Limestone

Limestone is a sedimentary rock composed largely of the mineral calcite (calcium carbonate: CaCO3). The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record. Calcium (along with nitrogen, phosphorus, and potassium) is a key mineral to plant nutrition: soils overlying limestone bedrock tend to be pre-fertilized with calcium. Limestone is an important stone for masonry and architecture, vying with only granite and sandstone to be the most commonly used architectural stone. Limestone is a key ingredient of quicklime, mortar, cement, and concrete. The solubility of limestone in water and weak acid solutions leads to important phenomena. Regions overlying limestone bedrock tend to have fewer visible groundwater sources (ponds and streams), as surface water easily drains downward through cracks in the limestone. While draining, water slowly (over thousands or millions of years) enlarges these cracks; dissolving the calcium-carbonate and carrying it away in solution. Most well-known natural cave systems are through limestone bedrock.

Description
Limestone often contains variable amounts of silica in the form of chert and/or flint, as well as varying amounts of clay, silt and sand as disseminations, nodules, or layers within the rock. The primary source of the calcite in limestone is most commonly marine organisms. These organisms secrete shells that settle out of the water column and are deposited on ocean floors as pelagic ooze or alternatively are conglomerated in a coral reef (see lysocline for information on calcite dissolution). Secondary calcite may also be deposited by supersaturated meteoric waters (groundwater that precipitates the material in caves). This produces speleothems such as stalagmites and stalactites. Another form taken by calcite is that of oolites (oolitic limestone) which can be recognized by its granular appearance.
Limestone makes up about 10% of the total volume of all sedimentary rocks. Limestones may also form in both lacustrine and evaporite depositional environments.
Calcite can be either dissolved by groundwater or precipitated by groundwater, depending on several factors including the water temperature, pH, and dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility in which it becomes less soluble in water as the temperature increases.
When conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together or it can fill fractures.
Karst topography and caves develop in carbonate rocks due to their solubility in dilute acidic groundwater. Cooling groundwater or mixing of different groundwaters will also create conditions suitable for cave formation.

Uses
Limestone is very common in architecture, especially in North America and Europe. Many landmarks across the world, including the Great Pyramid and its associated Complex in Giza, Egypt, are made of limestone. So many buildings in Kingston, Ontario, Canada were constructed from it that it is nicknamed the 'Limestone City'. On the island of Malta, a variety of limestone called Globigerina limestone was for a long time the only building material available, and is still very frequently used on all types of buildings and sculptures. Limestone is readily available and relatively easy to cut into blocks or more elaborate carving. It is also long-lasting and stands up well to exposure. However, it is a very heavy material, making it impractical for tall buildings, and relatively expensive as a building material.

Limestone was most popular in the late 19th and early 20th centuries. Train stations, banks and other structures from that era are normally made of limestone. Limestone is used as a facade on some skyscrapers, but only in thin plates for covering rather than solid blocks. In the United States, Indiana, most notably the Bloomington area, has long been a source of high quality quarried limestone, called Indiana limestone. Many famous buildings in London are built from Portland limestone.
Limestone was also a very popular building block in the Middle Ages in the areas where it occurred since it is hard, is durable, and commonly occurs in easily accessible surface exposures. Many medieval churches and castles in Europe are made of limestone. Beer stone was a popular kind of limestone for medieval buildings in southern England.
Limestone and marble are very reactive to acid solutions, making acid rain a significant problem. Many limestone statues and building surfaces have suffered severe damage due to acid rain. Acid-based cleaning chemicals can also etch limestone, which should only be cleaned with a neutral or mild alkaline-based cleaner.

Other uses include:
1) The manufacture of quicklime (calcium oxide) and slaked lime (calcium hydroxide);
2) Cement and mortar;
3) Pulverized limestone is used as a soil conditioner to neutralize acidic soil conditions;
4) Crushed for use as aggregate—the solid base for many roads;
5) Geological formations of limestone are among the best petroleum reservoirs;
6) As a reagent in desulfurizations;
7) Glass making, in some circumstances;
8) Added to paper, plastics, paint, tiles, and other materials as both white pigment and a cheap
filler.
9) Toothpaste
10) Suppression of methane explosions in underground coal mines
11) Added to bread and cereals as a source of calcium
12) Calcium supplement for poultry (when ground up)

Courthouse built of limestone in Manhattan, Kansas

Alcázar of Segovia in Spain

Unta Patah Rancangan Abu Jahal

Setelah pelbagai usaha oleh kaum Quraisy untuk menyekat dan menghapuskan penyebaran agama Islam menemui kegagalan, maka Abu Jahal semakin benci terhadap Rasulullah S.A.W. Kebencian Abu Jahal ini tidak ada tolok bandingnya, malah melebihi kebencian Abu Lahab terhadap Rasulullah S.A.W. Melihatkan agama Islam semakin tersebar, Abu Jahal pun berkata kepada kaum Quraisy di dalam suatu perhimpunan, "Hai kaumku! Janganlah sekali-kali membiarkan Muhammad menyebarkan ajaran barunya dengan sesuka hatinya. Ini adalah kerana dia telah menghina agama nenek moyang kita, dia mencela tuhan yang kita sembah. Demi Tuhan, aku berjanji kepada kamu sekalian, bahawa esok aku akan membawa batu ke Masjidil Haram untuk dibalingkan ke kepala Muhammad ketika dia sujud. Selepas itu, terserahlah kepada kamu semua samada mahu menyerahkan aku kepada keluarganya atau kamu membela aku dari ancaman kaum kerabatnya. Biarlah orang-orang Bani Hasyim bertindak apa yang mereka sukai." Tatkala mendengar jaminan daripada Abu Jahal, maka orang ramai yang menghadiri perhimpunan itu berkata secara serentak kepadanya, "Demi Tuhan, kami tidak akan sekali-kali menyerahkan engkau kepada keluarga Muhammad. Teruskan niatmu." Orang ramai yang menghadiri perhimpunan itu merasa bangga mendengar kata-kata yang diucapkan oleh Abu Jahal bahawa dia akan menghapuskan Muhammad kerana jika Abu Jahal berjaya menghapuskan Nabi Muhammad S.A.W bererti akan terhapuslah segala keresahan dan kesusahan mereka selama ini yang disebabkan oleh kegiatan Rasulullah S.A.W menyebarkan agama Islam di kalangan mereka. Dalam pada itu, terdapat juga para hadirin di situ telah mengira-ngira perbelanjaan untuk mengadakan pesta sekiranya Nabi Muhammad S.A.W berjaya dihapuskan. Pada pandangan mereka adalah mudah untuk membunuh Nabi Muhammad S.A.W yang dikasihi oleh Tuhan Yang Maha Esa serta sekalian penghuni langit. Padahal Allah tidak akan sekali-kali membiarkan kekasih-Nya diancam dan diperlakukan seperti binatang. Dengan perasaan bangga, keesokan harinya di sebelah pagi, Abu Jahal pun terus pergi ke Kaabah iaitu tempat biasa Nabi Muhammad S.A.W bersembahyang. Dengan langkahnya seperti seorang satria, dia berjalan dengan membawa seketul batu besar di tangan sambil diiringi oleh beberapa orang Quraisy yang rapat dengannya. Tujuan dia mengajak kawan-kawannya ialah untuk menyaksikan bagaimana nanti dia akan menghempapkan batu itu di atas kepala Nabi Muhammad S.A.W. Sepanjang perjalanan itu dia membayangkan bagaimana keadaan Nabi Muhammad nanti setelah kepalanya dihentak oleh batu itu. Dia tersenyum sendirian apabila membayangkan kepala Nabi Muhammad S.A.W pecah dan tidak bergerak lagi. Dan juga membayangkan bagaimana kaum Quraisy akan menyambutnya sebagai pahlawan yang telah berjaya membunuh musuh nombor satu mereka. Sebaik sahaja Abu Jahal tiba di perkarangan Masjidil Haram, dilihatnya Rasulullah S.A.W baru sahaja sampai dan hendak mengerjakan sembahyang. Dalam pada itu, Nabi Muhammad S.A.W tidak menyedari akan kehadiran Abu Jahal dan kawan-kawannya di situ. Baginda tidak pernah terfikir apa yang hendak dilakukan oleh Abu Jahal terhadap dirinya pada hari itu. Sebaik-baik sahaja Abu Jahal melihat Rasulullah S.A.W telah mula bersembahyang, dia berjalan perlahan-lahan dari arah belakang menuju ke arah Nabi Muhammad S.A.W. Abu Jahal melangkah dengan berhati-hati, setiap pergerakannya dijaga, takut disedari oleh baginda. Dari jauh kawan-kawan Abu Jahal memerhatikan dengan perasaan cemas bercampur gembira. Dalam hati mereka berkata, "Kali ini akan musnahlah engkau hai Muhammad." Sebaik sahaja Abu Jahal hendak menghampiri Nabi Muhammad S.A.W dan menghayun batu yang dipegangnya itu, tiba-tiba secepat kilat dia berundur ke belakang. Batu yang dipegangnya juga jatuh ke tanah. Mukanya yang tadi merah kini menjadi pucat lesi seolah-olah tiada berdarah lagi. Rakan-rakannya yang amat ghairah untuk melihat Nabi Muhammad S.A.W terbunuh, tercengang dan saling berpandangan. Kaki Abu Jahal seolah-olah terpaku ke bumi. Dia tidak dapat melangkahkan kaki walaupun setapak. Melihatkan keadaan itu, rakan-rakannya segera menarik Abu Jahal dari situ sebelum disedari oleh baginda. Abu Jahal masih terpinga-pinga dengan kejadian yang dialaminya. Sebaik sahaja dia sedar dari kejutan peristiwa tadi, rakan-rakannya tidak sabar untuk mengetahui apakah sebenarnya yang telah berlaku. Kawannya bertanya, "Apakah sebenarnya yang terjadi kepada engkau, Abu Jahal? Mengapa engkau tidak menghempapkan batu itu ke kepala Muhammad ketika dia sedang sujud tadi?" Akan tetapi Abu Jahal tetap membisu, rakan-rakannya semakin kehairanan. Abu Jahal yang mereka kenali selama ini seorang yang lantang berpidato dan menyumpah seranah Nabi S.A.W, tiba-tiba sahaja diam membisu. Dalam pada itu, Abu Jahal masih terbayang-bayang akan kejadian yang baru menimpanya tadi. Dia seolah-olah tidak percaya dengan apa yang dilihatnya, malah dia sendiri tidak menyangka perkara yang sama akan berulang menimpa dirinya. Perkara yang sama pernah menimpa Abu Jahal sewaktu Rasulullah S.A.W pergi ke rumah Abu Jahal apabila seorang Nasrani mengadu kepada baginda bahawa Abu Jahal telah merampas hartanya. Pada masa itu Abu Jahal tidak berani berkata apa-apa pada baginda apabila dia terpandang dua ekor harimau menjadi pengawal peribadi Rasulullah S.A.W. Kemudian setelah habis mereka menghujani Abu Jahal dengan pelbagai soalan, maka Abu Jahal pun mula bersuara, "Wahai sahabatku! Untuk pengetahuan kamu semua, sebaik sahaja aku menghampiri Muhammad hendak menghempapkan batu itu ke kepalanya, tiba-tiba muncul seekor unta yang besar hendak menendang aku. Aku amat terkejut kerana belum pernah melihat unta yang sebegitu besar seumur hidupku. Sekiranya aku teruskan niatku, nescaya akan matilah aku ditendang oleh unta itu, sebab itulah aku berundur dan membatalkan niatku." Rakan-rakan Abu Jahal berasa amat kecewa mendengar penjelasan itu, mereka tidak menyangka orang yang selama ini gagah dan beria-ia hendak membunuh Nabi Muhammad S.A.W hanya tinggal kata-kata sahaja. Orang yang selama ini diharapkan boleh menghapuskan Nabi Muhammad S.A.W dan pengaruhnya hanya berupaya bercakap seperti tin kosong sahaja. Setelah mendengar penjelasan dari Abu Jahal yang tidak memuaskan hati itu, maka mereka pun berkata kepada Abu Jahal dengan perasaan kehairanan, "Ya Abu Jahal, semasa kau menghampiri Muhammad tadi, kami memerhatikan engkau dari jauh tetapi kai tidak napak akan unta yang engkau katakan itu. Malah bayangnya pun kami tidak nampak." Rakan-rakan Abu Jahal mula sangsi dengan segala keterangan yang diberikan oleh Abu Jahal. Mereka menyangka Abu Jahal sentiasa mereka-reka cerita yang karut itu, mereka mula hilang kepercayaan terhadapnya. Akhirnya segala kata-kata Abu Jahal mereka tidak berapa endahkan lagi.

Netizen @ Cybercitizen

A Netizen (a portmanteau of Internet and citizen) or cybercitizen is a person actively involved in online communities.

Description
Netizens can use the Internet to engage in activities of extended social groups, such as giving and receiving viewpoints, furnishing information, fostering the Internet as an intellectual and a social resource, and making choices for the self-assembled communities. Generally, a netizen can be any user of the worldwide, unstructured forums of the Internet. The word netizen itself was coined by Michael Hauben. Netizens are Internet users who utilize the networks from their home, workplace, or school (among other places). Netizens try to be conducive to the Internet's use and growth. Netizens, who use and know about the network of networks, usually have a self-imposed responsibility to make certain that it is improved in its development while encouraging free speech and open access. Netizens' use of the Internet around the world has been marked by:

Medium Description
E-mail : Delivery of letters by means of the Internet, as a
replacement to the traditional based paper
correspondence letters.

Online chat : Establishing of one-on-one or group conversations by
means of the Internet.

Instant messaging : Software which enables real time conversations without the
need of using a website (in contrast to online chats).

Internet fora : Web Sites which serve to hold discussions in defined
subjects.

Online games : Multiplayer Computer games which are played through the
Internet.

Blog : A kind of log in which the writer(s) writes in it in any
possible subject in which he or she desires to talk discuss at
any time the author(s) so desires, and in which the writer(s)
control access to.

Feedback comment system : A Mechanism used in web sites to post responses from the
internet users, which is mostly used in the news web sites, in
blogs and in the other additional sites

File sharing : A technology which enables the internet users to share files
from their computers with other internet users, and in
return the same internet user is capable of downloading files
from the computer of other internet users. This enables the
fast distribution, not always legal, of software, music, etc.

Gopher : A distributed document search and retrieval network protocol
designed for the Internet. Its goal is to function as an
improved form of Anonymous FTP, enhanced with
hyperlinking features similar to that of the World Wide Web.

Wiki : A collection of web pages designed to enable anyone who
accesses it to contribute or modify content, using a simplified
markup language.

Internet Commerce: Netizens are citizens of the internet community dedicated to
the participation and civic responsibility of providing Internet
commerce resources to the netizens of the global Internet
community.

The term has been used most frequently recently in Korea where there are vigorous netizens movements. The election of President Roh Moo-hyun of South Korea in 2002 is widely attributed to the support for him among South Korean netizens, especially OhmyNews.

Kuiper belt

The Kuiper belt (pronounced /ˈkaɪpər/, rhyming with "viper"), sometimes called the Edgeworth-Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 55 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20–200 times as massive. Like the asteroid belt, it consists mainly of small bodies (remnants from the Solar System's formation). But while the asteroid belt is composed primarily of rock and metal, the Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water. It is home to at least three dwarf planets – Pluto, Haumea and Makemake.
Since the first discovery in 1992, the number of known Kuiper belt objects (KBOs) has increased to over a thousand, and more than 70 000 KBOs over 100 km in diameter are believed to reside there. The Kuiper belt was initially believed to be the main repository for periodic comets, those with orbits lasting less than 200 years. However, studies since the mid-1990s have shown that the Kuiper belt is dynamically stable, and that it is the farther scattered disc, a dynamically active region created by the outward motion of Neptune 4.5 billion years ago, that is their true place of origin. Scattered disc objects such as Eris are KBO-like bodies with extremely large orbits that take them as far as 100 AU from the Sun. The centaurs, comet-like bodies that orbit among the gas giants, are believed to originate there. Neptune's moon Triton is believed to be a captured KBO. Pluto, a dwarf planet, is the largest known member of the Kuiper belt. Originally considered a planet, it is similar to many other objects of the Kuiper belt, and its orbital period is identical to that of the KBOs known as "Plutinos".
The Kuiper belt should not be confused with the hypothesized Oort cloud, which is a thousand times more distant. The objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects, are collectively referred to as trans-Neptunian objects (TNOs)

Known objects in the Kuiper belt, derived from data from the Minor Planet Center. Objects in the main belt are coloured green, while scattered objects are coloured orange. The four outer planets are blue. Neptune's few known Trojan asteroids are yellow, while Jupiter's are pink. The scattered objects between the Sun and the Kuiper belt are known as centaurs. The scale is in astronomical units. The pronounced gap at the bottom is due to obscuration by the band of the Milky Way.

The precise origins of the Kuiper belt and its complex structure are still unclear, and astronomers are awaiting the completion of several wide-field survey telescopes such as Pan-STARRS and the future LSST, which should reveal many currently unknown KBOs. These surveys will provide data that will help determine answers to these questions. The Kuiper belt is believed to consist of planetesimals; fragments from the original protoplanetary disc around the Sun that failed to fully coalesce into planets and instead formed into smaller bodies, the largest less than 3000 km in diameter.
Modern computer simulations show the Kuiper belt to have been strongly influenced by Jupiter and Neptune, and also suggest that neither Uranus nor Neptune could have formed in situ beyond Saturn, as too little primordial matter existed at that range to produce objects of such high mass. Instead, these planets are believed to have formed closer to Jupiter, and migrated outwards during the course of the Solar System's early evolution. Eventually, the orbits shifted to the point where Jupiter and Saturn existed in an exact 2:1 resonance; Jupiter orbited the Sun twice for every one Saturn orbit. The gravitational pull from such a resonance ultimately disrupted the orbits of Uranus and Neptune, causing Neptune's orbit to move outward into the primordial planetesimal disk, which sent the disk into temporary chaos. As Neptune traveled along this modified orbit, it excited and scattered many TNO planetesimals into higher and more eccentric orbits, depleting the primordial population. However, the present most popular model still fails to account for many of the characteristics of the distribution and, quoting one of the scientific articles, the problems "continue to challenge analytical techniques and the fastest numerical modeling hardware and software".

Simulation showing Outer Planets and Kuiper Belt: a)Before Jupiter/Saturn 2:1 resonance b)Scattering of Kuiper Belt objects into the solar system after the orbital shift of Neptune c)After ejection of Kuiper Belt bodies by Jupiter

Kisah Nabi Musa a.s dan Nabi Khidir a.s

Lalu mereka dapati seorang dari hamba-hamba Kami (Nabi Khidir) yang telah kami kurniakan kepadanya rahmat dari Kami, dan Kami telah mengajarnya sejenis ilmu; dari sisi Kami.
Nabi Musa berkata kepadanya: “Bolehkah aku mengikutmu, dengan syarat engkau mengajarku dari apa yang telah diajarkan oleh Allah kepadamu, ilmu yang menjadi petunjuk bagiku?” Dia menjawab: “Sesungguhnya engkau (wahai Musa), tidak sekali-kali akan dapat bersabar bersamaku. Dan bagaimana engkau akan sabar terhadap perkara yang engkau tidak mengetahuinya secara meliputi?”

Nabi Musa berkata: ”Engkau akan dapati aku, InsyaAllah, orang yang sabar; dan aku tidak akan membantah sebarang perintahmu.” Dia (Nabi Khidir) menjawab: “Sekiranya engkau mengikutku, maka janganlah engkau bertanya kepadaku akan sesuatupun sehingga aku ceritakan halnya kepadamu.”

Lalu berjalanlah keduanya sehingga apabila mereka menaiki sebuah perahu, dia (Nabi Khidir) membocorkannya. Nabi Musa berkata: “Patutkah engkau membocorkannya sedang akibat perbuatan itu menenggelamkan penumpang-penumpangnya? Sesungguhnya engkau telah melakukan satu perkara yang besar.”

Dia (Nabi Khidir) menjawab: “Bukankah aku telah katakan, bahawa engkau tidak sekali-kali akan dapat bersabar bersamaku?” Nabi Musa berkata: “Janganlah engkau marah akan daku disebabkan aku lupa (akan syaratmu); dan janganlah engkau memberati daku dengan sebarang kesukaran dalam urusanku (menuntut ilmu).”

Kemudian keduanya berjalan lagi sehingga apabila mereka bertemu dengan seorang pemuda lalu dia (Nabi Khidir) membunuhnya. Nabi Musa berkata “Patutkah engkau membunuh satu jiwa yang bersih, yang tidak berdosa membunuh orang? Sesungguhnya engkau telah melakukan satu perbuatan yang mungkar!”

Dia (Nabi Khidir) menjawab: “Bukankah, aku telah katakana kepadamu, bahawa engkau tidak sekali-kali akan dapat bersabar bersamaku?” Nabi Musa berkata: “Jika aku bertanya kepadamu tentang sebarang perkara sesudah ini, maka janganlah engkau jadikan daku sahabatmu lagi; sesungguhnya engkau telah cukup mendapat alasan-alasan berbuat demikian disebabkan pertanyaan-pertanyaan dan bantahanku.”

Kemudian keduanya berjalan lagi, sehingga apabila mereka sampai kepada penduduk sebuah bandar, mereka meminta makan kepada orang-orang di situ, lalu orang-orang itu enggan menjamu mereka. Kemudian mereka dapati di situ sebuah tembok yang hendak runtuh, lalu dia (Nabi Khidir) emmbinanya. Nabi Musa berkata: “Jika engkau mahu, tentulah engkau berhak mengambil upah mengenainya!”

Dia (Nabi Khidir) menjawab: “Inilah masanya perpisahan antaraku denganmu, aku akan terangkan kepadamu maksud (kejadian-kejadian yang dimusykilkan) yang engkau tidak dapat bersabar mengenainya.

Adapun perahu itu adalah dipunyai oleh orang-orang miskin yang bekerja di laut; oleh itu aku bocorkan dengan tujuan hendak mencacatkannya kerana di belakang mereka nanti ada seorang raja yang merampas tiap-tiap sebuah perahu yang tidak cacat.

Adapun pemuda itu, kedua ibu bapanya adalah orang-orang yang beriman, maka kami bimbang bahawa dia akan mendesak mereka melakukan perbuatan yang zalim dan kufur. Oleh itu kami ingin dan berharap supaya Tuhan mereka gantikan bagi mereka anak yang lebih baik daripadanya tentang kebersihan jiwa dan lebih mesra kasih sayangnya.

Adapun tembok itu pula, adalah ia dipunyai oleh dua orang anak yatim di bandar itu; dan di bawahnya ada harta terpendam kepunyaan mereka; dan bapa mereka pula adalah orang yang shalih. Maka Tuhanmu menghendaki supaya mereka cukup umur dan dapat mengeluarkan harta mereka yang terpendam itu, sebagai satu rahmat dari Tuhanmu (kepada mereka). Dan (ingatlah) aku tidak melakukannya menurut fikiranku sendiri. Demikianlah penjelasan tentang maksud dan tujuan perkara-perkara yang engkau tidak dapat bersabar mengenainya.” (sumber - Al-Kahfi : 65-82)

Fata Morgana (mirage)

Fata Morgana are usually seen in the morning after a cold night which has resulted in the radiation of heat into space. In this form of mirage, objects on the horizon or even beyond the horizon, such as islands, cliffs, ships or icebergs, appear elongated and elevated, like "fairy tale castles".
This is how the mirage is caused: in calm weather, when warm air lies over cold dense air near the surface of the ground, the undisturbed interface between these two air masses can act as a refracting lens, producing an upside-down image, over which the distant direct image appears to hover.
The first mention of the "Fata Morgana" phenomenon in English was in 1818, when this type of mirage was observed in the Strait of Messina, between Calabria and Sicily. It is also commonly seen in high mountain valleys, such as the San Luis Valley of Colorado where the effect is exaggerated due to the curvature of the floor of the valley canceling out the curvature of the Earth. These mirages are also seen in Arctic seas on very still mornings, and are common on Antarctic ice shelves.

Superior mirage

A superior mirage occurs when the air below the line of sight is colder than that above. This is called a temperature inversion, since it does not represent the normal equilibrium temperature gradient of the atmosphere. Since in this case the light rays are bent down, the image appears above the true object, hence the name superior. They are in general less common than inferior mirages, but when they do occur they tend to be more stable, as cold air has no tendency to move up and warm air no tendency to move down.
Superior mirages are most common in polar regions, especially over large sheets of ice with a uniform low temperature. They also occur at more moderate latitudes, however, although in that case they are weaker and not so smooth. For example a distant shoreline may be made towering, looking higher (and thus perhaps closer) than it is in reality, but because of the turbulences there seem to be dancing spikes, towers and so forth. This type of mirage is also called the Fata Morgana or, in Icelandic, halgerndingar.
Superior images can be right-side-up or upside down, depending on the distance of the true object and the temperature gradient. Often the image appears as a distorted mixture of up and down parts.
If the Earth were flat, superior images would not be as interesting. Light rays which bent down would soon hit the ground, and only close objects would be affected. Since the Earth is round, if the amount of downward bending is about equal to the curvature of the Earth, light rays can travel large distances, perhaps from beyond the horizon. This was observed for the first time in 1596, when a ship under the command of Willem Barents looking for the Northeast passage got stuck in the ice at Novaya Zemlya, and the crew had to endure the polar winter there. They saw their midwinter night ending with the rise of a distorted sun about 2 weeks earlier than expected. It was not until the 20th century that Europeans understood the reason: the real sun had still been under their horizon, but its light rays followed the curvature of the Earth. This effect is often called a Novaya Zemlya mirage. For every 100 kilometres (62 mi) the light rays can travel parallel to the Earth's surface, the sun will appear 1° higher on the horizon. The inversion layer must have just the right temperature gradient over the whole distance to make this possible. In the same way, ships which are in reality so far away that they should not be visible above the geometric horizon may appear on the horizon, or even above the horizon, as superior mirages. This may explain some stories about flying ships or coastal cities in the sky, as described by some polar explorers. These are examples of so-called Arctic mirages, or hillingar in Icelandic.
If the vertical temperature gradient is +11°C per 100 meters (reminder: positive means getting hotter when going up), horizontal light rays will just follow the curvature of the Earth, and the horizon will appear flat. If the gradient is less the rays are not bent enough and get lost in space. That is the normal situation of a spherical, convex horizon. But if the gradient gets larger, say 18°C per 100 meters, the observer will see the horizon turned upwards, being concave, as if he were standing at the bottom of a saucer.

A hot road mirage, "fake water" on the road, the most
common example of an inferior mirage

An inferior mirage on the Mojave Desert in spring

Speed of light

The speed of light normally refers the speed of light in a vacuum, and is an important physical constant in modern physics. Light travels at different speeds through different materials, but in vacuum light travels fastest, and the speed does not vary with the color, intensity, or direction of travel. Perhaps more surprisingly, the speed also does not depend on the motion of the light emitter or the observer. Therefore it makes sense to speak of the speed of light, which is the speed of light in vacuum and usually written as c. The speed of light factors into much of modern physics, including special relativity, general relativity, and quantum mechanics.

By everyday standards, light travels very rapidly - approximately 300,000 km each second, in vacuum or air. This is roughly a million times faster than sound, and fast enough to circle the Earth more than 7 times in one second Such a rapid speed is very hard to measure without specialized techniques, and in ancient times the speed of light was the subject of speculation. The first effective measurements of the speed of light were made in the seventeenth century, and were progressively refined. Today, time intervals can be measured extremely precisely, to the point where the metre is now defined officially as the distance light travels in "vacuum" in 1⁄299,792,458 of a second. As a consequence, according to NIST: "… the effect of this definition is to fix the speed of light in vacuum at exactly 299 792 458 m/s."

Speed of light in different units
metres per second : 299,792,458 (exact)
km per hour : 1,079,252,848.8 (exact)
miles per hour : ≈ 670,616,629.3844
miles per second : ≈ 186,282.39705122

Approximate length of time for light to travel...
One foot :0.98 nanoseconds
One metre : 3.3 nanoseconds
One km : 3.3 microseconds
One mile : 5.4 microseconds
Around Earth's equator : 0.13 seconds
From Earth to geostationary orbit and back : 0.24 seconds
From Earth to the moon : 1.3 seconds
From Earth to the sun : 8.3 minutes
To Earth from Alpha Centauri : 4.4 years
From edge to edge of the Milky Way : 100,000 years

Practical effect of the finite speed of light

The speed of light plays an important part in many modern sciences and technologies. Radar systems measure the distance to a target by measuring the time taken for an echo of the light pulse to return. Similarly, a global positioning system (GPS) receiver measures its distance to satellites based on how long it takes for a radio signal to arrive from the satellite. The distances to the moon, planets, and spacecraft are determined by measuring the round-trip travel time.
Another effect of the finite speed of light is stellar aberration. Suppose we look at a star with a telescope idealized as a narrow tube. The light enters the tube from a star at angle θ and travels at speed c taking a time h/c to reach the bottom of the tube, where our eye detects the light. Suppose observations are made from Earth, which is moving with a speed v. During the transit of the light, the tube moves a distance vh/c. Consequently, for the photon to reach the bottom of the tube, the tube must be inclined at an angle φ different from θ , resulting in an apparent position of the star at angle φ.
In astronomy beyond the solar system, distances are often measured in light-years, the distance light travels in a year.
In electronic systems, despite their small size, the speed of light can become a limiting factor in their maximum speed of operation.

As light propagates down the telescope, the telescope moves requiring a tilt to the telescope that depends on the speed of light. The apparent angle of the star φ differs from its true angle θ, a phenomenon called stellar aberration

The blue glow in this "swimming pool" nuclear reactor is Čerenkov radiation, emitted as a result of electrons traveling faster than the speed of light in water.

Vortex

A vortex (plural: vortices) is a spinning, often turbulent, flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex. The speed and rate of rotation of the fluid are greatest at the center, and decrease progressively with distance from the center.

Vortex created by the passage of an aircraft wing, revealed by colored smoke

Properties

Vortices display some special properties:
The fluid pressure in a vortex is lowest in the center where the speed is greatest, and rises progressively with distance from the center.This is in accordance with Bernoulli's Principle. The core of a vortex in air is sometimes visible because of a plume of water vapor caused by condensation in the low pressure of the core. The spout of a tornado is a classic and frightening example of the visible core of a vortex. A dust devil is also the core of a vortex, made visible by the dust drawn upwards by the turbulent flow of air from ground level into the low pressure core.
The core of every vortex can be considered to contain a vortex line, and every particle in the vortex can be considered to be circulating around the vortex line. Vortex lines can start and end at the boundary of the fluid or form closed loops. They cannot start or end in the fluid. (See Helmholtz's theorems.) Vortices readily deflect and attach themselves to a solid surface. For example, a vortex usually forms ahead of the propeller disk or jet engine of a slow-moving airplane. One end of the vortex line is attached to the propeller disk or jet engine, but when the airplane is taxiing the other end of the vortex line readily attaches itself to the ground rather than end in midair. The vortex can suck water and small stones into the core and then into the propeller disk or jet engine.
Two or more vortices that are approximately parallel and circulating in the same direction will quickly merge to form a single vortex. The circulation of the merged vortex will equal the sum of the circulations of the constituent vortices. For example, a sheet of small vortices flows from the trailing edge of the wing or propeller of an airplane when the wing is developing lift or the propeller is developing thrust. In less than one wing chord downstream of the trailing edge of the wing these small vortices merge to form a single vortex. If viewed from the tail of the airplane, looking forward in the direction of flight, there is one wingtip vortex trailing from the left-hand wing and circulating clockwise, and another wingtip vortex trailing from the right-hand wing and circulating anti-clockwise. The result is a region of downwash behind the wing, between the pair of wingtip vortices. These two wingtip vortices do not merge because they are circulating in opposite directions.
Vortices contain a lot of energy in the circular motion of the fluid. In an ideal fluid this energy can never be dissipated and the vortex would persist forever. However, real fluids exhibit viscosity and this dissipates energy very slowly from the core of the vortex. (See Rankine vortex). It is only through dissipation of a vortex due to viscosity that a vortex line can end in the fluid, rather than at the boundary of the fluid. For example, the wingtip vortices from an airplane dissipate slowly and linger in the atmosphere long after the airplane has passed. This is a hazard to other aircraft and is known as wake turbulence.

Observations
A vortex can be seen in the spiraling motion of air or liquid around a center of rotation. Circular current of water of conflicting tides form vortex shapes. Turbulent flow makes many vortices. A good example of a vortex is the atmospheric phenomenon of a whirlwind or a tornado or dust devil. This whirling air mass mostly takes the form of a helix, column, or spiral. Tornadoes develop from severe thunderstorms, usually spawned from squall lines and supercell thunderstorms, though they sometimes happen as a result of a hurricane.
In atmospheric physics, a mesovortex is on the scale of a few miles (smaller than a hurricane but larger than a tornado). [2] On a much smaller scale, a vortex is usually formed as water goes down a drain, as in a sink or a toilet. This occurs in water as the revolving mass forms a whirlpool. This whirlpool is caused by water flowing out of a small opening in the bottom of a basin or reservoir. This swirling flow structure within a region of fluid flow opens downward from the water surface.

Instances
1. In the hydrodynamic interpretation of the behaviour of electromagnetic fields, the acceleration of electric fluid in a particular direction creates a positive vortex of magnetic fluid. This in turn creates around itself a corresponding negative vortex of electric fluid.
2. Smoke ring : A ring of smoke which persists for a surprisingly long time, illustrating the slow rate at which viscosity dissipates the energy of a vortex.
3. Lift-induced drag of a wing on an aircraft.
4. The primary cause of drag in the sail of a sloop.
5. Whirlpool: a swirling body of water produced by ocean tides or by a hole underneath the vortex where the water would drain out, such as a bathtub. A large, powerful whirlpool is known as a maelstrom. In popular imagination, but only rarely in reality, can they have the dangerous effect of destroying boats. Examples are Scylla and Charybdis of classical mythology in the Straits of Messina, Italy; the Naruto whirlpools of Nankaido, Japan; the Maelstrom, Lofoten, Norway.
6. Tornado : a violent windstorm characterized by a twisting, funnel-shaped cloud. A less violent version of a tornado, over water, is called a waterspout.
7. Hurricane : a much larger, swirling body of clouds produced by evaporating warm ocean water and influenced by the Earth's rotation. Similar, but far greater, vortices are also seen on other planets, such as the permanent Great Red Spot on Jupiter and the intermittent Great Dark Spot on Neptune.
8. Polar vortex : a persistent, large-scale cyclone centered near the Earth's poles, in the middle and upper troposphere and the stratosphere.
9. Sunspot : dark region on the Sun's surface (photosphere) marked by a lower temperature than its surroundings, and intense magnetic activity.
10. The accretion disk of a black hole or other massive gravitational source.
11. Spiral galaxy : a type of galaxy in the Hubble sequence which is characterized by a thin, rotating disk. Earth's galaxy, the Milky Way, is of this type.

Gemstone

Sapphire
Sapphire (Greek: sappheiros) refers to gem varieties of the mineral corundum, an aluminium oxide (α-Al2O3), when it is a color other than red, in which case the gem would instead be a ruby. Trace amounts of other elements such as iron, titanium, or chromium can give corundum blue, yellow, pink, purple, orange, or greenish color. Pink-orange corundum are also sapphires, but are instead called padparadscha.
Because it is a gemstone, sapphire is commonly worn as jewelry. Sapphire can be found naturally, or manufactured in large crystal boules. Because of its remarkable hardness, sapphire is used in many applications, including infrared optical components, watch crystals, high-durability windows, and wafers for the deposition of semiconductors.

Sapphire from Madagascar

Natural sapphires
Sapphire is one of the two gem varieties of corundum, the other being the red ruby. Although blue is the most well known hue, sapphire is any color of corundum except red. Sapphire may also be colorless, and it also occurs in the non-spectral shades gray and black. Pinkish-orange sapphire is known as padparadscha.
The cost of natural sapphire varies depending on their color, clarity, size, cut, and overall quality as well as geographic origin. Significant sapphire deposits are found in Eastern Australia, Thailand, Sri Lanka, Madagascar, East Africa and in the United States at various locations (Gem Mountain) and in the Missouri River near Helena, Montana. Sapphire and rubies are often found together in the same area, but one gem is usually more abundant.

Blue sapphire
Color in gemstones breaks down into three components: hue, saturation, and tone. Hue is most commonly understood as the "color" of the gemstone. Saturation refers to the vividness or brightness or "colorfulness" of the hue, and tone is the lightness to darkness of the hue. Blue sapphire exists in various mixtures of its primary and secondary hues, various tonal levels (shades) and at various levels of saturation (brightness): the primary hue must, of course, be blue.
Blue sapphires are evaluated based upon the purity of their primary hue. Purple, violet and green are the normal secondary hues found in blue sapphires. Violet and purple can contribute to the overall beauty of the color, while green is considered a distinct negative. Blue sapphires with no more than 15% violet or purple are generally said to be of fine quality. Blue sapphires with any amount of green as a secondary hue are not considered to be fine quality. Gray is the normal saturation modifier or mask found in blue sapphires. Gray reduces the saturation or brightness of the hue and therefore has a distinctly negative effect.
The color of fine blue sapphires can be described as a vivid medium dark violet to purplish blue where the primary blue hue is at least 85% and the secondary hue no more than 15% without the least admixture of a green secondary hue or a gray mask. The 422.99 carats (84.60 g) Logan sapphire in the National Museum of Natural History, Washington D.C. is one of the largest faceted gem-quality blue sapphires in the world.

The 422.99 carats (84.60 g) blue Logan sapphire

Color change sapphire
A rare variety of sapphire, known as color change sapphire, exhibits different colors in different light. Color change sapphires are blue in outdoor light and purple under incandescent indoor light. Color changes may also be pink in daylight to greenish under fluorescent light. Some stones shift color well and others only partially, in that some stones go from blue to bluish purple. While color change sapphires come from a variety of locations, the gem gravels of Tanzania is the main source.
Certain synthetic color-change sapphires are sold as “lab” or “synthetic” alexandrite, which is accurately called an alexandrite simulant (also called alexandrium) since the latter is actually a type of chrysoberyl---an entirely different substance whose pleochroism is different and much more pronounced than color-change corundum (sapphire).

Treatments

Sapphires may be treated by several methods to enhance and improve their clarity and color. It is common practice to heat natural sapphires to improve or enhance color. This is done by heating the sapphires to temperatures between 500 and 1800 °C for several hours, or by heating in a nitrogen-deficient atmosphere oven for seven days or more. Low Tube heating is where the stone is placed in a ceramic pot over charcoal, in which a man blows air through a bamboo tube to the charcoal creating more heat. The stone becomes a more blue in color but loses some of the silk. When high heat temperatures are used, the stone loses all of the silk and becomes clear under magnification. Evidence of sapphire and other gemstones being subjected to heating goes back to, at least, Roman times. Un-heated stones are quite rare and will often be sold accompanied by a certificate from an independent gemological laboratory attesting to "no evidence of heat treatment".
Diffusion treatments are somewhat more controversial as they are used to add elements to the sapphire for the purpose of improving colors. Typically beryllium (Be) is diffused into a sapphire with very high heat, just below the melting point of the sapphire. Initially (c. 2000) orange sapphires were created with this process, although now the process has been advanced and many colors of sapphire are often treated with beryllium. It is unethical to sell beryllium-treated sapphires without disclosure, and the price should be much lower than a natural gem or one that has been enhanced by heat alone.
Treating stones with surface diffusion is generally frowned upon; as stones chip or are repolished/refaceted the 'padparadscha' colored layer can be removed. (There are some diffusion treated stones in which the color goes much deeper than the surface, however.) The problem lies in the fact that treated padparadschas are at times very difficult to detect, and they are the reason that getting a certificate from a reputable gemological lab (e.g. Gubelin, SSEF, AGTA, etc.) is recommended before investing in a padparadscha.
According to Federal Trade Commission guidelines, in the United States, disclosure is required of any mode of enhancement that has a significant effect on the gem's value

Extrasolar planet

Gliese 581 c

Gliese 581 c (pronounced /ˈɡliːzə/) or Gl 581 c is a "Super-Earth", a large extrasolar terrestrial planet, orbiting the red dwarf star Gliese 581. Assuming that the planet's mass is close to the lower limit determined by radial velocity measurements (the true mass is unknown), it was the smallest known extrasolar planet around a main sequence star, but on April 21, 2009, another planet orbiting Gliese 581, Gliese 581 e, was announced with an approximate mass of 1.9 earth masses, which is now the smallest known extrasolar planet around a main sequence star. Gliese 581 c generated interest because it was initially reported to be the first potentially Earth-like planet in the habitable zone of its star, with a temperature right for liquid water on its surface, and by extension, potentially capable of supporting extremophile forms of Earth-like life. However, further research on the potential effects of the planetary atmosphere casts doubt upon the habitability of Gliese 581 c and indicates that the fourth planet in the system, Gliese 581 d, is a better candidate for habitability. In astronomical terms, the Gliese 581 system is relatively close to Earth, at 20.3 light years (192 trillion km or 119 trillion miles) in the direction of the constellation of Libra. This distance, along with the declination and right ascension coordinates, give its exact location in our galaxy. It is identified as Gliese 581 by its number in the Gliese Catalogue of Nearby Stars; it is the 87th closest known star system to the Sun.


Discovery
The discovery of the planet by the team of Stéphane Udry University of Geneva's Observatory in Switzerland was announced on April 24, 2007. The team used the HARPS instrument (an echelle spectrograph) on the European Southern Observatory 3.6 m Telescope in La Silla, Chile, and employed the radial velocity technique to identify the planet's influence on the star. The Canadian-built MOST space telescope was used to conduct a follow-up study over the next six weeks. No transit was detected over this time, so a direct measurement of the planet has not yet been possible; however, the star's apparent magnitude changed very little, indicating that it provides a stable source of light and heat to Gliese 581 c.
The team released a paper of their findings dated April 27, 2007, published in the July, 2007 journal Astronomy and Astrophysics. In the paper they also announced the discovery of another planet in the system, Gliese 581 d, with a minimum mass of 7.7 Earth masses and a semi-major axis of 0.25 astronomical units.

Physical characteristics

Mass
The existence of Gliese 581 c and its mass have been measured by the radial velocity method of detecting extrasolar planets. The mass of a planet is calculated by the small periodic movements around a common centre of mass between the host star Gliese 581 and its planets. When all four planets are fit with a Keplerian solution, the minimum mass of the planet is determined to be 5.36 Earth masses.

Radius
Since Gliese 581 c has not been detected directly, there are no measurements of its radius. Furthermore, the radial velocity method used to detect it only puts a lower limit on the planet's mass, which means theoretical models of planetary radius and structure can only be of limited use. However, assuming a random orientation of the planet's orbit, the true mass is likely to be close to the measured minimum mass.

Scale comparison of the relative sizes of the Earth and Gliese 581c, assuming Gliese 581c is a rocky body with a mass close to the minimum mass determined by the radial velocity method.

Orbit
Gliese 581 c has an orbital period ("year") of 13 Earth days and its orbital radius is only about 7% that of the Earth, about 11 million km, while the Earth is 150 million kilometres from the Sun. Since the host star is smaller and colder than the Sun—and thus less luminous—this distance places the planet on the "warm" edge of the habitable zone around the star according to Udry's team. Note that in astrophysics, the "habitable zone" is defined as the range of distances from the star at which a planet could support liquid water on its surface: it should not be taken to mean that the planet's environment would be suitable for humans, a situation which requires a more restrictive range of parameters. A typical radius for an M0 star of Gliese 581's age and metallicity is 0.00128 AU, against the sun's 0.00465 AU. This proximity means that the primary star should appear 3.75 times wider and 14 times larger in area for an observer on the planet's surface looking at the sky than the Sun appears to be from Earth's surface.

Habitability and Climate
The study of Gliese 581 c by the von Bloh et al. team has been quoted as concluding "The super-Earth Gl 581c is clearly outside the habitable zone, since it is too close to the star. "The study by Selsis et al. claims even "a planet in the habitable zone is not necessarily habitable" itself, and this planet "is outside what can be considered the conservative habitable zone" of the parent star, and further that if there was any water there and it was lost when the red dwarf was a strong X-ray and EUV emitter, it could have surface temperatures ranging from 700 K to 1000 K (427 to 727 °C). Temperature speculations by other scientists are based on the temperature of (and heat from) the parent star Gliese 581 and have been calculated without factoring in the wide margin of error (96 °C/K) for the star's temperature of 3432 K to 3528 K.

Planetary habitable zones of the Solar System and the Gliese 581 system compared.

Source from Wikipedia

Nuclear Radiation

The release of radiation is a phenomenon unique to nuclear explosions. There are several kinds of radiation emitted; these types include gamma, neutron, and ionizing radiation, and are emitted not only at the time of detonation (initial radiation) but also for long periods of time afterward (residual radiation).

Initial Nuclear Radiation

Initial nuclear radiation is defined as the radiation that arrives during the first minute after an explosion, and is mostly gamma radiation and neutron radiation.
The level of initial nuclear radiation decreases rapidly with distance from the fireball to where less than one roentgen may be received five miles from ground zero. In addition, initial radiation lasts only as long as nuclear fission occurs in the fireball. Initial nuclear radiation represents about 3 percent of the total energy in a nuclear explosion.
Though people close to ground zero may receive lethal doses of radiation, they are concurrently being killed by the blast wave and thermal pulse. In typical nuclear weapons, only a relatively small proportion of deaths and injuries result from initial radiation.

Residual Nuclear Radiation

The residual radiation from a nuclear explosion is mostly from the radioactive fallout. This radiation comes from the weapon debris, fission products, and, in the case of a ground burst, radiated soil.
There are over 300 different fission products that may result from a fission reaction. Many of these are radioactive with widely differing half-lives. Some are very short, i.e., fractions of a second, while a few are long enough that the materials can be a hazard for months or years. Their principal mode of decay is by the emission of beta particles and gamma radiation.

Radiation Effects on Humans

Certain body parts are more specifically affected by exposure to different types of radiation sources. Several factors are involved in determining the potential health effects of exposure to radiation. These include:

1) The size of the dose (amount of energy deposited in the body)
2) The ability of the radiation to harm human tissue
3) Which organs are affected

The most important factor is the amount of the dose - the amount of energy actually deposited in your body. The more energy absorbed by cells, the greater the biological damage. Health physicists refer to the amount of energy absorbed by the body as the radiation dose. The absorbed dose, the amount of energy absorbed per gram of body tissue, is usually measured in units called rads. Another unit of radation is the rem, or roentgen equivalent in man. To convert rads to rems, the number of rads is multiplied by a number that reflects the potential for damage caused by a type of radiation. For beta, gamma and X-ray radiation, this number is generally one. For some neutrons, protons, or alpha particles, the number is twenty.

Hair

The losing of hair quickly and in clumps occurs with radiation exposure at 200 rems or higher.

Brain

Since brain cells do not reproduce, they won't be damaged directly unless the exposure is 5,000 rems or greater. Like the heart, radiation kills nerve cells and small blood vessels, and can cause seizures and immediate death.

Thyroid

The certain body parts are more specifically affected by exposure to different types of radiation sources. The thyroid gland is susceptible to radioactive iodine. In sufficient amounts, radioactive iodine can destroy all or part of the thyroid. By taking potassium iodide can reduce the effects of exposure.

Blood System

When a person is exposed to around 100 rems, the blood's lymphocyte cell count will be reduced, leaving the victim more susceptible to infection. This is often refered to as mild radiation sickness. Early symptoms of radiation sickness mimic those of flu and may go unnoticed unless a blood count is done.According to data from Hiroshima and Nagaski, show that symptoms may persist for up to 10 years and may also have an increased long-term risk for leukemia and lymphoma. For more information, visit Radiation Effects Research Foundation.

Heart

Intense exposure to radioactive material at 1,000 to 5,000 rems would do immediate damage to small blood vessels and probably cause heart failure and death directly.

Gastrointestinal Tract

Radiation damage to the intestinal tract lining will cause nausea, bloody vomiting and diarrhea. This is occurs when the victim's exposure is 200 rems or more. The radiation will begin to destroy the cells in the body that divide rapidly. These including blood, GI tract, reproductive and hair cells, and harms their DNA and RNA of surviving cells.

Reproductive Tract

Because reproductive tract cells divide rapidly, these areas of the body can be damaged at rem levels as low as 200. Long-term, some radiation sickness victims will become sterile.

Dose-rem Effects
5-20 Possible late effects; possible chromosomal damage.

20-100 Temporary reduction in white blood cells.

100-200 Mild radiation sickness within a few hours: vomiting,
diarrhea, fatigue; reduction in resistance to infection.

200-300 Serious radiation sickness effects as in 100-200 rem and
hemorrhage; exposure is a Lethal Dose to 10-35% of the
population after 30 days (LD 10-35/30).

300-400 Serious radiation sickness; also marrow and intestine
destruction; LD 50-70/30.

400-1000 Acute illness, early death; LD 60-95/30.

1000-5000 Acute illness, early death in days; LD 100/10.

Effects of Nuclear Weapons

The Fireball

The fireball, an extremely hot and highly luminous spherical mass of air and gaseous weapon residues, occurs within less than one millionth of one second of the weapon's detonation. Immediately after its formation, the fireball begins to grow in size, engulfing the surrounding air. This growth is accompanied by a decrease in temperature because of the accompanying increase in mass. At the same time the fireball rises, like a hot-air balloon. Within seven-tenth of one millisecond from detonation, the fireball from a 1-megaton weapon is about 134 meters across, and this increases to a maximum value of about 1.73 kilometers in 10 seconds. It is then rising at rate of 76 - 106 meters per second. After a minute, the fireball has cooled to such an extent that it no longer emits visible radiation. It has then risen roughly 7.2 kilometers from the point of burst.

Illustrated components of a nuclear explosion.

The Mushroom Cloud

As the fireball increases in size and cools, the vapors condense to form a cloud containing solid particles of the weapon debris, as well as many small drops of water derived from the air sucked into the rising fireball.

The early formation of the mushroom cloud.

Depending on the height of burst, a strong updraft with inflowing winds, called "afterwinds," are produced. These afterwinds can cause varying amount of dirt and debris to be sucked up from the earth's surface into the cloud. In an air burst with a moderate (or small) amount of dirt and debris drawn up into the cloud, only a relatively small proportion become contaminated with radioactivity. For a burst near the ground, however, large amounts of dirt and debris are drawn into the cloud during formation.

The color of cloud is initially red or reddish brown, due to the presence of nitrous acid and oxides of nitrogen. As the fireball cools and condensation occurs, the color changes to white, mainly due to water droplets (as in an ordinary cloud)

The cloud consists chiefly or very small particles of radioactive fiision products and weapon residues, water droplets, and larger particles of dirt and debris carried up by the afterwinds.

The eventual height reached by the radioactive cloud depends upon the heat energy of the weapon and upon the atmospheric conditions. If the cloud reaches the tropopause, about 9.6 - 12.87 kilometer above the Earth's surface, there is a tendency for it to spread out. But if sufficient energy remains in the radioactive cloud at this height, a portion of it will ascend into the more stable air of the stratosphere.

The mushroom cloud forming at the Nevada Test Site.

The cloud attains its maximum height after about 10 minutes and is then said to be "stabilized". It continue to grow laterally, however, to produce the characteristic mushroom shape. The cloud may continue to be visible for about and hour or more before being dispersed by the winds into the surrounding atmosphere where it merges with natural clouds in the sky.

Thermal Pulse Effects

One of the important differences between a nuclear and conventional weapon is the large proportion of a nuclear explosion's energy that is released in the form of thermal energy. This energy is emitted from the fireball in two pulses. The first is quite short, and carries only about 1 percent of the energy; the second pulse is more significant and is of longer duration (up to 20 seconds).

The thermal pulse charring the paint

The energy from the thermal pulse can initiate fires in dry, flammable materials, such as dry leaves, grass, old newspaper, thin dark flammable fabrics, etc. The incendiary effect of the thermal pulse is also substantially affected by the later arrival of the blast wave, which usually blows out any flames that have already been kindled. However, smoldering material can reignite later.

The major incendiary effect of nuclear explosions is caused by the blast wave. Collapsed structures are much more vulnerable to fire than intact ones. The blast reduces many structures to piles of kindling, the many gaps opened in roofs and walls act as chimneys, gas lines are broken open, storage tanks for flammable materials are ruptured. The primary ignition sources appear to be flames and pilot lights in heating appliances (furnaces, water heaters, stoves, etc.). Smoldering material from the thermal pulse can be very effective at igniting leaking gas.
Thermal radiation damage depends very strongly on weather conditions. Cloud cover, smoke, or other obscuring material in the air can considerably reduce effective damage ranges versus clear air conditions.

The energy from the thermal pulse can initiate fires in dry, flammable materials, such as dry leaves, grass, old newspaper, thin dark flammable fabrics, etc. The incendiary effect of the thermal pulse is also substantially affected by the later arrival of the blast wave, which usually blows out any flames that have already been kindled. However, smoldering material can reignite later.

Effects of the thermal pulse on clothing

Thermal radiation also affects humans both directly - by flash burns on exposed skin - and indirectly - by fires started by the explosion.