The Rainbow Formation

A rainbow is most often viewed as a circular arc in the sky. An observer on the ground observes a half-circle of color with red being the color perceived on the outside or top of the bow. Those who are fortunate enough to have seen a rainbow from an airplane in the sky may know that a rainbow can actually be a complete circle. Observers on the ground only view the top half of the circle since the bottom half of the circular arc is prevented by the presence of the ground (and the rather obvious fact that suspended water droplets isn’t present below ground). Yet observers in an airborne plane can often look both upward and downward to view the complete circular bow.

The circle (or half-circle) results because there are a collection of suspended droplets in the atmosphere that are capable concentrating the dispersed light at angles of deviation of 40-42 degrees relative to the original path of light from the sun. These droplets actually form a circular arc, with each droplet within the arc dispersing light and reflecting it back towards the observer. Every droplet within the arc is refracting and dispersing the entire visible light spectrum (ROYGBIV). As described above, the red light is refracted out of a droplet at steeper angles towards the ground than the blue light. Thus, when an observer sights at a steeper angle with respect to the ground, droplets of water within this line of sight are refracting the red light to the observer's eye. The blue light from these same droplets is directed at a less steep angle and is directed along a trajectory that passes over the observer's head. Thus, it is the red light that is seen when looking at the steeper angles relative to the ground. Similarly, when sighting at less steep angles, droplets of water within this line of sight are directing blue light to the observer's eye while the red light is directed downwards at a steeper angle towards the observer's feet. This discussion explains why it is the red light that is observed at the top and on the outer perimeter of a rainbow and the blue light that is observed on the bottom and the inner perimeter of the rainbow.

Rainbows are not limited to the dispersion of light by raindrops. The splashing of water at the base of a waterfall caused a mist of water in the air that often results in the formation of rainbows. A backyard water sprinkler is another common source of a rainbow. Bright sunlight, suspended droplets of water and the proper angle of sighting are the three necessary components for viewing one of nature's most splendid masterpieces.

A collection of suspended water droplets in the atmosphere serves as a refractor of light. The water represents a medium with a different optical density than the surrounding air. Light waves refract when they cross over the boundary from one medium to another. The decrease in speed upon entry of light into a water droplet causes a bending of the path of light towards the normal. And upon exiting the droplet, light speeds up and bends away from the normal. The droplet causes a deviation in the path of light as it enters and exits the drop.

There are countless paths by which light rays from the sun can pass through a drop. Each path is characterized by this bending towards and away from the normal. One path of great significance in the discussion of rainbows is the path in which light refracts into the droplet, internally reflects, and then refracts out of the droplet. The diagram at the right depicts such a path. A light ray from the sun enters the droplet with a slight downward trajectory. Upon refracting twice and reflecting once, the light ray is dispersed and bent downward towards an observer on earth's surface. Other entry locations into the droplet may result in similar paths or even in light continuing through the droplet and out the opposite side without significant internal reflection. But for the entry location shown in the diagram at the right, there is an optimal concentration of light exiting the airborne droplet at an angle towards the ground. As in the case of the refraction of light through prisms with nonparallel sides, the refraction of light at two boundaries of the droplet results in the dispersion of light into a spectrum of colors. The shorter wavelength blue and violet light refract a slightly greater amount than the longer wavelength red light. Since the boundaries are not parallel to each other, the double refraction results in a distinct separation of the sunlight into its component colors.

The angle of deviation between the incoming light rays from the sun and the refracted rays directed to the observer's eyes is approximately 42 degrees for the red light. Because of the tendency of shorter wavelength blue light to refract more than red light, its angle of deviation from the original sun rays is approximately 40 degrees. As shown in the diagram, the red light refracts out of the droplet at a steeper angle toward an observer on the ground. There are a multitude of paths by which the original ray can pass through a droplet and subsequently angle towards the ground. Some of the paths are dependent upon which part of the droplet the incident rays contact. Other paths are dependent upon the location of the sun in the sky and the subsequent trajectory of the incoming rays towards the droplet. Yet the greatest concentration of outgoing rays is found at these 40-42 degree angles of deviation. At these angles, the dispersed light is bright enough to result in a rainbow display in the sky. Now that we understand the path of light through an individual droplet, we can approach the topic of how the rainbow forms.

Red Sky in the Afternoon

As the sun begins to set, the light must travel farther through the atmosphere before it gets to you. More of the light is reflected and scattered. As less reaches you directly, the sun appears less bright. The color of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes.

Another explanation is When sunlight hits the molecules in the atmosphere, the red and yellow wavelengths tend to pass straight through, or get absorbed by the molecules (which warms the air and gives rise to the world's climate). The blue wavelength is shorter and more energetic than the red and yellow wavelengths, and it reacts much more with the air molecules. It causes the air molecules to vibrate and’re-emit' the light.. Because the molecules vibrate in all directions, the light is emitted in all directions (called 'scattering') , The re-emitted radiation may then have the same reaction with other molecules, and it gets 'bounced around' from molecule to molecule. It turns red, because of the low angle of the sunlight, the blue light has already been scattered away, and we see more of the red and yellow wavelength, hence the colors of the setting sun.

Why sky is blue?

The atmosphere is the mixture of gas molecules and other materials surrounding the earth. It is made mostly of the gases nitrogen (78%), and oxygen (21%). Argon gas and water (in the form of vapor, droplets and ice crystals) are the next most common things. There are also small amounts of other gases, plus many small solid particles, like dust, soot and ashes, pollen, and salt from the oceans.

The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.

However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue.

A pot of gold at the end of a rainbow

There is an old tradition that a pot of gold lies under the end of every rainbow, and that the first person to reach the spot may secure the treasure. Unfortunately, it is impossible to reach the end of a rainbow—when the observer moves he sees a new rainbow that appears to be the same distance away.

It is also explained as a metaphor that after all the life you live, and after everything you have went through, rewards become apparent after work, and though they may seem to be impossible to reach, they are there if only you believe they are, such as growing old. You may find this as a burden, a burden at the end of a long journey, but if you just believe in the good that growing old has brought you, you see the immense knowledge you have gained, the family that surrounds you, the impacts you have made on the world.

NEWS: Earthquake

New earthquake strikes Pakistan's BalochistanQuake measuring 6.8 in magnitude in Awaran, with epicentre about 30km away from previous tremor, claims more lives.

At least 15 people have died in a new earthquake in the southwestern Pakistani province of Balochistan, where hundreds of people have already died and thousands more been affected by a previous tremor.

Saturday's earthquake measured 6.8 in magnitude, and occurred about 96km northeast of the city of Awaran, the US Geological Survey (USGS) said on Saturday.

The quake occurred at a depth of 14.8km, which was similar to Tuesday's 7.7 magnitude earthquake. The epicentres of the two tremors were about 30km apart, according to USGS data.

"It was an aftershock it was an independent earthquake," Zahid Rafi, director of the National Seismic Centre of Pakistan, told local news television station Geo TV.not

An AFP news agency reporter in Awaran said that hundreds of patients being treated in the aftermath of the previous quake fled a hospital in panic as the new tremor hit.

At least 359 people were killed, and another 765 injured, when Tuesday's earthquake struck the impoverished region of southwestern Pakistan, according to the Provincial Disaster Management Authority's official figures.

Unofficial estimates put the number of dead at more than 500.

The government says that more than 185,000 people have been affected by the tremor, and that rescue and relief activities are being undertaken by the civil administration in conjunction with the army.

The population of Awaran district is scattered over more than 21,000 square kilometres of remote and rugged terrain, where infrastructure is limited, with few medical facilities or even roads.

While Saturday's earthquake was recorded as a fresh event, there have been 16 aftershocks in the region of Awaran since the first major earthquake on Tuesday, USGS data showed.

NEWS: Tsunami

Western Indian Ocean Earthquake and Tsunami Hazard Potential Greater Than Previously Though

Earthquakes similar in magnitude to the 2004 Sumatra earthquake could occur in an area beneath the Arabian Sea at the Makran subduction zone, according to recent research published in Geophysical

Research Letters.

The research was carried out by scientists from the University of Southampton based at the National Oceanography Centre Southampton (NOCS), and the Pacific Geoscience Centre, Natural Resources Canada.The study suggests that the risk from undersea earthquakes and associated tsunami in this area of the Western Indian Ocean -- which could threaten the coastlines of Pakistan, Iran, Oman, India and potentially further afield -- has been previously underestimated. The results highlight the need for further investigation of pre-historic earthquakes and should be fed into hazard assessment and planning for the region.

Subduction zones are areas where two of Earth's tectonic plates collide and one is pushed beneath the other. When an earthquake occurs here, the seabed moves horizontally and vertically as the pressure is released, displacing large volumes of water that can result in a tsunami.The Makran subduction zone has shown little earthquake activity since a magnitude 8.1 earthquake in 1945 and magnitude 7.3 in 1947. Because of its relatively low seismicity and limited recorded historic earthquakes it has often been considered incapable of generating major earthquakes.Plate boundary faults at subduction zones are expected to be prone to rupture generating earthquakes at temperatures of between 150 and 450 °C. The scientists used this relationship to map out the area of the potential fault rupture zone beneath the Makran by calculating the temperatures where the plates meet. Larger fault rupture zones result in larger magnitude earthquakes."Thermal modelling suggests that the potential earthquake rupture zone extends a long way northward, to a width of up to 350 kilometres which is unusually wide relative to most other subduction zones,"

says Gemma Smith, lead author and PhD student at University of Southampton School of Ocean and Earth Science, which is based at NOCS.

The team also found that the thickness of the sediment on the subducting plate could be a contributing factor to the magnitude of an earthquake and tsunami there."If the sediments between the plates are too weak then they might not be strong enough to allow the strain between the two plates to

build up," says Smith. "But here we see much thicker sediments than usual, which means the deeper sediments will be more compressed and warmer. The heat and pressure make the sediments stronger. This results in the shallowest part of the subduction zone fault being potentially capable of slipping during an earthquake."These combined factors mean the Makran subduction zone is potentially capable of producing major earthquakes, up to magnitude 8.7-9.2. Past assumptions may have significantly underestimated the earthquake and tsunami hazard in this region."

NEWS: Typhoon

Floods, landslides kill 20 in Zambales

MANILA, Philippines – At least 20 persons were killed in landslides and floods in Zambales Monday at the height of heavy rains spawned by the storm-enhanced southwest monsoon, reports said.Subic Mayor Jun Khonghun said in a radio interview that 10 people were killed in a landslide in Cawag village in Subic that occurred around 6 a.m. Monday. Two were still missing.Meanwhile, in San Isidro village, five were killed from a landslide and one was still missing. A report from First Lieutenant Yvonne Ricaforte, Civil Military Operations officer of the Army’s 24th Infantry Battalion said that a landslide in San Marcelino at about 6a.m., town killed four persons.Based on reports from the Office of Civil Defense in Central Luzon, a 67-year-old woman drowned in Olongapo.

In a separate television interview, Khonghun said that roads in Subic were already impassable and “isolated.” Speedboats were used to get around the area.

“We are asking for help from the national government. Please assist us,” he said.The heavy rains in Luzon were caused by the prevailing southwest monsoon that is strengthened by a storm over the Pacific Ocean. “Pabuk” will not enter the country but it will trigger heavy rains.

Rainy weather persists in Luzon due to ‘habagat’

MANILA, Philippines – Luzon continues to experience light to moderate rainshowers Sunday morning due to the southwest monsoon (habagat), as super typhoon Odette (international name Usagi) moves away from the country.The Philippine weather bureau said Metro Manila, Cavite, Batangas and portions of Rizal and Quezon should expect light to moderate with occasional heavy rains.Meanwhile, moderate to occasional heavy rains are expected to prevail over Pampanga, Zambales, Bataan and parts of Bulacan in the next three hours, the Philippine Atmospheric, Geophysical and Astronomical Services Administration said in its 8:30 a.m. rainfall advisory.Pagasa said Western Visayas and the rest of Luzon will have light to moderate rains and thunderstorms while Mindanao and other parts of Visayas will have cloudy weather with isolated rainshowers.

Meteor, Meteorites, Comets and Asteroids

METEORS

A meteor or "shooting star" is the visible streak of light from a meteoroid or micrometeoroid, heated and glowing from entering the Earth's atmosphere, as it sheds glowing material in its wake. Meteors typically occur in the mesosphere at altitudes between 76 km to 100 km (46–62 miles). The root word meteor comes from the Greek meteōros, meaning "suspended in the air".

Millions of meteors occur in the Earth's atmosphere daily. Most meteoroids that cause meteors are about the size of a pebble. Meteors may occur in showers, which arise when the Earth passes through a stream of debris left by a comet, or as "random" or "sporadic" meteors, not associated with a specific stream of space debris. A number of specific meteors have been observed, largely by members of the public and largely by accident, but with enough detail that orbits of the meteoroids producing the meteors have been calculated. All of the orbits passed through the asteroid belt. The atmospheric velocities of meteors result from the movement of Earth around the Sun at about 30 km/s (18 miles/second),[20] the orbital speeds of meteoroids, and the gravity well of Earth.

Meteors become visible between about 75 to 120 km (34–70 miles) above the Earth. They usually disintegrate at altitudes of 50 to 95 km (31–51 miles).[21] Meteors have roughly a fifty percent chance of a daylight (or near daylight) collision with the Earth. Most meteors are, however, observed at night, when darkness allows fainter objects to be recognised. For bodies with a size scale larger than (10 cm to several meters) meteor visibility is due to the atmospheric ram pressure (not friction) that heats the meteoroid so that it glows and creates a shining trail of gases and melted meteoroid particles. The gases include vaporized meteoroid material and atmospheric gases that heat up when the meteoroid passes through the atmosphere. Most meteors glow for about a

second. A relatively small percentage of meteoroids hit the Earth's atmosphere and then pass out again: these are termed Earth-grazing fireballs (for example The Great Daylight 1972 Fireball). The visible light produced by a meteor may take on various hues, depending on the chemical composition of the meteoroid, and the speed of its movement through the atmosphere. As layers of the meteoroid abrade and ionize, the color of the light emitted may change according to the layering of minerals. Possible colors (and elements producing them) include:

Orange/yellow (sodium) Yellow (iron) Blue/green (copper) Purple (potassium) Red (silicate)

METEORITES

A meteorite originates in outer space as a solid piece of debris from such sources as asteroids or comets that survives its impact with the Earth's surface. It is called a meteoroid before its impact. A meteorite's size can range from small to extremely large. When a meteoroid enters the atmosphere, frictional, pressure, and chemical interactions with the atmospheric gases cause it to heat up and radiate that energy, thus forming a fireball, also known as a meteor or shooting/falling star. The term bolide refers to either an extraterrestrial body that collides with the Earth, or to an exceptionally bright, fireball-like meteor regardless of whether it ultimately impacts the surface.

More generally, a meteorite on the surface of any celestial body is a natural object that has come from somewhere in space. Meteorites have been found on the Moon and Mars. Meteorites that are recovered after being observed as they transit the atmosphere or impact the Earth is called Meteorite fall. All other meteorites are known as finds. As of February 2010, there are approximately 1,086 witnessed falls having specimens in the world's collections. In contrast, there are more than 38,660 well-documented meteorite finds.

Meteorites have traditionally been divided into three broad categories: stony meteorites are rocks, mainly composed of silicate minerals; iron meteorites that are largely composed of metallic iron-nickel; and, stony-iron meteorites that contain large amounts of both metallic and rocky material. Modern classification schemes divide meteorites into groups according to their structure, chemical and isotopic composition and mineralogy. Meteorites smaller than 2mm are classified as micrometeorites.

TYPES OF METEORITES:

Iron Meteorites

Iron meteorites were once part of the core of a long-vanished planet or large asteroid and are believed to have originated within the Asteroid Belt between Mars and Jupiter. They are among the densest materials on earth and will stick very strongly to a powerful magnet. Iron meteorites are far heavier than most earth rocks-if you've ever lifted up a cannon ball or a slab or iron or steel, you'll get the idea.

These alloys grow into a complex interlocking crystalline pattern known as the Widmanstätten Pattern, after Count Aloys de Widmanstätten who described the phenomenon in the 19th Century. This remarkable lattice-like arrangement can be very beautiful and is normally only visible when iron meteorites are cut into slabs, polished, and then etched with a mild solution of nitric acid. The kamacite crystals revealed by this process are measured and the average bandwidth is used to subdivide iron meteorites into a number of structural classes. An iron with very narrow bands, less than 1mm, would be a "fine octahedrite" and those with wide bands would be called "coarse octahedrites."

Stone Meteorites

The largest group of meteorites is the stones, and they once formed part of the outer crust of a planet or asteroid. Many stone meteorites-particularly those that have been on the surface of our planet for an extended period of time-frequently look much like terrestrial rocks, and it can take a skilled eye to spot them when meteorite hunting in the field. Freshly fallen stones will exhibit a black fusion crust, created as the surface literally burned during flight, and the vast majority of stones contain enough iron for them to easily adhere to a powerful magnet.

Some stone meteorites contain small, colorful, grain-like inclusions known as "chondrules." These tiny grains originated in the solar nebula, and therefore pre-date the formation of our planet and the rest of the solar system, making them the oldest known matter available to us for study. Stone meteorites that contain these chondrules are known as "chondrites."

Space rocks without chondrites are known as "achondrites." These are volcanic rocks from space which formed from igneous activity within their parent bodies where melting and recrystallization eradicated all trace of ancient chondrules. Achondrites contain little or no extraterrestrial iron, making them much more difficult to find than most other meteorites, though specimens often display a remarkable glossy fusion crust which looks almost like enamel paint.

Stony-Iron Meteorites

The least abundant of the three main types, the stony-irons, account for less than 2% of all known meteorites. They are comprised of roughly equal amounts of nickel-iron and stone and are divided into two groups: pallasites and mesosiderites. The stony-irons are thought to have formed at the core/mantle boundary of their parent bodies.

Pallasites are perhaps the most alluring of all meteorites, and certainly of great interest to private collectors. Pallasites consist of a nickel-iron matrix packed with olivine crystals. When olivine crystals are of sufficient purity, and display an emerald-green color, they are known as the gemstone peridot. Pallasites take their name from a German zoologist and explorer, Peter Pallas, who described the Russian meteorite Krasnojarsk, found near the Siberian capital of the same name in the 18th Century. When cut and polished into thin slabs, the crystals in pallasites become translucent giving them a remarkable otherworldly beauty.

The mesosiderites are the smaller of the two stony-iron groups. They contain both nickel-iron and silicates and usually show an attractive, high-contrast silver and black matrix when cut and polished-the seemingly random mixture of inclusions leading to some very striking features. The word mesosiderite is derived from the Greek for "half" and "iron," and they are very rare. Of the thousands of officially cataloged meteorites, fewer than one hundred are mesosiderites.

COMETS

A comet is an icy small Solar System body that, when passing close to the Sun, displays a visible atmosphere or coma and sometimes also a tail. These phenomena are due to the effects of solar radiation and the solar wind upon the nucleus of the comet. Comet nuclei range from a few hundred metres to tens of kilometres across and are composed of loose collections of ice, dust, and small rocky particles. Comets have been observed and recorded since ancient times by many different cultures.Comets have a wide range of orbital periods, ranging from several years to several millions of years. Short-period comets originate in the Kuiper belt or its associated scattered disc, which lie beyond the orbit of Neptune. Longer-period comets are thought to originate in the Oort cloud, a spherical cloud of icy bodies extending from outside the Kuiper Belt to halfway to the next nearest star. Long-period comets plunge toward the Sun from the Oort cloud because of gravitational perturbations caused by either the massive outer planets of the Solar System (Jupiter, Saturn, Uranus, and Neptune) or passing stars. Rare hyperbolic comets pass once through the inner Solar System before being flung out to interstellar space along hyperbolic trajectories.

Comets are distinguished from asteroids by the presence of an extended, gravitationally unbound atmosphere surrounding their central nucleus. This atmosphere has parts termed the coma (the central atmosphere immediately surrounding the nucleus) and the tail (a typically linear section consisting of dust or gas blown out from the coma by the Sun's light pressure or outstreaming solar wind plasma). However, extinct comets that have passed close to the Sun many times have lost nearly all of their volatile ices and dust and may come to resemble small asteroids. Asteroids are thought to have a different origin from comets, having formed inside the orbit of Jupiter rather than in the outer Solar System. The discovery of main-belt comets and active centaurs has blurred the distinction between asteroids and comets.

As of July 2013 there were 4,894 known comets, and this number is steadily increasing. However, this represents only a tiny fraction of the total potential comet population, as the reservoir of comet-like bodies in the outer Solar System may number one trillion.[6] Roughly one comet per year is visible to the naked eye, though many of these are faint and unspectacular. Particularly bright examples are called "Great Comets".

CHARACTERISTIC OF A COMET:

Physical Characteristics

The solid nucleus or core of a comet consists mostly of ice and dust coated with dark organic material, with the ice composed mainly of frozen water but perhaps other frozen substances as well, such as ammonia, carbon dioxide, carbon monoxide and methane. The nucleus might have a small rocky core.

As a comet gets closer to the sun, the ice on the surface of the nucleus begins turning into gas, forming a cloud known as the coma. Radiation from the sun pushes dust particles away from the coma, forming a dust tail, while charged particles from the sun convert some of the comet's gases into ions, forming an ion tail. Since comet tails are shaped by sunlight and the solar wind, they always point away from the sun.

The nuclei of most comets are thought to measure 10 miles (16 km) or less. Some comets have comas that can reach nearly 1 million miles (1.6 million kilometers) wide, and some have tails reaching 100 million miles (160 million kilometers) long.

We can see a number of comets with the naked eye when they pass close to the sun because their comas and tails reflect sunlight or even glow because of energy they absorb from the sun. However, most comets are too small or too faint to be seen without a telescope.

Comets leave a trail of debris behind them that can lead to meteor showers on Earth. For instance, the Perseid meteor shower occurs every year between August 9 and 13 when the Earth passes through the orbit of the Swift-Tuttle comet.

Orbital Characteristics

Asteroids classify comets based on the durations of their orbits around the sun. Short-period comets need roughly 200 years or less to complete one orbit, long-period comets take more than 200 years, and single-apparition comets are not bound to the sun, on orbits that take them out of the solar system. Recently, scientist have also discovered comets in the main asteroid belt — these main-belt comets might be a key source of water for the inner terrestrial planets.

Scientists think short-period comets, also known as periodic comets, originate from a disk-shaped band of icy objects known as the Kuiper belt beyond Neptune's orbit, with gravitational interactions with the outer planets dragging these bodies inward, where they become active comets. Long-period comets are thought to come from the nearly spherical Oort cloud even further out, which get slung inward by the gravitational pull of passing stars.

Some comets, called sun-grazers, smash right into the sun or get so close that they break up and evaporate.

ASTEROIDS

Asteroids are minor planets, especially those of the inner Solar System. Especially the larger ones have also been called planetoids. These terms have historically been applied to any astronomical object orbiting the Sun that did not show the disk of a planet and was not observed to have the characteristics of an active comet, but as small objects in the outer Solar System were discovered, their volatile-based surfaces were found to more closely resemble comets and so were often distinguished from traditional asteroids. Thus the term asteroid has come increasingly to refer specifically to the small bodies of the inner Solar System out to the orbit of Jupiter. They are grouped with the outer bodies—centaurs, Neptune trojans, and trans-Neptunian objects—as m inor planets, which is the term preferred in astronomical circles. In this article the term "asteroid" refers to the minor planets of the inner Solar System.

ASTEROID BELT

There are millions of asteroids, many thought to be the shattered remnants of planetesimals, bodies within the young Sun's solar nebula that never grew large enough to become planets. The large majority of known asteroids orbit in the asteroid belt between the orbits of Mars and Jupiter or co-orbital with Jupiter (the Jupiter Trojans). However, other orbital families exist with significant populations, including the near-Earth asteroids. Individual asteroids are classified by their characteristic spectra, with the majority falling into three main groups: C-type, S-type, and M-type. These were named after and are generally identified with carbon-rich, stony, and metallic compositions, respectively.Only one asteroid, 4 Vesta, which has a relatively reflective surface, is normally visib le to the naked eye, and this only in very dark skies when it is favorably positioned. Rarely, small asteroids passing close to Earth may be visible to the naked eye for a short time.

The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets. The asteroid belt is also termed the main asteroid belt or main belt to distinguish its members from other asteroids in the Solar System such as near-Earth asteroids and trojan asteroids.

About half the mass of the belt is contained in the four largest asteroids, Ceres, Vesta, Pallas, and Hygiea. These have mean diameters of more than 400 km, whereas Ceres, the asteroid belt's only dwarf planet, is about 950 km in diameter.The remaining bodies range down to the size of a dust particle. The asteroid material is so thinly distributed that numerous unmanned spacecraft have traversed it without incident. Nonetheless, collisions between large asteroids do occur, and these can form an asteroid family whose members have similar orbital characteristics and compositions. It was once thought that collisions of asteroids produce a fine dust that forms a major component of the zodiacal light. However, Nesvorny and Jenniskens (2010 Astrophysical Journal) attributed 85 percent of the Zodiacal Light dust to fragmentations of Jupiter-family comets, rather than from comets and collisions between asteroids in the asteroid belt. Individual asteroids within the asteroid belt are categorized by their spectra, with most falling into three basic groups: carbonaceous (C-type), silicate (S-type), and metal-rich (M-type).

The asteroid belt formed from the primordial solar nebula as a group of planetesimals, the smaller precursors of the planets, which in turn formed protoplanets. Between Mars and Jupiter, however, gravitational perturbations from Jupiter imbued the protoplanets with too much orbital energy for them to accrete into a planet. Collisions became too violent, and instead of fusing together, the planetesimals and most of the protoplanets shattered. As a result, 99.9% of the asteroid belt's original mass was lost in the first 100 million years of the Solar System's history.[5] Some fragments can eventually find their way into the inner Solar System, leading to meteorite impacts with the inner planets. Asteroid orbits continue to be appreciably perturbed whenever their period of revolution about the Sun forms an orbital resonance with Jupiter. At these orbital distances, a Kirkwood gap occurs as they are swept into other orbits.

Contrary to popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that it would be improbable to reach an asteroid without aiming carefully. Nonetheless, hundreds of thousands of asteroids are currently known, and the total number ranges in the millions or more, depending on the lower size cutoff. Over 200 asteroids are known to be larger than 100 km,[42] and a survey in the infrared wavelengths has shown that the asteroid belt has 0.7–1.7 million asteroids with a diameter of 1 km or more. The apparent magnitudes of most of the known asteroids are 11–19, with the median at about 16.

Asteroid misses Earth by 17,000 miles after meteor strikes Russia

After skimming closer to the earth than any other asteroid of its size, space rock 2012 DA14 missed us by about 17,100 miles on Friday evening, a margin closer than some satellites.

Experts had given assurances there was nothing to fear from the asteroid, which was too small to see with the naked eye even at its closest approach over the Indian Ocean, near Sumatra.

At 50 metres across (about the size of an Olympic swimming pool), the object was a relative tiddler in comparison with the one measuring 5 miles across that wiped out the dinosaurs 65 million years ago.

Anenometer

An anemometer is a device used for measuring wind speed, and is a common weather station instrument. The term is derived from the Greek word anemos, meaning wind, and is used to describe any airspeed measurement instrument used in meteorology or aerodynamics. The first known description of an anemometer was given by Leon Battista Alberti around 1450.Anemometers can be divided into two classes: those that measure the wind's speed, and those that measure the wind's pressure; but as there is a close connection between the pressure and the speed, an anemometer designed for one will give information about both.

The anemometer has changed little since its development in the 15th century. Leon Battista Alberti is said to have invented the first mechanical anemometer around 1450. In following centuries, numerous others, including Robert Hooke and the Mayans, developed their own versions, with some being mistakenly credited as the inventor. In 1846, John Thomas Romney Robinson improved upon the design by using four hemispherical cups and mechanical wheels. Later, in 1926, John Patterson developed a three cup anemometer, which was improved by Brevoort and Joiner in 1935. In 1991, Derek Weston added the ability to detect wind direction. Most recently, in 1994, Dr. Andrews Pflitsch developed the sonic anemometer.

CLASSIFICATION OF ANENOMETERS

VELOCITY ANEMOMETERS

Cup anemometers

A simple type of anemometer was invented in 1846 by Dr. John Thomas Romney Robinson, of Armagh Observatory. It consisted of four hemispherical cups each mounted on one end of four horizontal arms, which in turn were mounted at equal angles to each other on a vertical shaft. The air flow past the cups in any horizontal direction turned the shaft in a manner that was proportional to the wind speed. Therefore, counting the turns of the shaft over a set time period produced the average wind speed for a wide range of speeds. On an anemometer with four cups it is easy to see that since the cups are arranged symmetrically on the end of the arms, the wind always has the hollow of one cup presented to it and is blowing on the back of the cup on the opposite end of the cross.

Windmill anemometers

The other forms of mechanical velocity anemometer may be described as belonging to the windmill type or propeller anemometer. In the Robinson anemometer the axis of rotation is vertical, but with this subdivision the axis of rotation must be parallel to the direction of the wind and therefore horizontal. Furthermore, since the wind varies in direction and the axis has to follow its changes, a wind vane or some other contrivance to fulfill the same purpose must be employed. An aerovane combines a propeller and a tail on the same axis to obtain accurate and precise wind speed

and direction measurements from the same instrument. In cases where the direction of the air motion is always the same, as in the ventilating shafts of mines and buildings for instance, wind vanes, known as air meters are employed, and give most satisfactory results.

Hot-wire anemometers

Hot wire anemometers use a very fine wire (on the order of several micrometres) electrically heated up to some temperature above the ambient. Air flowing past the wire has a cooling effect on the wire. As the electrical resistance of most metals is dependent upon the temperature of the metal (tungsten is a popular choice for hot-wires), a relationship can be obtained between the resistance of the wire and the flow speed.

Laser Doppler anemometers

In Laser Doppler velocimetry, Laser Doppler anemometers use a beam of light from a laser that is divided into two beams, with one propagated out of the anemometer. Particulates (or deliberately introduced seed material) flowing along with air molecules near where the beam exits reflect, or backscatter, the light back into a detector, where it is measured relative to the original laser beam. When the particles are in great motion, they produce a Doppler shift for

measuring wind speed in the laser light, which is used to calculate the speed of the particles, and therefore the air around the anemometer.

Sonic anemometers

Sonic anemometers, first developed in the 1950s, use ultrasonic sound waves to measure wind velocity. They measure wind speed based on the time of flight of sonic pulses between pairs of transducers. Measurements from pairs of transducers can be combined to yield a measurement of velocity in 1-, 2-, or 3-dimensional flow. The spatial resolution is given by the path length between transducers, which is typically 10 to 20 cm. Sonic anemometers can take measurements with very fine temporal resolution, 20 Hz or better, which makes them well suited for turbulence measurements. The lack of moving parts makes them appropriate for long term use in exposed automated weather stations and weather buoys where the accuracy and reliability of traditional cup-and-vane anemometers is adversely affected by salty air or large amounts of dust. Their main disadvantage is the distortion of the flow itself by the structure supporting the transducers, which requires a correction based upon wind tunnel measurements to minimize the effect.

Acoustic resonance anemometers

Acoustic resonance technology enables measurement within a small cavity, the sensors therefore tend to be typically smaller in size than other ultrasonic sensors. The small size of acoustic resonance anemometers makes them physically strong and very easy to heat and therefore resistant to icing. This combination of features means that they achieve high levels of data availability and are well suited to wind turbine control and to other uses that require small robust sensors such as battlefield meteorology. One issue with this sensor type is measurement accuracy when compared to a calibrated mechanical sensor. For many end uses this weakness is compensated for by the sensors’ longevity and the fact that it does not require re-calibrating once installed.

Ping-pong ball anemometers

A common anemometer for basic use is constructed from a ping-pong ball attached to a string. When the wind blows horizontally, it presses on and moves the ball; because ping-pong balls are very lightweight, they move easily in light winds. Measuring the angle between the string-ball apparatus and the vertical gives an estimate of the wind speed.

This type of anemometer is mostly used for middle-school level instruction which most students make themselves.

PRESSURE ANEMOMETERS

Plate anemometers

These are the earliest anemometers and are simply a flat plate suspended from the top so that the wind deflects the plate. In 1450, the Italian art architect Leon Battista Alberti invented the first mechanical anemometer; in 1664 it was re-invented by Robert Hooke (who is often mistakenly considered the inventor of the first anemometer). Later versions of this form consisted of a flat plate, either square or circular, which is kept normal to the wind by a wind vane.

Tube anemometers

The great advantage of the tube anemometer lies in the fact that the exposed part can be mounted on a high pole, and requires no oiling or attention for years; and the registering part can be placed in any convenient position. Two connecting tubes are required. It might appear at first sight as though one connection would serve, but the differences in pressure on which these instruments depend are so minute, that the pressure of the air in the room where the recording part is placed has to be considered. Thus if the instrument depends on the pressure or suction effect alone, and this pressure or suction is measured against the air pressure in an ordinary room, in which the doors and windows are carefully closed and a newspaper is then burnt up the chimney, an effect may be produced equal to a wind of 10 mi/h (16 km/h); and the opening of a window in rough weather, or the opening of a door, may entirely alter the registration.

RAIN GUAGE

The rain gauge measures the amount of liquid precipitation that falls. It can measure either rain or, with added steps, the liquid equivalent of snow. The rain gauge has an outer cylinder, a measuring tube, and a funnel. The measuring tube measures to a hundredth inch. When it is full, it contains one inch of rain. When more than an inch falls, the extra flows into the outer cylinder. By carefully pouring the rain from the outer cylinder back into the measuring tube, a total rainfall amount can be accurately measured.

Most rain gauges generally measure the precipitation in millimeters. The level of rainfall is sometimes reported as inches or centimeters.Rain gauge amounts are read either manually or by automatic weather station (AWS). The frequency of readings will depend on the requirements of the collection agency. Some countries will supplement the paid weather observer with a network of volunteers to obtain precipitation data (and other types of weather) for sparsely populated areas.In most cases the precipitation is not retained, however some stations do submit rainfall (and snowfall) for testing, which is done to obtain levels of pollutants.Rain gauges have their limitations. Attempting to collect rain data in a hurricane can be nearly impossible and unreliable (even if the equipment survives) due to wind extremes. Also, rain gauges only indicate rainfall in a localized area. For virtually any gauge, drops will stick to the sides or funnel of the collecting device, such that amounts are very slightly underestimated, and those of .01 inches or .25 mm may be recorded as a trace.Another problem encountered is when the temperature is close to or below freezing. Rain may fall on the funnel and ice or snow may collect in the gauge and not permit any subsequent rain to pass through.Rain gauges should be placed in an open area where there are no obstacles, such as building or trees, to block the rain. This is also to prevent the water collected on the roofs of buildings or the leaves of trees from dripping into the rain gauge after a rain, resulting in inaccurate readings.

TYPES OF RAIN GUAGE

Standard rain gauge

The standard NWS rain gauge, developed at the start of the 20th century, consists of a funnel emptying into a graduated cylinder, 2 cm in diameter, which fits inside a larger container which is 20 cm in diameter and 50 cm tall. If the rainwater overflows the graduated inner cylinder, the larger outer container will catch it. When measurements are taken, the height of the water in the small graduated cylinder is measured, and the excess overflow in the large container is carefully poured into another graduated cylinder and measured to give the total

rainfall. In locations using the metric system, the cylinder is usually marked in mm and will measure up to 250 millimetres (9.8 in) of rainfall. Each horizontal line on the cylinder is 0.5 millimetres (0.02 in). In areas using Imperial units each horizontal line represents 0.01 inch.

Weighing precipitation gauge

A weighing-type precipitation gauge consists of a storage bin, which is weighed to record the mass. Certain models measure the mass using a pen on a rotating drum, or by using a vibrating wire attached to a data logger. The advantages of this type of gauge over tipping buckets are that it does not underestimate intense rain, and it can measure other forms of precipitation, including rain, hail and snow. These gauges are, however, more expensive and require more maintenance than tipping bucket gauges.

Tipping bucket rain gauge

The tipping bucket rain gauge consists of a funnel that collects and channels the precipitation into a small seesaw-like container. After a pre-set amount of precipitation falls, the lever tips, dumping the collected water and sending an electrical signal. An old-style recording device may consist of a pen mounted on an arm attached to a geared wheel that moves once with each signal sent from the collector. In this design, the wheel turns the pen arm moves either up or down leaving a trace on the graph and at the same time making a loud click. Each jump of the arm is sometimes

referred to as a 'click' in reference to the noise. The chart is measured in 10 minute periods (vertical lines) and 0.4 mm (0.015 in) (horizontal lines) and rotates once every 24 hours and is powered by a clockwork motor that must be manually wound.

Optical rain gauge

These have a row of collection funnels. In an enclosed space below each is a laser diode and a photo transistor detector. When enough water is collected to make a single drop, it drops from the bottom, falling into the laser beam path. The sensor is set at right angles to the laser so that enough light is scattered to be detected as a sudden flash of light. The flashes from these photo detectors are then read and transmitted or recorded.

BAROMETER

A barometer is a scientific instrument used in meteorology to measure atmospheric pressure. Pressure tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used within surface weather analysis to help find surface troughs, high pressure systems, and frontal boundaries.

TYPES OF BAROMETER

Water-based barometers

The Water Based barometer is made of a tube that is of glass and this tube is sealed in a way that it consists of water. This water is the main operating feature of the weather based barometer. There is a narrow spot that is placed in this glass container, this spout actually connects with the body that is present below the level of water and also it rises to a level that is above the level of water. This narrow spot that is present in the glass container is let out in the open air. The changes in the pressure can be estimated by the fluctuating levels of the water. The water present in the glass tube changes with the change in the pressure in the atmosphere. Also this instrument depends on the location and the altitude. The altitude matters a lot in every way. The pressure of the air changes rapidly with the rapid change in the atmosphere. As the location changes the gases present in the air also reacts differently. They all change to the atmosphere. As the pressure present in the air goes to the higher mode the ultimately this will change the level of water in the glass tube and so the level of water drops of the standard limit that is prescribed for every limit. The water based barometer is so easily available in every market and also this can be made at home with convenience.

Mercury barometers

A mercury barometer has a glass tube with a height of at least 84 cm, closed at one end, with an open mercury-filled reservoir at the base. The weight of the mercury creates a vacuum in the top of the tube. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir. Since higher temperature at the instrument will reduce the density of the mercury, the scale for reading the height of the mercury is adjusted to compensate for this effect.

Vacuum pump oil barometer

Using vacuum pump oil as the working fluid in a barometer has led to the creation of the new "World's Tallest Barometer" in February 2013. The barometer at Portland State University (PSU) uses doubly distilled vacuum pump oil and has a nominal height of ~12.4 m for the oil column height; expected excursions are in the range of ±0.4 m over the course of a year. Vacuum pump oil has very low vapor pressure and it is available in a range of densities; the lowest density vacuum oil was chosen for the PSU barometer to maximize the oil column height.

Aneroid barometers

An aneroid barometer, invented in 1843 by French scientist Lucien Vidi uses a small, flexible metal box called an aneroid cell (capsule), which is made from an alloy of beryllium and copper. The evacuated capsule (or usually more capsules) is prevented from collapsing by a strong spring. Small changes in external air pressure cause the cell to expand or contract. This expansion and contraction drives mechanical levers such that the tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer.

WIND VANE

A wind vane, also known as a weather vane, is a tool used to determine the direction the wind is blowing from. These instruments have been in use for centuries, dating from around 50 BCE. They come in many designs, from sleek and professional to ornamentally fun, but they usually follow similar aerodynamic design rules. A number of giant wind vanes exist, and there is a dispute about the largest one in the world.

The current wind direction, combined with knowledge of the geography of a region, can often provide a good indication of how the weather may change over the next day or two. For this reason, a wind vane is often used in weather forecasting. If the wind is blowing from a warm ocean, for example, mild, cloudy, and wet conditions can be expected. Wind vanes at weather stations record wind direction electronically, to provide permanent records. They are used in conjunction with anemometers, which measure wind speed.Weather vanes are also used at airports, as wind direction is an important factor for incoming and outgoing airplanes. Another use is in sailing: a type of automatic steering system employs a sailing wind vane connected to a rudder to keep the boat or yacht on course

Design and Construction

The modern version is usually constructed of lightweight metals in order to provide responsiveness to light winds, durability in a variety of weather conditions, and also because of metal's ability to be forged precisely. This precision is important to vane developers because the weight of the device needs to be distributed evenly in order for it to function properly. The simplest design involves an arrow on a rotating axis with the points of the compass labeled below. The arrow's design is crucial for the success of the vane, because it has one large end and one smaller, point ed end so that it will point in the direction from which the wind is blowing.

Using this basic setup, many more elaborate forms — shaped like animals, ships, and people, for example — can be constructed. Such designs may not give a very accurate indication of wind direction if there is only a light breeze, however, and they tend to be ornamental rather than functional in purpose. A simple weather vane can easily be constructed from everyday materials, making it a popular school science project.

Positioning

To provide accurate wind directions, weather vanes need to be high above the ground. Near the ground, obstacles such as buildings and trees cause winds be deflected or swirl around, so that it is not possible to determine the correct wind direction. The vanes are usually placed on top of tall buildings or on purpose-built structures, where no nearby taller objects can interfere with the air movement.

History

The earliest recorded wind vane is thought to be a large bronze statue of the god Triton placed on top of the Tower of the Winds in Athens, Greece. It was able to rotate and indicated the wind direction. Below it were representations of the gods associated with the eight wind directions, along with sundials and a water clock. It is thought to date from around 50 BCE.

In Christian countries, there is a long tradition of installing weather vanes on top of church towers. Generally, they incorporated a cock, or rooster, into the design. The practice is thought to date from the 9th century, when the animals seems to have become a Christian symbol, following papal pronouncements. Later, many other, often very elaborate designs began to appear, some of which are much sought after as antiques.

Modern wind vanes are often of much simpler and strictly functional design to maximize their sensitivity and the accuracy of the readings. They also dispense with directional indicators, as the data is recorded automatically by computer. Decorative ones are still available, however, for individuals who simply like their look.

Notable Wind Vanes

The Guinness Book of World Records has recognized a weather vane advertising a brand of sherry in Jerez, Spain, as the world’s largest. This is disputed, however, and there are a number of other candidates for the record. Perhaps the most remarkable is located at a transportation museum next to Whitehorse International Airport in Yukon, Canada. It consists of an actual DC-3 airplane mounted on a rotating column. Despite its bulk and weight, it will respond to a wind of just 5 knots (5.75 mph or 9.26 kph).

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