Spiral arms

Spiral arms

spiral arms are regions of  that extend from the center of and   These long, thin regions resemble a spiral and thus give spiral galaxies their name. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very "loose" arms, whereas Sa and SBa galaxies tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain a great many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so remarkable.

Using the Hubble classification, the bulge of Sa galaxies is usually composed of population II stars, that is old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are a great deal lesser, and are composed of young, blue, Population I stars. Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity), and others simply appear as higher density centers of disks, with properties similar to disk galaxies.

Many bulges are thought to host a at their center. Such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object calledis believed to be a supermassive black hole. 

Spiral galaxy

Spiral galaxies are named for the (usually two-armed)ructures that extend from the center into the disk. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disk because of the young, hot  that inhabit them. Roughly half of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. Our own  has recently (in the 1990s) been confirmed to be a although the bar itself is difficult to observe from our position within the Galactic disk. The most convincing evidence for its existence comes from a recentperformed by the of stars in the Galactic center.

Together with , spiral galaxies make up approximately 60% of galaxies in the local They are mostly found in low-density regions and are rare in the centers of galaxy clusters

A spiral galaxy is a f  originally described by in his 1936 work The Realm of the Nebulae and, as such, forms part of theSpiral galaxies consist of a flat, rotating containingand a central concentration of stars known as the . These are surrounded by a much fainter of stars, many of which reside in

 

Types and morphology

Types and morphology

Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the  Since the Hubble sequence is entirely based upon visual morphological type, it may miss certain important characteristics of galaxies such as  rate (in starburst galaxies) and activity in the core (ingalaxies)

The Hubble classification system rates elliptical galaxies on the basis of their ellipticity, ranging from E0, being nearly spherical, up to E7, which is highly elongated. These galaxies have an profile, giving them an elliptical appearance regardless of the viewing angle. Their appearance shows little structure and they typically have relatively little  . Consequently these galaxies also have a low portion of and a reduced rate of new star formation. Instead they are dominated by generally older, more that are orbiting the common center of gravity in random directions. In this sense they have some similarity to the much smaller .

Physical modeling debate

Physical modeling debate

Former Chair of the IPCC Working Group I Report, has commented on Lindzen's criticisms of the IPCC:,saying that Lindzen, having previously expressed satisfaction with the report, had "gone on to express his view that the conclusions of the Policymakers Summary did not faithfully represent the chapters. But he has never provided any supporting evidence for that statement." He emphasized that the Policymaker's Summaries were "agreed unanimously at intergovernmental meetings involving over 200 government delegates from around 100 countries...after several days of scientific debate (only scientific arguments not political ones are allowed) the main purpose of which is to challenge the scientific chapter authors regarding the accuracy, clarity and relevance of the summary and most especially its consistency with the underlying chapters."

document before the professor, one of the lead authors of the IPCC Working Group I Report, has criticised the IPCC  Taking Chapter 7, Physical Processes, on which he worked, he said that it "found numerous problems with model treatments – including those of clouds and water vapor", the chapter was summarized in the single sentence "Understanding of climate processes and their incorporation in climate models have improved, including water vapor, sea-ice dynamics, and ocean heat transport."

Debate over value of a statistical life

Debate over value of a statistical life

This information was presented in the full Second Assessment Report, however, dispute arose over the Report's Summary for Policymakers. The Summary for Policymakers (SPM) is prepared with the input of government delegates and IPCC experts. Governments were unhappy with thevaluation of human life, and this was implied in the SPM. , the IPCC convening lead author who oversaw the relevant chapter of the Report, officially dissented on this summary, commenting that:

The relevant chapter [of the Report] values of statistical life based on actual studies in different countries. Whether the values used remain as in Chapter 6 or whether a common global average is used makes no difference to the results. What the authors of Chapter 6 did not accept, and still do not accept, was the call from a few [government] delegates for a common valuation based on the highest number for willingness to pay.

The Second Assessment Report was controversial in its treatment of the economic value of human life In environmental economics, it is customary to value the health impacts of climate change on the basis of willingness to pay for. An advantage of this method is that health risks of climate change are treated like any other health risk. Some have commented on the difficultly of calculating the costs of climate change impacts such as human mortality. For example, in calculations based on risk reduction, the value of a statistical life is assessed to be much higher in rich countries than in poor countries.

 

Debate

Debate

president emeritus of Rockefeller University, past president of the National Academy of Sciences, and former health consultant forpublicly denounced the IPCC report, writing "I have never witnessed a more disturbing corruption of the peer-review process than the events that led to this IPCC report". He opposed it in the of s 

In turn, Seitz's comments were vigorously opposed by the presidents of the and   who wrote about a "systematic effort by some individuals to undermine and discredit the scientific process that has led many scientists working on understanding climate to conclude that there is a very real possibility that humans are modifying Earth's climate on a global scale. Rather than carrying out a legitimate scientific debate... they are waging in the public media a vocal campaign against scientific results with which they disagree".

1. All revisions were made with the sole purpose of producing the best-possible and most clearly explained assessment of the science, and were under the full scientific control of the Convening Lead Author of Chapter 8.
2. None of the changes were politically motivated.

Santer's position was supported by fellow IPCC authors and senior figures of the American Meteorological Society and University Corporation for Atmospheric ResearchIn 1997, Paul Edwards and IPCC author published a paper rebutting criticisms of the IPCC report

 

Intergovernmental Panel on Climate Change

The IPCC does not carry out its own original research, nor does it do the work of monitoring climate or related phenomena itself. A main activity of the IPCC is publishing special reports on topics relevant to the implementation of the (UNFCCC), an international treaty that acknowledges the possibility of harmful climate change. Implementation of the UNFCCC led eventually to the  The IPCC bases its assessment mainly on peer reviewed and published scientific literature. The IPCC is only open to member states of the WMO and UNEP. IPCC reports are widely cited in almost any debate related to climate change.National and international responses to climate change generally regard the UN climate panel as authoritative

The Intergovernmental Panel on Climate Change (IPCC) is a scientific  body tasked with reviewing and assessing the most recent scientific, technical and socio-economic information produced worldwide relevant to the understanding of climate change. It provides the world with a clear scientific view on the current state of climate change and its potential environmental and socio-economic consequences, notably the  of  change caused by human activity. The panel was established in 1988 by the (WMO) and the  Programme (UNEP), two organizations of the . The IPCC shared the 2007 with former

Magnetic field reversals

Magnetic field reversals

There is no clear theory as to how the geomagnetic reversals might have occurred . Some scientists have produced models for the core of the Earth wherein the magnetic field is only quasi-stable and the poles can spontaneously migrate from one orientation to the other over the course of a few hundred to a few thousand years. Other scientists propose that the geodynamo first turns itself off, either spontaneously or through some external action like a come and then restarts itself with the magnetic "North" pole pointing either North or South. External events are not likely to be routine causes of magnetic field reversals due to the lack of a correlation between the age of impact craters and the timing of reversals. Regardless of the cause, when the magnetic pole flips from one hemisphere to the other this is known as a reversal, whereas temporary dipole tilt variations that take the dipole axis across the equator and then back to the original polarity are known as excursions.

As an example of how this property of igneous rocks allows us to determine that the Earth's field has reversed in the past, consider measurements of magnetism across  Before exits the  through a fissure, it is at an extremely high temperature, above the of any ferrous oxide that it may contain. The lava begins to cool and solidify once it enters the ocean, allowing these ferrous oxides to eventually regain their magnetic properties, specifically, the ability to hold a remnant magnetization. Assuming that the only magnetic field present at these locations is that associated with the Earth itself, this solidified rock becomes magnetized in the direction of the geomagnetic field. Although the strength of the field is rather weak and the iron content of typical rock samples is small, the relatively small remnant magnetization of the samples is well within the resolution of modern  The age and magnetization of solidified lava samples can then be measured to determine the orientation of the geomagnetic field during ancient eras.

Magnetic field variations

Magnetic field variations

The distortion was recognized by Icelandic mariners as early as the late 18th century. More important, because the presence of magnetite gives the basalt measurable magnetic properties, these magnetic variations have provided another means to study the deep ocean floor. When newly formed rock cools, such magnetic materials record the Earth's magnetic field.

The currents in the core of the Earth that create its magnetic field started up at least million years ago

detect minute deviations in the Earth's magnetic field caused by iron  kilns, some types of stone structures, and even ditches and  in  Using magnetic instruments adapted from airbornedetectors developed during World War II to detect submarines, the magnetic variations across the ocean floor have been mapped. The the iron-rich, volcanic rock making up the ocean floor — contains a strongly magnetic mineral  and can locally distort compass readings. 

Field characteristics

Field characteristics

Convection of molten iron within the outer liquid core, along with a  caused by the overall planetary rotation, tends to organize these "electric currents" in rolls aligned along the north-south polar axis. When conducting fluid flows across an existing magnetic field, electric currents are induced, which in turn creates another magnetic field. When this magnetic field reinforces the original magnetic field, is created that sustains itself. This is called the and it explains how the Earth's magnetic field is sustained.

Another feature that distinguishes the magnetically from a bar magnet is its  At large distances from the planet, this dominates the surface magnetic field. Electric currents induced in the  also generate magnetic fields. Such a field is always generated near where the atmosphere is closest to the Sun, causing daily alterations that can deflect surface magnetic fields by as much as one degree. Typical daily variations of field strength are about 25 nanoteslas (nT) (i.e. ~ 1:2,000), with variations over a few seconds of typically around 1 nT (i.e. ~ 1:50,000)

The strength of the field at the Earth's surface ranges from less than  in an area including most of South America and South Africa to over  around the magnetic poles in northern Canada and south of Australia, and in part of Siberia. The average magnetic field strength in the Earth's outer core was measured to be 25 Gauss, 50 times stronger than the magnetic field at the surface.

The field is similar to that of a bar The Earth's magnetic field is mostly caused by in the liquid core. The Earth's core is hotter than 1043 , the  above which the orientations of  within iron become randomized. Such randomization causes the substance to lose its magnetization.

 

Importance

Importance

A smaller number of particles from the solar wind manage to travel, as though on an electromagnetic energy transmission line, to the Earth's upper atmosphere andin the auroral zones. The only time the solar wind is observable on the Earth is when it is strong enough to produce phenomena such as the and . Bright auroras strongly heat the ionosphere, causing its plasma to expand into the  increasing the size of the plasma , and causing escape of atmospheric matter into the solar wind. result when the pressure of plasmas contained inside the magnetosphere is sufficiently large to inflate and thereby distort the geomagnetic field.

Earthis largely protected from the , a stream of energetic charged particles emanating from the  by its magnetic field, which deflects most of the charged particles. Some of the charged particles from the solar wind are trapped in the  Allen radiation belt. 

Earth's magnetic field

This makes the usable for navigation. The cause of the field can be explained by  A field extends infinitely, though it weakens with distance from its source. The Earth's magnetic field, also called the geomagnetic field, which effectively extends several tens of thousands of kilometres into  forms the Earth's magnetosphere. A paleomagnetic study of Australian red dacite and pillow basalt has estimated the magnetic field to be at least 3.5 billion years old.

 

Earth's magnetic field (and the surface magnetic field) is approximately a   with the  Snear the 's geographic see  and the other magnetic field N pole near the Earth's geographic .  

Pressure and thickness

Pressure and thickness

If atmospheric density were to remain constant with height the atmosphere would terminate abruptly at 8.50 km (27,900 ft). Instead, density decreases with height, dropping by 50% at an altitude of about 5.6 km (18,000 ft). As a result the pressure decrease is approximately exponential with height, so that pressure decreases by a factor of two approximately every 5.6 km (18,000 ft) and by a factor of e = 2.718… approximately every 7.64 km (25,100 ft), the latter being the average of Earth's atmosphere below 70 km (43 mi; 230,000 ft). However, because of changes in temperature, average molecular weight, and gravity throughout the atmospheric column, the dependence of atmospheric pressure on altitude is modeled by separate equations for each of the layers listed above. Even in the exosphere, the atmosphere is still present. This can be seen by the effects ,

The average atmospheric pressure at is about 1 atmosphere (atm) = 101.3 kPa (kilopascals) = 14.7 psi (pounds per square inch) = 760 torr = 29.9 inches of mercury (symbol Hg). Total atmospheric mass is 5.1480×10¹⁸ kg (1.135×10¹⁹about 2.5% less than would be inferred naively from the average sea level pressure and the Earth's area of 51007.2 megahectares, this defect having been displaced by the Earth's mountainous terrain. Atmospheric pressure is the total weight of the air above unit area at the point where the pressure is measured. Thus air pressure varies with location and time, because the amount of air above the Earth's surface varies. lb)

Density and mass

Density and mass

The average mass of the atmosphere is about 5 quadrillion (5×10¹⁵) or 1/1,200,000 the mass of Earth. According to the"The total mean mass of the atmosphere is 5.1480×10¹⁸ kg with an annual range due to water vapor of 1.2 or 1.5×10¹⁵ kg depending on whether surface pressure or water vapor data are used; somewhat smaller than the previous estimate. The mean mass of water vapor is estimated as 1.27×10¹⁶ kg and the dry air mass as 5.1352 ±0.0003×10¹⁸ kg."

The density of air at sea level is about 1.2  kg/m³ (1.2 g/L). Density is not measured directly but is calculated from measurements of temperature, pressure and humidity using the equation of state for air (a form of the . Atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the  More sophisticated models are used to predict orbital decay of satellites.

Physical properties

Physical properties

If atmospheric density were to remain constant with height the atmosphere would terminate abruptly at 8.50 km (27,900 ft). Instead, density decreases with height, dropping by 50% at an altitude of about 5.6 km (18,000 ft). As a result the pressure decrease is approximately exponential with height, so that pressure decreases by a factor of two approximately every 5.6 km (18,000 ft) and by a factor of e = 2.718… approximately every 7.64 km (25,100 ft), the latter being the average of Earth's atmosphere below 70 km (43 mi; 230,000 ft). However, because of changes in temperature, average molecular weight, and gravity throughout the atmospheric column, the dependence of atmospheric pressure on altitude is modeled by separate equations for each of the layers listed above. Even in the exosphere, the atmosphere is still present. This can be seen by the effects of on

The average atmospheric pressure at is about 1 atmosphere (atm) = 101.3 kPa (kilopascals) = 14.7 psi (pounds per square inch) = 760 torr = 29.9 inches of mercury (symbol Hg). Total atmospheric mass is 5.1480×10¹⁸ kg (1.135×10¹⁹ lb), about 2.5% less than would be inferred naively from the average sea level pressure and the Earth's area of 51007.2 megahectares, this defect having been displaced by the Earth's mountainous terrain. Atmospheric pressure is the total weight of the air above unit area at the point where the pressure is measured. Thus air pressure varies with location and time, because the amount of air above the Earth's surface varies.

 

Structure of the atmosphere

Structure of the atmosphere

Principal layers

Earth's atmosphere can be divided into five main layers. These layers are mainly determined by whether temperature increases or decreases with altitude. From highest to lowest, these layers are:

The outermost layer of Earth's atmosphere extends from the  upward. It is mainly composed of hydrogen and helium. The particles are so far apart that they can travel hundreds of kilometres without colliding with one another. Since the particles rarely collide, the atmosphere no longer behaves like a fluid. These free-moving particles follow ballistic trajectories and may migrate into and out of the or the 

 Temperature increases with height in the thermosphere from the mesopause up to the then is constant with height. The temperature of this layer can rise to 1,500 °C (2,730 °F), though the gas molecules are so far apart that  sense is not well defined. The orbits in this layer, between 320 and 380 km (200 and 240 mi). The top of the thermosphere is the bottom of the exosphere, called the  Its height varies with solar activity and ranges from about 350–800 km (220–500 mi; 1,100,000–2,600,000 ft).

The mesosphere extends from the stratopause to 80–85 km (50–53 mi; 260,000–280,000 ft). It is the layer where most  burn up upon entering the atmosphere. Temperature decreases with height in the mesosphere. The  the temperature minimum that marks the top of the mesosphere, is the coldest place on Earth and has an average temperature around −85  (−121 ; 188.1 Due to the cold temperature of the mesophere, water vapor is frozen, forming ice clouds (or A type of lightning referred to as either or  form many miles above thunderclouds in the troposphere.

Atmosphere of Earth

Air is the name given to atmosphere used in and Dry air contains roughly (by volume) 78.09%, 20.95% 0.93% 0.039% and small amounts of other gases. Air also contains a variable amount of on average around 1%. While air content and varies at different layers, air suitable for the survival of and is currently known only to be found in Earth's and artificial atmospheres.

The atmosphere of Earth is a layer of surrounding the planet that is retained by Earth's The protects by absorbing, warming the surface through heat retention (and reducing extremes between and

describes the structure of the atmosphere, dividing it into distinct layers, each with specific characteristics such as temperature or composition. The atmosphere has a mass of about 5×10¹⁸ kg, three quarters of which is within about 11 km (6.8 mi; 36,000 ft) of the surface. The atmosphere becomes thinner and thinner with increasing altitude, with no definite boundary between the atmosphere and An altitude of 120 km (75 mi) is where atmospheric effects become noticeable during of spacecraft. The, at 100 km (62 mi), also is often regarded as the boundary between atmosphere and outer space.

Effects on human health

Effects on human health

On the other hand, excessive sunlight exposure has been linked to all types of caused by the part of radiation from sunlight or sunlamps.^[ can have mild to severe effects on skin; this can be avoided by using a proper cream or lotion or by gradually building up melanocytes with increasing exposure. Another detrimental effect of UV exposure is accelerated skin aging (also called which produces a difficult to treat effect. Some people are concerned that is increasing the incidence of such health hazards. A 10% decrease in ozone could cause a 25% increase in skin cancer

A lack of sunlight, on the other hand, is considered one of the primary causes of (SAD), a serious form of the "winter blues". SAD occurrence is more prevalent in locations further from the tropics, and most of the treatments (other than prescription drugs) replicating sunlight via lamps tuned to specific (visible, not ultra-violet) wavelengths of light or full-spectrum bulbs.

A recent study indicates that more exposure to sunshine early in a person’s life relates to less risk from (MS) later in life.

The body produces sunlight (specifically from the UVB band of light), and excessive seclusion from the sun can lead to deficiency unless adequate amounts are obtained through diet. This very common deficiency leaves the body generally vulnerable to cancers.

 

Cultural aspects

Cultural aspects

Many people find direct sunlight to be too bright for comfort, especially when reading from white paper upon which the sun is directly shining. Indeed, looking directly at the sun can cause long-term vision damage. To compensate for the brightness of sunlight, many people wear , many  and are equipped with o block the sun from direct vision when the sun is at a low angle.

In colder countries, many people prefer sunnier days and often avoid the . In hotter countries the converse is true; during the midday hours many people prefer to stay inside to remain cool. If they do go outside, they seek shade which may be provided by trees, and so on.

Sunshine is often blocked from entering buildings through the use of or

The more recent discoveries ofand  are modern extensions of this trend. These are the remnants of ancient plant and animal matter, formed using energy from sunlight and then trapped within the earth for millions of years. Because the stored energy in theseas accumulated over many millions of years, they have allowed modern humans to massively increase the production and consumption of  As the amount of fossil fuel is large but finite, this cannot continue indefinitely, and various theories exist as to what will follow this stage of human civilization  .

 

Life on Earth

Life on Earth

During the  the domestication of plants and animals further increased human access to solar energy. Fields devoted to crops were enriched by inedible plant matter, providing sugars anfor future harvests. Animals which had previously only provided humans with meat and tools once they were killed were now used for labour throughout their lives, fueled by  inedible to humans.

The more recent discoveries of and  are modern extensions of this trend. These are the remnants of ancient plant and animal matter, formed using energy from sunlight and then trapped within the earth for millions of years. Because the stored energy in these  has accumulated over many millions of years, they have allowed modern humans to massively increase the production and consumption of . As the amount of fossil fuel is large but finite, this cannot continue indefinitely, and various theories exist as to what will follow this stage of human civilization (e.g.

The existence of nearly allis fueled by light from the sun. Mos, such as plants, use the energy of sunlight, combined with carbon dioxide and water, to produce simple sugars—a process known as These sugars are then used as building blocks and in other synthetic pathways which allow the organism to grow.

such as animals, use light from the sun indirectly by consuming the products of autotrophs, either directly or by consuming other heterotrophs. The sugars and other molecular components produced by the autotrophs are then broken down, releasing stored solar energy, and giving the heterotroph the energy required for survival. This process is known as

In humans began to further extend this process by putting plant and animal materials to other uses. They used animal skins for warmth, for example, or wooden weapons to hunt. These skills allowed humans to harvest more of the sunlight than was possible through glycolysis alone, and human population began to grow.

 

Composition

Composition

The spectrum o striking the  is 100 to 10⁶  This can be divided into five regions in increasing order of

The of the's solar radiation is close to that of a y with a temperature of about 5,80 About half that lies in the short-wave part of the  and the other half mostly in the near- part. Some also lies in the  part of the spectrum When ultraviolet radiation is not absorbed by the atmosphere or other protective coating, it can cause damage to the skin known as or trigger an adaptive change in human

• Ultraviolet C or (UVC) range, which spans a range of 100 to 280 nm. The term ultraviolet refers to the fact that the radiation is at higher frequency than violet light (and, hence also invisible to the human eye). Owing to absorption by the atmosphere very little reaches the Earth's surfaceThis spectrum of radiation has  properties, and is used in 
• Ultraviolet B or (UVB) range spans 280 to 315 nm. It is also greatly absorbed by the atmosphere, and along with UVC is responsible for the eading to the production of the 
• Ultraviolet A or (UVA) spans 315 to 400 nm. It has been traditionally held as less damaging to the , and hence used inand  for
• Visible range orspans 380 to 780 nm. As the name suggests, it is this range that is visible to the naked human eye.

 

Sunlight

Sunlight, in the broad sense, is the total of  given off by the On , sunlight ithrough the atmosphere, and solar radiation is obvious as when the Sun is above the

When the direct solar radiation is not blocked by clouds, it is experienced as sunshine, a combination of bright light and . When it is blocked by the clouds or reflects off of other objects, it is experienced as diffused light.

Direct sunlight has a luminous efficacy of about 93per watt of radiant flux, which includes  , and light. Bright sunlight provides of approximately 100,000 or per square meter at the Earth's surface.

Sunlight is a key factor in , a process vital foron Earth.

Theuses the term "sunshine duration" to mean the cumulative time during which an area receives direct  from the Sun of at least 120per square

Sunlight may be recorded using a or . Sunlight takes about 8.3 minutes to reach the Earth.

 

Fauna and flora

Fauna and flora

Some fauna includes the and many These animals adapted to live in deserts are called. Many desert animals (and plants) show especially clear evolutionary adaptations for water conservation or heat tolerance, and so are often studied in, and One well-studied example is the specializations of mammalian kidneys shown by desert-inhabiting species. Many examples o have been identified in desert organisms, including between and and and lizards.
Some flora includes shrubs, and the Most desert are drought- or salt-tolerant, such as Some store water in their leave roots, and stems. Other desert plants have long that penetrate to the water table if present, or have adapted to the weather by having wide-spreading to absorb water from a greater of the ground. Another adaptation is the development of small, spiny which shed less moisture than leaves with greater surface areas. The stems and leaves of some plants lower the surface velocity of sand-carrying winds and protect the ground from erosion. Even small fungi and microscopic plant organisms found on the soil surface (so-called can be a vital link in preventing erosion and providing support for other living organisms.
Although often thought of as characteristic desert plants, other types of plants have adapted well to the arid environment. They include the and families. Cold deserts have grasses and shrubs as dominant vegetation.
 

Definition

 Definition

Potential supplements the measurement of rainfall in providing a scientific measurement-based definition of a desert. The water budget of an area can be calculated using the formula P − PE ± S, wherein P is precipitation, PE is potential evapotranspiration rates and S is amount of surface storage of water. Evapotranspiration is the combination of water loss through atmospheric and through the life processes of plants. Potential evapotranspiration, then, is the amount of water that could evaporate in any given region. As an example receives about 300 millimeters (12 in) of rain per year, however about 2500 millimeters (100 in) of water could evaporate over the course of a year.^[ In other words, about 8 times more water could evaporate from the region than actually falls. Rates of evapotranspiration in cold regions such as Alaska are much lower because of the lack ofheat to aid in the evaporation process.

Measurement of alone cannot provide an accurate definition of what a desert is because being arid also depends on evaporation, which depends in part on For example, receives less than 250 millimeters (10 in) of precipitation per year, and is immediately recognized as being located in a desert due to its arid adapted plants. The of Alaska's also receives less than 250 millimeters (10 in) of precipitation per year and is often classified as a cold desert. Other regions of the world have cold deserts, including areas of the and other high altitude areas in other parts of the world. Polar deserts cover much of the ice free areas of the arctic and Antarctic.

Etymology

Etymology

English desert and its cognates (including and  desertodésert and desierto) all come from the dēsertum (originally "an abandoned place"), a participle of dēserere, "to abandon." (See  The correlation between aridity and sparse population is complex and dynamic, varying by culture, era, and technologies; thus the use of the word desert can cause confusion. In English prior to the 20th century, desert was often used in the sense of "unpopulated area", without specific reference to aridity; but today the word is most often used in its climate-science sense (an area of low precipitation)—and a desert may be quite heavily populated, with millions of inhabitants. Phrases such as "" and "" in previous centuries did not necessarily imply sand or aridity; their focus was the sparse population. However, the connotation of a hot, parched, and sandy place often influences today's popular interpretation of those phrases.

Geography

Geography

Many deserts are formed by; mountains blocking the path of precipitation to the desert (on the lee side of the mountain). Deserts are often composed of and surfaces. called and stony surfaces called surfaces compose a minority of desert surfaces. Exposures of terrain are typical, and reflect minimal soil development and sparseness of The soil is rocky because of the low chemical weathering, and the relative absence of a fraction.

Deserts are part of a wide classification of regions that, on an average annual basis, have a moisture deficit (i.e. they can potentially lose more than is received). Deserts are located where vegetation cover is sparse to almost nonexistent. Deserts take up about one fifth (20%) of the Earth's land surface Hot deserts usually have a large and seasonal temperature range, with high daytime temperatures, and low nighttime temperatures (due to extremely low In hot deserts the temperature in the daytime can reach 45 °C/113 °F or higher in the summer, and dip to 0 °C/32 °F or lower at nighttime in the winter. Water vapor in the atmosphere acts to trap long wave radiation from the ground, and dry desert air is incapable of blocking during the day (due to absence of clouds) or during the night. Thus, during daylight most of the s heat reaches the ground, and as soon as the sun sets the desert cools quickly by radiating its heat into space.

 

Mayan references to B'ak'tun 13

Mayan references to B'ak'tun 13

The present-day Maya, as a whole, do not attach much significance to b'ak'tun 13. Although the Calendar Round is still used by some Maya tribes in the Guatemalan highlands, the Long Count was employed exclusively by the classic Maya, and was only recently rediscovered by archaeologists.Mayan elder Apolinario Chile Pixtun and Mexican archaeologist Guillermo Bernal both note that "apocalypse" is a Western concept that has little or nothing to do with Mayan beliefs. Bernal believes that such ideas have been foisted on the Maya by Westerners because their own myths are "exhausted".Mayan archaeologist Jose Huchm has stated that "If I went to some Mayan-speaking communities and asked people what is going to happen in 2012, they wouldn't have any idea. That the world is going to end? They wouldn't believe you. We have real concerns these days, like rain"

What significance the classic Maya gave the 13th b'ak'tun is uncertain. Most classic Maya inscriptions are strictly historical and do not make any prophetic declarations.Two items in the Maya historical corpus, however, may mention the end of the 13th b'ak'tun:  Monument 6 and, possibly, the chilam balam.

 

Objections

Objections

Maya inscriptions occasionally reference future predicted events or commemorations that would occur on dates that lie beyond the completion of the 13th b'ak'tun. Most of these are in the form of "distance dates" where some Long Count date is given, together with a Distance Number that is to be added to the Long Count date to arrive at this future date. On the west panel at the in , a section of the text projects into the future to the 80th Calendar Round anniversary of the Palenque ruler  s accession to the throne (Pakal's accession occurred on 9.9.2.4.8; equivalent to 27 July 615 CE in the proleptic Gregorian calendar). It does this by commencing with Pakal's birthdate of 9.8.9.13.0 (24 March 603 CE Gregorian) and adding to it the Distance Number 10.11.10.5.8.This calculation arrives at the 80th Calendar Round since his accession, which lies over 4,000 years in the future from Pakal's time—the 21st of October in the year AD 4772

 

Coe's apocalyptic interpretation was repeated by other scholars through the early 1990s. In contrast, later researchers said that, while the end of the 13th b'ak'tun would perhaps be a cause for celebrationt did not mark the end of the calendar. "There is nothing in the Maya or Aztec or ancient Mesoamerican prophecy to suggest that they prophesied a sudden or major change of any sort in 2012," says Mayanist scholar Mark Van Stone. "The notion of a "Great Cycle" coming to an end is completely a modern invention." In 1990, Mayanist scholars  and  argued that the Maya "did not conceive this to be the end of creation, as many have suggested." Susan Milbrath, curator of Latin American Art and Archaeology at the  stated that "We have no record or knowledge that [the Maya] would think the world would come to an end" in 2012."For the ancient Maya, it was a huge celebration to make it to the end of a whole cycle," says Sandra Noble, executive director of the Foundation for the Advancement of Mesoamerican Studies in  To render December 21, 2012, as aor moment of cosmic shifting, she says, is "a complete fabrication and a chance for a lot of people to cash in.""There will be another cycle," says E. Wyllys Andrews V, director of the Middle American Research Institute (MARI). "We know the Maya thought there was one before this, and that implies they were comfortable with the idea of another one after this."

Mesoamerican Long Count calendar

Mesoamerican Long Count calendar

unlike the 52-year still used today among the Maya, the Long Count was linear, rather than cyclical, and kept time roughly in units of 20: 20 days made a uinal, 18 uinals (360 days) made a tun, 20 tuns made a k'atun, and 20 k'atuns (144,000 days) made up a b'ak'tun. Thus, the Mayan date of 8.3.2.10.15 represents 8 b'ak'tuns, 3 k'atuns, 2 tuns, 10 uinals and 15 days.

 

December 2012 marks the conclusion of a; a great cycle of years in the , which was used in  prior to the arrival of Europeans. Though the Long Count was most likely invented by the it has become closely associated with the  whose classic period lasted from 250 to 900 ADThe of the classic Maya has been substantially deciphered, meaning that a corpus of their written and inscribed material has survived from before the ^days.

2012 phenomenon

 2012 phenomenon

Scholars from various disciplines have dismissed the idea of catastrophe in 2012. Mainstream scholars state that predictions of impending doom are not found in any of the existing, and that the idea that the Long Count calendar "ends" in 2012 misrepresents  The  do not consider the date significant, and the classical sources on the subject are scarce and contradictory, suggesting that there was little if any universal agreement among them about what, if anything, the date might mean.

The 2012 phenomenon comprises a range of eschatological beliefs that cataclysmic or transformative events will occur on December 21, which is said to be the end-date of a 5,125-year-long cycle in the Count calendar. Various astronomical alignments and numerological formulae related to this date have been proposed.

A  interpretation of this transition posits that during this time Earth and its inhabitants may undergo a positive physical or  and that 2012 may mark the beginning of a new era.Others suggest that the 2012 date marks the r a similar catastrophe. Scenarios posited for the end of the world include the Earth's collision with a passing planet (often referred to as ) or or the arrival of the next solar maximum.

Earthing system

A protective earth (PE) connection ensures that all exposed conductive surfaces are at the same electrical potential as the surface of the Earth, to avoid the risk of electrical shock if a person touches a device in which an insulation fault has occurred. It ensures that in the case of an insulation fault (a "short circuit"), a very high current flows, which will trigger an overcurrent protection device  that disconnects the power supply.

A functional earth connection serves a purpose other than providing protection against  In contrast to a protective earth connection, a functional earth connection may carry a current during the normal operation of a device. Functional earth connections may be required by devices such as and  filters, some types of and various measurement instruments. Generally the protective earth is also used as a functional earth, though this requires care in some situations.

In systems, an  system defines the of the conductors relative to that of the Earth's conductive surface. The choice of earthing system has implications for the and of the power supply. Note that regulations for earthing (grounding) systems vary considerably among different countries.

 

Basic configuration

Basic configuration

Rainwater harvesting systems channel rainwater that falls on to a roof into storage via a system of gutters and pipes. The first flush of rainwater after a dry season should be allowed to run to waste as it will be contaminated with dust, bird droppings etc. Roof gutters should have sufficient incline to avoid standing water. They must be strong enough, and large enough to carry peak flows. should be covered to prevent mosquito breeding and to reduce evaporation losses, contamination and algal growth. Rainwater harvesting systems require regular maintenance and cleaning to keep the system hygienic.

 

Bibliography

Bibliography

• Hemenway, Toby. Gaia’s Garden: A Guide to Home-Scale Permaculture. Vermont: Chelsea Green Publishing Company, 2000.
• Lowes, P. (1987). "The Water Decade: Half Time". In in John Pickford (ed.). Developing World Water. London: Grosvenor Press International. pp. 16–17. 

• Ludwig, Art. Create an Oasis With Greywater: Choosing, Building, and Using Greywater Systems. California: Oasis Design, 1994.

• Pacey, Arnold, and Adrian Cullis. Rainwater Harvesting. UK: Intermediate Technology Publications, 1986.

·  Frasier, Gary, and Lloyd Myers. Handbook of Water Harvesting. Washington D.C.: U.S. Dept. of Agriculture, Agricultural Research Service, 1983

·  Geerts, S., Raes, D. (2009). Agric. Water Manage 96, 1275–1284

·  Gould, John, and Erik Nissen-Peterson. Rainwater Catchment Systems. UK: Intermediate Technology Publications, 1999.

 

Around the world

Around the world

In Australia rainwater harvesting is typically used to supplement the reticulated mains supply. In south east Queensland, households that harvested rainwater doubled each year from 2005 to 2008, reaching 40% penetration at that time (White, 2009 (PhD)).

·  Currently in China and Brazil, rooftop rainwater harvesting is being practiced for providing drinking water, domestic water, water for livestock, water for small irrigation and a way to replenish ground water levels. province in China and semi-arid   have the largest rooftop rainwater harvesting projects ongoing.

·  In rainwater harvesting has traditionally been practiced by the people of th.

·  In the law requires all new construction to include rainwater harvesting adequate for the residents.

·  The have a similar law.

·  In the Elephanta Caves and Kanheri Caves in rainwater harvesting alone has been used to supply in their water requirements.

·  In and, the houses of the people are frequently equipped with homebrew rainwater harvesters made from local, organic materials.

·  In the are often found in domestic gardens to collect rainwater which is then used to water the garden. However, the British government's Code For Sustainable Homes encourages fitting large underground tanks to new-build homes to collect rainwater for flushing toilets, washing clothes, watering the garden and washing cars. This reduces by 50% th of the groundwater is saline and communities rely on mud-lined rainwater

·  ponds to meet their drinking water needs throughout the dry season. Some of these ponds are centuries old and are treated with great reverence and respect.

·  Until 2009 in water rights laws almost completely restricted rainwater harvesting; a property owner who captured rainwater was deemed to be stealing it from those who have to take water from the watershed. Now, residential well owners that meet certain criteria may obtain a permit to install a rooftop precipitation collection system (SB 09-080). to 10 large scale pilot studies may also be permitted (HB 09-1129The main factor in persuading the Colorado Legislature to change the law was a 2007 study that found that in an average year, 97% of the precipitation that fell in, in the southern suburbs of never reached a stream—it was used by plants or evaporated on the ground. In New Mexico, rainwater catchment is mandatory for new dwellings in water.

 

Advantages in urban areas

Advantages in urban areas

Rainwater harvesting Rainwater harvesting can be adopted in cities to supplement the city's other water supplies, to increase soil moisture levels for urban greenery, to raise the through artificial recharge, to mitigate urban flooding and to improve the quality of groundwater. In urban areas of the developed world, at a household level, non-potable uses of harvested rainwater include bathroom (i.e. shower/bath/basin), flushing toilets and washing laundry. Indeed in areas it is superior to municipal water for laundry because of its compatibility with detergents and soaps. Rainwater may require treatment prior to use for drinking, depending on anthropogenic (e.g. vehicle exhaust) and natural (e.g. Coal.) contaminants,it is one of the most useful methods of saving water

In many houses away from the larger towns and cities routinely rely on rainwater collected from roofs as the only source of water for all household activities. This is almost inevitably the case for many

Rainwater harvesting can (a) assure an independent water supply during water restrictions, that is though somewhat dependent on end use and maintenance, (b)usually of acceptable quality for household needs and (c) renewable at acceptable volumes despite forecast climate change (CSIRO, 2003). It produces beneficial by reducing peak stormwater runoff and processing costs. In municipalities with systems, reducing storm runoff is especially important, because excess runoff during heavy storms leads to the discharge of raw sewage from when treatment plant capacity cannot handle the combined flow. RH systems are simple to install and operate. Running costs are negligible, and they provide water at the point of consumption.We are consuming this water for our basic needs.

Rain water harvesting law.-- Who Owns the Rain? Check water rights and water right law very well before buying land in USA. Some states have water law in favor of land owners,and some states own all water rights, leaving you only water rights they are willing to grant though permits. In Colorado you may not catch , collect or harvest rain water from your roof unless you first buy a permit. Depending on where you live, And if the government owns your rain water rights.

 

 

Rainwater harvesting

There are a number of types of systems to harvest rainwater ranging from very simple to the complex industrial systems. The rate at which water can be collected from either system is dependent on the plan area of the system, its efficiency, and the intensity of rainfall (i.e annual precipitation (mm per annum) x square meter of catchment area = litres per annum yield) ... a 200 square meter roof catchment catching 1,000mm PA yields 200 kLPA.

Rainwater harvesting is the accumulating and storing, of rainwater. It has been used to provide water for water for or to refill in a process called Rainwater collected from the roofs of houses, tents and local institutions, can make an important contribution to the availability of drinking water. Water collected from the ground, sometimes from areas which are especially prepared for this purpose, is called. In some cases, rainwater may be the only available, or economical, water source. Rainwater harvesting systems can be simple to construct from inexpensive local materials, and are potentially successful in most habitable locations. Roof rainwater can be of good quality and may not require treatment before consumption. Although some rooftop materials may produce rainwater that is harmful to human health, it can be useful in flushing toilets, washing clothes, watering the garden and washing cars; these uses alone halve the amount of water used by a typical home. Household rainfall catchment systems are appropriate in areas with an average rainfall greater than 200 mm (7.9 in) per year, and no other accessible water sources (Skinner and Cotton, 1992).

Culture

Mythology

In the rainbow is called meaning "the and dhanush is bow) of the god of and ". Another Indian mythology says the rainbow is the bow of the incarnation of It is called Ramdhonu in dhonuArabian Peninsula, the rainbow, called Qaus Quzaħ in Arabic, is the war bow of the god Quzaħ. (dhanush) meaning bow. Likewise, in mythology of

In a rainbow called th Bridge connects the realms of and homes of the gods and humans, respectively. The Irish's secret hiding place for his pot of gold is usually said to be at the end of the rainbow. This place is impossible to reach, because the rainbow is an optical effect which depends on the location of the viewer. When walking towards the end of a rainbow, it will appear to "move" further away (two people who simultaneously observe a rainbow at different locations will disagree about where a rainbow is).

The rainbow has a place in legend owing to its beauty and the historical difficulty in explaining the phenomenon.

In the rainbow was the rainbow was a slit in the sky sealed by goddess using stones of five different colours.

Then Ishtar arrived. She lifted up the necklace of great jewels that her father, Anu, had created to please her and said, "Heavenly gods, as surely as this jewelled necklace hangs upon my neck, I will never forget these days of the great flood. Let all of the gods except Enlil come to the offering. Enlil may not come, for without reason he brought forth the flood that destroyed my people."