rate of ice-age-recovery warming.

              It is possible to simulate the conditions of certain climates inside a greenhouse. Glass and other materials can be used to create a space within which the heat and light from the Sun is intensified, making it much warmer than it is outside. The temperature, humidity and air movement can be controlled, recreating the atmosphere of a particular climate.

           Climate simulators (or climate models) are complex computer programmers which simulate the Earth's climate system, including the atmosphere, ocean, land surface and ice, and the interactions between them. The computer programme represents the climate in terms of key quantities such as atmospheric temperature, pressure, wind, and humidity at locations on a three dimensional grid. The atmospheric grid covers the Earth’s surface and extends from the surface to the upper atmosphere. A similar grid for the ocean extends from the ocean’s surface to the ocean floor. By solving the relevant mathematical equations the computer is able to calculate how the state of the atmosphere and ocean evolves in time.

                 At present, a typical simulator of global climate has grid boxes with horizontal dimensions of approximately 100-200 km; this is known as the "spatial resolution". Simulators used to predict daily weather use much higher spatial resolution, but typically only simulate a specific region (e.g. the UK). Simulators with higher resolution are more accurate, but they also take longer to run and require larger computers.

To test scientific understanding

              Scientists use climate simulators to test and improve their understanding of the climate system. By comparing the simulated climate with observations of the real world, scientists can identify where a simulator needs improvement.

To predict future climate

              Climate simulators are used to predict how climate may change in the future. For example, scientists can implement expected future conditions, such as a higher concentration of greenhouse gases in the atmosphere, and use the simulator to predict how such a change may affect the climate.

              Climate simulators are not perfect and scientists are careful to study, quantify and communicate their accuracy and reliability along with particular results. However, climate scientists are confident that climate simulators can accurately represent many fundamental aspects of the climate system for several reasons.

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            Parts of India and Southeast Asia have a monsoon climate. In these areas, it changes very suddenly from a wet to a dry season, according to the direction of the prevailing wind. The dry period is extremely hot, and the powerful monsoon winds that blow in from the sea bring torrential rain, often without warning. Such violent extremes of weather can make daily life very difficult, with heavy flooding, damage to property and loss of life commonplace.

            A monsoon often brings about thoughts of torrential rains, similar to a hurricane or typhoon. But there is a difference: a monsoon is not a single storm; rather, it is a seasonal wind shift over a region. The shift may cause heavy rains in the summer, but at other times, it may cause a dry spell.

            A monsoon (from the Arabic mawsim, which means “season”) arises due to a difference in temperatures between a land mass and the adjacent ocean, according to the National Weather Service. The sun warms the land and ocean differently, according to Southwest Climate Change, causing the winds to play "tug of war" eventually switching directions bringing the cooler, moister air from over the ocean. The winds reverse again at the end of the monsoon season. 

            A wet monsoon typically occurs during the summer months (about April through September) bringing heavy rains, according to National Geographic. On average, approximately 75 percent of India's annual rainfall and about 50 percent of the North American monsoon region (according to a 2004 NOAA study) comes during the summer monsoon season. The wet monsoon begins when winds bringing cooler, more humid air from above the oceans to the land, as described above.

            A dry monsoon typically occurs between October and April. Instead of coming from the oceans, the winds tend to come from drier, warmer climates such as from Mongolia and northwestern China down into India, according to National Geographic. Dry monsoons tend to be less powerful than their summer counterparts. Edward Guinan, an astronomy and meteorology professor at Villanova University, states that the winter monsoon occurs when "the land cools off faster than the water and a high pressure develops over the land, blocking any ocean air from penetrating." This leads to a dry period. 

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          The polar climate is very dry and windy, as well as being exceptionally cold. Inland, it is nearly always below freezing, and temperatures often reach -40°C (-40°F). Only near the coasts do temperatures reach about 10°C (50°F) in the summer.

          Both the Arctic (North Pole) and the Antarctic (South Pole) are cold because they don’t get any direct sunlight. The Sun is always low on the horizon, even in the middle of summer. In winter, the Sun is so far below the horizon that it doesn’t come up at all for months at a time. So the days are just like the nights—cold and dark.

          Even though the North Pole and South Pole are “polar opposites,” they both get the same amount of sunlight. But the South Pole is a lot colder than the North Pole. Why? Well, the Poles are polar opposites in other ways too.

          The Arctic is ocean surrounded by land. The Antarctic is land surrounded by ocean.

          The ocean under the Arctic ice is cold, but still warmer than the ice! So the ocean warms the air a bit.

          Antarctica is dry—and high. Under the ice and snow is land, not ocean. And it’s got mountains. The average elevation of Antarctica is about 7,500 feet (2.3 km). And the higher you go, the colder it gets.

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          Some relatively small areas have their own climate, which differs slightly from the climate surrounding it — a microclimate. Cities often have a microclimate, due to the concentration of buildings, people and vehicles generating heat. This creates a “heat island” — a warm mass of air that sits over the city, making it up to 6°C (11°F) warmer than the surrounding area.

          Microclimate, any climatic condition in a relatively small area, within a few metres or less above and below the Earth’s surface and within canopies of vegetation. The term usually applies to the surfaces of terrestrial and glaciated environments, but it could also pertain to the surfaces of oceans and other bodies of water.

          The strongest gradients of temperature and humidity occur just above and below the terrestrial surface. Complexities of microclimate are necessary for the existence of a variety of life forms because, although any single species may tolerate only a limited range of climate, strongly contrasting microclimates in close proximity provide a total environment in which many species of flora and fauna can coexist and interact.

          Microclimatic conditions depend on such factors as temperature, humidity, wind and turbulence, dew, frost, heat balance, and evaporation. The effect of soil type on microclimates is considerable. Sandy soils and other coarse, loose, and dry soils, for example, are subject to high maximum and low minimum surface temperatures. The surface reflection characteristics of soils are also important; soils of lighter colour reflect more and respond less to daily heating. Another feature of the microclimate is the ability of the soil to absorb and retain moisture, which depends on the composition of the soil and its use. Vegetation is also integral as it controls the flux of water vapour into the air through transpiration. In addition, vegetation can insulate the soil below and reduce temperature variability. Sites of exposed soil then exhibit the greatest temperature variability.

          Topography can affect the vertical path of air in a locale and, therefore, the relative humidity and air circulation. For example, air ascending a mountain undergoes a decrease in pressure and often releases moisture in the form of rain or snow. As the air proceeds down the leeward side of the mountain, it is compressed and heated, thus promoting drier, hotter conditions there. An undulating landscape can also produce microclimatic variety through the air motions produced by differences in density.

          The microclimates of a region are defined by the moisture, temperature, and winds of the atmosphere near the ground, the vegetation, soil, and the latitude, elevation, and season. Weather is also influenced by microclimatic conditions. Wet ground, for example, promotes evaporation and increases atmospheric humidity. The drying of bare soil, on the other hand, creates a surface crust that inhibits ground moisture from diffusing upward, which promotes the persistence of the dry atmosphere. Microclimates control evaporation and transpiration from surfaces and influence precipitation, and so are important to the hydrologic cycle—i.e., the processes involved in the circulation of the Earth’s waters.

          The initial fragmentation of rocks in the process of rock weathering and the subsequent soil formation are also part of the prevailing microclimate. The fracturing of rocks is accomplished by the frequent freezing of water trapped in their porous parts. The final weathering of rocks into the clay and mineral constituents of soils is a chemical process, where such microclimatic conditions as relative warmth and moisture influence the rate and degree of weathering.

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          In the most mountainous regions of the world, the climate will often be very different from that of the land that surrounds them. The freezing climate of the Himalayas, for example, is surrounded by desert, warm temperate, and monsoon climates.

          Lower down, the climate may be milder (temperate), suitable for lots of different plants and trees to grow, which in turn, provide food for a wide variety of animals. Higher up, plants and animals are fewer: they have to be highly adapted to survive, as the climate becomes much harsher. It’s windy and cold. Frozen ground means that there is not much water available and the soil is shallow. Humans also struggle to cope at high altitude (a fancy word for great height), because the air becomes much thinner, meaning that there is less oxygen available for your body to use.

          Mountain weather conditions can change in a split second! Well, maybe not quite that quickly but in just a few minutes clouds can gather and a thunderstorm begins. That’s why mountaineers have to be ready for anything; they pack their rucksacks really carefully and carry emergency kit like tents and extra food. Professional climbers always tell other people what their plans are so, if they go missing, search and rescue teams know where to look!

          Mountains also receive lots and lots of rainfall. This is because air travelling over land is forced up and over any mountains in its path – it can’t tunnel! This air cools as it rises causing the condensation of any water vapour it was carrying into huge clouds (made up of tiny droplets) ready to burst at any moment.

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            Area with a Tropical climate have high temperatures (24°C to 27°C (75°F to 81°F)) throughout the year. The atmosphere is very humid (full of moisture) and the levels of rainfall are very high — at least 150cm (59in) — particularly in those regions close to the Equator.

          Some people assume the word tropical climate refers to their favorite warm vacation spot. However, this is a bit far from the truth since the word tropical is defined differently in meteorology. A tropical climate is identified as a climate characteristic to the tropics; that is from the equator to the Tropic of Capricorn in the south and from the Equator to the Tropic of Cancer in the north. The Koppen climate classification defines a tropical climate as a non-arid climate in which the mean temperature is about 64°F throughout the year. Unlike subtropical regions which are characterized by variations in temperature to different degrees and day length, temperatures in tropical climates, remain relatively constant all year long as variations for different seasons are dominated by rainfall. Tropical climates comprise of only two seasons which are the dry season and the wet season. Changes in solar angle are small in tropical climates which happen to be frost-free. There are different varieties of tropical climates within the tropical climate zone. The different varieties are based on precipitation. Here are the three subtypes of tropical climates.

The Tropical Wet and Dry Climate

          Also known as the Savannah climate, the tropical wet and dry climate experiences a long dry period and less annual rainfall. The driest month in a wet and dry tropical climate has precipitation of less than 2.4 inches and less than 3.9 inches total annual precipitation. The tropical wet and dry climate are mainly found in Lagos, Nigeria; Bangalore, India; Dar es Salaam, Tanzania; Barquisimeto, Venezuela; Darwin, Australia; Honolulu, US; Fort Myers, Florida; Rio de Janeiro, Brazil; and Kupang, Indonesia among others.

The Tropical Monsoon Climate

          A tropical monsoon climate is the type of climate found in the Southern and Central regions of America and the Southeast and Southern parts of Asia as well as parts of Australia and Africa. The tropical monsoon climate is influenced by the monsoon winds which according to the seasons change directions. For this part of the equator, the driest month of the tropical monsoon climate occurs either soon after or at the ‘winter’ solstice. Rainfall is usually less than 2.4 inches but exceeds 3.9 inches total annual precipitation. Examples of the area that experience tropical monsoon climates include Jakarta, Indonesia; Miami, Florida; Abidjan, Ivory Coast; Puerto Ayacucho, Venezuela; Chittagong, Bangladesh; Yangon, Myanmar; Cairns, Australia; and Macapa, Brazil among others.

Tropical Rainforest Climate

          The tropical rainforest climate is found in places that are around the equatorial region usually between 5° to 10° latitude of the equator. However, in several eastern coastal regions, such climates might extend beyond 26° from the equator. Tropical rainforest climates are mainly characterized by low-pressure systems since they are dominated by doldrums thus receiving rainfall all year long. There is no specific season found in the tropical rainforest climate. All 12 months in this type of climate have an average precipitation of at least 2.4 inches. Examples of areas with tropical rainforest climates include Mbandaka, Congo; Singapore; Klang, Malaysia; Hilo, Hawaii; Innisfail, Australia; Apia Samoa, Davao, Philippines; Bogor, Indonesia among others.

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            There are two types of temperate climate — cool and warm. Cool temperate areas have rainfall throughout the year, warm summers, and winters with temperatures often below freezing. The warm temperate climate features mild, wet winters where the temperature rarely gets below 4°C (39°F). The summers are hot and dry, with temperatures averaging 20°C to 27°C (68°F to 81°F).

            In geography, temperate latitudes of the Earth lie between the subtropics and the polar circles. Average yearly temperatures in these regions are not extreme, not burning hot nor freezing cold. Temperate means moderate.

            Unlike in the tropics, temperatures can change greatly here, between summer and winter. So, most places with a temperate climate have four seasons: summer, autumn, winter and spring. Other areas with a temperate climate can have very unpredictable weather. One day it may be sunny, the next may be rainy, and after that it may be cloudy. This is normal in summer as well as in winter. These are the main types of temperate climate:

• A maritime climate is generally for places near the sea. That includes London, Dublin, Melbourne and Auckland. Most places do not have a rainy season and a dry season. Prevailing winds in the temperate zone are from the west. The western edge of temperate continents usually get this maritime climate. Examples are Western Europe, and western North America at latitudes between 40° and 60° north (65°N in Europe).


• Some parts of the temperate zone have a Mediterranean climate, which have a dry summer – for example Madrid, and Adelaide.


• Some parts of the temperate zone, especially in the northern part of the continental climate, have severe winters – for example Moscow and Minnesota – this is called a hemiboreal climate.


• Some places in the temperate zone have hot summers and cold winters, for example Chicago, Budapest and Almaty.

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            A region’s climate is the general pattern of weather that it experiences over a long period of time. Climate depends on a number of factors. The position of the area on the Earth's surface, and its height above sea level are two factors. Warmth carried around the world by ocean currents affects the climate on land, and those areas far from the sea will have a different climate from those on the coast. There are eight main types of climate, but there are variations to be found within them.

           Climate is the average weather conditions in a place over a long period of time—30 years or more. And as you probably already know, there are lots of different types of climates on Earth.

            For example, hot regions are normally closest to the equator. The climate is hotter there because the Sun’s light is most directly overhead at the equator. And the North and South Poles are cold because the Sun’s light and heat are least direct there.

            Using this information, in the late 1800s and early 1900s a German climate scientist named Wladimir Koppen divided the world's climates into categories. His categories were based on the temperature, the amount of precipitation, and the times of year when precipitation occurs. The categories were also influenced by a region’s latitude—the imaginary lines used to measure our Earth from north to south from the equator.

            Today, climate scientists split the Earth into approximately five main types of climates. They are:

A: Tropical. In this hot and humid zone, the average temperatures are greater than 64°F (18°C) year-round and there is more than 59 inches of precipitation each year.

B: Dry. These climate zones are so dry because moisture is rapidly evaporated from the air and there is very little precipitation.

C: Temperate. In this zone, there are typically warm and humid summers with thunderstorms and mild winters.

D. Continental. These regions have warm to cool summers and very cold winters. In the winter, this zone can experience snowstorms, strong winds, and very cold temperatures—sometimes falling below -22°F (-30°C)!

E: Polar. In the polar climate zones, it’s extremely cold. Even in summer, the temperatures here never go higher than 50°F (10°C)!

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            When the Sun is directly overhead at its most northern or southern position, it is called the solstice. The Northern Hemisphere's summer solstice occurs when the Sun is above the Tropic of Cancer — on 20, 21 or 22 June — and marks the beginning of summer. Its winter solstice (the Southern Hemisphere's summer solstice) is on 21 or 22 December. The summer solstice is the longest day of the year; the winter equivalent is the shortest.

            Solstice, either of the two moments in the year when the Sun’s apparent path is farthest north or south from Earth’s Equator. The situation is exactly the opposite in the Southern Hemisphere, where the seasons are reversed. At the winter solstice the day is the year’s shortest, and at the summer solstice it is the year’s longest. The term solstice also is used in reference to either of the two points of greatest deviation of the ecliptic (the Sun’s apparent annual path) from the celestial equator.

            At the time of the summer solstice in the Northern Hemisphere, the North Pole is tilted about 23.4° (23°27´) toward the Sun. Because the Sun’s rays are shifted northward by the same amount, the vertical noon rays are directly overhead at the Tropic of Cancer (23°27´ N). Six months later the South Pole is inclined about 23.4° toward the Sun. On this day of the summer solstice in the Southern Hemisphere the Sun’s vertical overhead rays progress to their southernmost position, the Tropic of Capricorn (23°27´ S). Compare equinox. See also season.

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