SUMMARY / RELATED TOPICS

Snowflake

A snowflake is a single ice crystal that has achieved a sufficient size, may have amalgamated with others falls through the Earth's atmosphere as snow. Each flake nucleates around a dust particle in supersaturated air masses by attracting supercooled cloud water droplets, which freeze and accrete in crystal form. Complex shapes emerge as the flake moves through differing temperature and humidity zones in the atmosphere, such that individual snowflakes differ in detail from one another, but may be categorized in eight broad classifications and at least 80 individual variants; the main constituent shapes for ice crystals, from which combinations may occur, are needle, column and rime. Snow appears white in color despite being made of clear ice; this is due to diffuse reflection of the whole spectrum of light by the small crystal facets of the snowflakes. Snowflakes nucleate around mineral or organic particles in moisture-saturated, subfreezing air masses, they grow by net accretion to the incipient crystals in hexagonal formations.

The cohesive forces are electrostatic. In warmer clouds, an aerosol particle or "ice nucleus" must be present in the droplet to act as a nucleus; the particles that make ice nuclei are rare compared to nuclei upon which liquid cloud droplets form. Clays, desert dust, biological particles may be effective, although to what extent is unclear. Artificial nuclei include particles of silver iodide and dry ice, these are used to stimulate precipitation in cloud seeding. Experiments show that "homogeneous" nucleation of cloud droplets only occurs at temperatures lower than −35 °C. Once a droplet has frozen, it grows in the supersaturated environment, one where air is saturated with respect to ice when the temperature is below the freezing point; the droplet grows by deposition of water molecules in the air onto the ice crystal surface where they are collected. Because water droplets are so much more numerous than the ice crystals due to their sheer abundance, the crystals are able to grow to hundreds of micrometers or millimeters in size at the expense of the water droplets.

This process is known as the Wegener–Bergeron–Findeisen process. The corresponding depletion of water vapor causes the droplets to evaporate, meaning that the ice crystals grow at the droplets' expense; these large crystals are an efficient source of precipitation, since they fall through the atmosphere due to their mass, may collide and stick together in clusters, or aggregates. These aggregates are the type of ice particle that falls to the ground. Guinness World Records lists the world's largest snowflakes as those of January 1887 at Fort Keogh, Montana. Although this report by a farmer is doubtful, aggregates of three or four inches width have been observed. Single crystals the size of a dime have been observed. Snowflakes encapsulated in rime form balls known as graupel. Although ice by itself is clear, snow appears white in color due to diffuse reflection of the whole spectrum of light by the scattering of light by the small crystal facets of the snowflakes of which it is comprised; the shape of the snowflake is determined broadly by the temperature and humidity at which it is formed.

At a temperature of around −2 °C, snowflakes can form in threefold symmetry — triangular snowflakes. The most common snow particles are visibly irregular, although near-perfect snowflakes may be more common in pictures because they are more visually appealing, it is unlikely that any two snowflakes are alike due to the estimated 1019 water molecules which make up a typical snowflake, which grow at different rates and in different patterns depending on the changing temperature and humidity within the atmosphere that the snowflake falls through on its way to the ground. Snowflakes that look identical, but may vary at the molecular level, have been grown under controlled conditions. Although snowflakes are never symmetrical, a non-aggregated snowflake grows so as to exhibit an approximation of six-fold radial symmetry; the symmetry gets started due to the hexagonal crystalline structure of ice. At that stage, the snowflake has the shape of a minute hexagon; the six "arms" of the snowflake, or dendrites grow independently from each of the corners of the hexagon, while either side of each arm grows independently.

The microenvironment in which the snowflake grows changes dynamically as the snowflake falls through the cloud and tiny changes in temperature and humidity affect the way in which water molecules attach to the snowflake. Since the micro-environment are nearly identical around the snowflake, each arm tends to grow in nearly the same way. However, being in the same micro-environment does not guarantee that each arm grow the same. Empirical studies suggest less than 0.1% of snowflakes exhibit the ideal six-fold symmetric shape. Twelve branched snowflakes are observed. Snowflakes form in a wide variety of intricate shapes, leading to the notion that "no two are alike". Although nearly-identical snowflakes have been made in laboratory, they are unlikely to be found in nature. Initial attempts to find identical snowflakes by photographing thousands of them with a microscope from 1885 onward by Wilson Alwyn Bentley found the wide variety of snowflakes we know about today. Ukichiro Nakaya developed a crystal morphology diagram, relating crystal shape to the temperature and moisture

Arg (Kabul)

The Arg serves as the presidential palace in Kabul, Afghanistan. It sits on a 34-hectare site in District 2, between Deh Afghanan and the affluent neighbourhood of Wazir Akbar Khan; the Arg was built after the destruction of the Bala Hissar in 1880 by the British Indian troops. It has been used by many Afghan kings and presidents, from Emir Abdur Rahman Khan to current President Ashraf Ghani; the foundation of the Arg was laid by Emir Abdur Rahman Khan in 1880 after assuming the throne. It was designed as a castle with water-filled trench around it. Abdur Rahman Khan named it Arg-e-Shahi and included among other buildings a residence for his family, army barracks, the national treasury; the Bala Hissar served as the citadel or the headquarters of the emirs until it was destroyed by the British Indian troops during the Second Anglo-Afghan war. The Arg has served as the royal and presidential palace for all of the kings and presidents of Afghanistan. However, Hafizullah Amin used Tajbeg Palace as the residence for his family.

It has undergone revitalization under the different rulers. During the 1978 Saur Revolution, Mohammed Daoud Khan and his family were assassinated by members of the People's Democratic Party of Afghanistan inside the Arg; the Arg today consists of the following: The Gul Khana which serves as the offices for President Ashraf Ghani and the President's Protocol Office. Offices for the Afghan National Security Forces. Building for the Administrative Office of the President. Various buildings for receiving delegations or hosting large meetings. Other Palaces in Kabul: Bagh-e Bala, a former royal palace in Kabul Bala Hissar, an ancient fortress located in the south of the old city of Kabul, Darul Aman, former royal palace Herat Citadel, former main royal palace Tajbeg Palace, former royal palace

10830 Desforges

10830 Desforges, provisional designation 1993 UT6, is a background or Eunomian asteroid from the central regions of the asteroid belt 7 kilometers in diameter. It was discovered on 20 October 1993, by Belgian astronomer Eric Elst at the La Silla Observatory in northern Chile; the elongated S-type asteroid has a rotation period of 8.8 hours. It was named after aviation visionary Jacques Desforges. Desforges is a non-family asteroid of the main belt's background population when applying the hierarchical clustering method to its proper orbital elements. Based on osculating Keplerian orbital elements, the asteroid has been classified as a member of the Eunomia family, a prominent family of stony S-type asteroid and the largest one in the intermediate main belt with more than 5,000 members, it orbits the Sun in the central main-belt at a distance of 2.2–3.1 AU once every 4 years and 4 months. Its orbit has an inclination of 11 ° with respect to the ecliptic; the body's observation arc begins with its first observations as 1982 FS2 at Klet Observatory in March 1982, more than 11 years prior to its official discovery observation at La Silla.

Desforges is an assumed stony S-type asteroid, the overall spectral type for members of the Eunomia family. Observations with the Wide-field Infrared Survey Explorer, gave a low geometric albedo more typical for an X- or C-type asteroid. In August 2010, a rotational lightcurve of Desforges was obtained from photometric observations in the R-band by astronomers at the Palomar Transient Factory in California. Lightcurve analysis gave a rotation period of 8.804 hours with a high brightness amplitude of 0.69 magnitude, which indicates that the body has a non-spherical shape. According to the survey carried out by the NEOWISE mission of NASA's WISE telescope, Desforges measures between 7.692 and 9.390 kilometers in diameter and its surface has an albedo between 0.0635 and 0.080. The Collaborative Asteroid Lightcurve Link assumes an albedo of 0.21 – derived from the Eunomia family's parent body, 15 Eunomia – and calculates a diameter of 4.21 kilometers based on an absolute magnitude of 14.19. This minor planet was named after a French priest Jacques Desforges, imprisoned for eight months in 1758 in the Bastille, during which time he planned the construction of a flying machine.

The official naming citation was published by the Minor Planet Center on 18 March 2003. Asteroid Lightcurve Database, query form Dictionary of Minor Planet Names, Google books Discovery Circumstances: Numbered Minor Planets - – Minor Planet Center 10830 Desforges at AstDyS-2, Asteroids—Dynamic Site Ephemeris · Observation prediction · Orbital info · Proper elements · Observational info 10830 Desforges at the JPL Small-Body Database Close approach · Discovery · Ephemeris · Orbit diagram · Orbital elements · Physical parameters