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2. A natural disaster is a major adverse eventresulting from natural processes of the Earth.Examples: floods, volcaniceruptions, earthquakes, tsunamis, and othergeologic processes.A natural disaster can cause loss of life orproperty damage and typically leaves someeconomic damage in its wake, the severity ofwhich depends on the affectedpopulation's resilience or ability to recover. 3. An adverse event will not rise to the levelof a disaster if it occurs in an area withoutvulnerable population. In a vulnerablearea, however, such as San Francisco,an earthquake can have disastrousconsequences and leave lasting damage,requiring years to repair. 4. Catastrophes Statistics for the year 2012Total count 905Meteorological (Storms) 45%Hydrological (Floods), 36%Climatological (Heat waves, coldWaves, Droughts, Wildfires) 12%Geophysical events (Earthquakesand Volcanic eruptions). 7%Catastrophes 93%Cost in Billion US $ 170Insured Losses US $ 7093%Between 1980 and 2011 geophysical eventsaccounted for 14% of all natural catastrophes. 5. Droughts Hailstorms Heat waves Tornadoes Wild fires Health disasters Epidemics Space disasters Impact events Solar flare Gamma-ray burst Protection by international law 6. Avalanches 7. An avalanche (also called a snowslide or snowslip)is a rapid flow of snow down a slope. Avalanches aretypically triggered in a starting zone from amechanical failure in the snowpack (slab avalanche)when the forces on the snow exceed its strength butsometimes only with gradually widening (loose snowavalanche). After initiation, avalanches usuallyaccelerate rapidly and grow in mass and volume asthey entrain more snow. If the avalanche moves fastenough some of the snow may mix with the airforming a powder snow avalanche, which is a typeof gravity current. 8. Slides of rocks or debris, behaving in a similar way to snow,are also referred to as avalanches (see rockslide). Theremainder of this article refers to snow avalanches.The load on the snowpack may be only due to gravity, inwhich case failure may result either from weakening in thesnowpack or increased load due to precipitation.Avalanches that occur in this way are known asspontaneous avalanches. Avalanches can also be triggeredby other loads such as skiers, snowmobilers, animals orexplosives. Seismic activity may also trigger failure in thesnowpack and avalanches. 9. Although primarily composed of flowing snow andair, large avalanches have the capability to entrainice, rocks, trees, and other material on the slope,and are distinct from mudslides, rock slides,and serac collapses on an icefall. Avalanches arenot rare or random events and are endemic to anymountain range that accumulates a standingsnowpack. Avalanches are most common duringwinter or spring but glacier movements may causeice and snow avalanches at any time of year. 10. In mountainous terrain, avalanches are among themost serious objective natural hazards to life andproperty, with their destructive capability resultingfrom their potential to carry enormous masses ofsnow at high speeds.There is no universally accepted classification ofavalanchesdifferent classifications are useful fordifferent purposes. Avalanches can be described bytheir size, their destructive potential, theirinitiation mechanism, their composition and theirdynamics. 11. Formation and classification Loose snow avalanches Slab avalanches Powder snow avalanches Dry snow avalanches Terrain, snowpack, weather Terrain Snowpack structure and characteristics Weather Dynamics Modeling Human involvement Prevention Mitigation Survival, rescue, and recovery 12. FormationandclassificationMost avalanchesoccur spontaneouslyduring storms underincreased load due tosnowfall. The secondlargest cause ofnatural avalanches ismetamorphicchanges in thesnowpack such asmelting due to solarradiation. 13. Loose snowavalanchesLoose snowavalanches(far left)and slab avalanches(near center)nearMountShuksan inthe North Cascadesmountains.Fracturepropagation isrelatively limited. 14. SlabavalanchesA crown fracturefrom a slabavalanche nearthe Neve Glacierin the NorthCascadesmountains.Extensive fracturepropagation isevident. 15. PowdersnowavalanchesA powder snowavalanche inthe HimalayasnearMountEverest. 16. Dry snowavalanchesDry snowavalanchewith apowdercloud 17. Terrain,snowpack,weatherDoug Fesler andJill Fredstondeveloped aconceptualmodel of thethree primaryelements ofavalanches:terrain, weather,and snowpack. 18. TerrainIn steepavalanche-proneterrain,traveling onridges isgenerally saferthan traversingthe slopes. 19. Drought is an extended period when a region notes a deficiency in its watersupply whether surface or underground water. A drought can last for months oryears, or may be declared after as few as 15 days. Generally, this occurs when aregion receives consistently below average precipitation. It can have a substantialimpact on the ecosystem and agriculture of the affected regionMany plant species, such as cacti, have adaptations such as reduced leaf area andwaxy cuticles to enhance their ability to tolerate drought. Some others survivedry periods as buried seeds. Semi-permanent drought produces arid biomes suchas deserts and grasslands. Most arid ecosystems have inherently lowproductivity.This global phenomenon has a widespread impact on agriculture. Lengthy periodsof drought have long been a key trigger for mass migration and played a key rolein a number of ongoing migrations and other humanitarian crises in the Horn ofAfrica and the Sahel.According to F. Bagouls and Henri Gaussen's definition, a month is dry when themean monthly precipitation in millimeters is equal to or lower than twice the meanmonthly temperature in C. 20. ConsequencesGlobally. Regions affectedCausesTypesProtection and relief 21. ConsequencesA Mongolian gazelle dead due to drought. Periods of droughts can havesignificant environmental, agricultural, health, economic and socialconsequences. The effect varies according to vulnerability. For example,subsistence farmers are more likely to migrate during drought because theydo not have alternative food sources. Areas with populations that depend on asa major food source are more vulnerable to famine.Drought can also reduce water quality, because lower water flows reducedilution of pollutants and increase contamination of remaining water sources.Common consequences of drought include:Diminished crop growth or yield productions and carrying capacity forlivestockDust bowls, themselves a sign of erosion, which further erode the landscapeDust storms, when drought hits an area suffering from desertification anderosion 22. Famine due to lack of water for irrigationHabitat damage, affecting both terrestrial and aquatic wildlifeHunger, drought provides too little water to support food crops.Malnutrition, dehydration and related diseasesMass migration, resulting in internal displacement and international refugeesReduced electricity production due to reduced water flow throughhydroelectric damsShortages of water for industrial usersSnake migration and increases in snakebitesSocial unrestWar over natural resources, including water and food 23. GloballyDrought is a normal, recurring feature of the climate in most parts of theworld. It is among the earliest documented climatic events, present in the Epicof Gilgamesh and tied to the biblical story of Joseph's arrival in and the laterExodus from Ancient Egypt.Hunter-gatherer migrations in 9,500 BC Chile have been linked to thephenomenon, as has the exodus of early humans out of Africa and into the restof the world around 135,000 years ago.Modern people can effectively mitigate much of the impact of drought throughirrigation and crop rotation. Failure to develop adequate drought mitigationstrategies carries a grave human cost in the modern era, exacerbated by ever-increasingpopulation densities.Regions affectedRecurring droughts leading to desertification in the Horn of Africa havecreated grave ecological catastrophes, prompting massive food shortages, stillrecurring. To the north-west of the Horn, the Darfur conflict in neighbouring 24. Sudan, also affecting Chad, was fueled by decades of drought and overpopulationare among the causes of the Darfur conflict, because the Arab Baggara nomadssearching for water have to take their livestock further south, to land mainlyoccupied by non-Arab farming peoples.Approximately 2.4 billion people live in the drainage basin of the Himalayanrivers. India, China, Pakistan, Bangladesh, Nepal and Myanmar could experiencefloods followed by droughts in coming decades. Drought in India affecting theGanges is of particular concern, as it provides drinking water and agriculturalirrigation for more than 500 million people. The west coast of North America,which gets much of its water from glaciers in mountain ranges such as the RockyMountains and Sierra Nevada, also would be affected.In 2005, parts of the Amazon basin experienced the worst drought in 100 years.A 23 July 2006 article reported Woods Hole Research Center results showingthat the forest in its present form could survive only three years of drought.Scientists at the Brazilian National Institute of Amazonian Research argue inthe article that this drought response, coupled with the effects ofdeforestation on regional climate, are pushing the rainforest towards a "tippingpoint" where it would irreversibly start to die. 25. It concludes that the rainforest is on the brink of being turned into savanna ordesert, with catastrophic consequences for the world's climate. According tothe WWF, the combination of climate change and deforestation increases thedrying effect of dead trees that fuels forest fires.By far the largest part of Australia is desert or semi-arid lands commonly knownas the outback. A 2005 study by Australian and American researchersinvestigated the desertification of the interior, and suggested that oneexplanation was related to human settlers who arrived about 50,000 years ago.Regular burning by these settlers could have prevented monsoons from reachinginterior Australia.In June 2008 it became known that an expert panel had warned of long term,maybe irreversible, severe ecological damage for the whole Murray-Darling basinif it does not receive sufficient water by October. Australia could experiencemore severe droughts and they could become more frequent in the future, agovernment-commissioned report said on July 6, 2008. Australianenvironmentalist Tim Flannery, predicted that unless it made drastic changes,Perth in Western Australia could become the worlds first ghost metropolis, anabandoned city with no more water to sustain its population 26. CausesGenerally, rainfall is related to the amount and dew point [determined by airtemperature] of water vapour carried by regional atmosphere, combined withthe upward forcing of the air mass containing that water vapour. If thesecombined factors do not support precipitation volumes sufficient to reach thesurface, the result is a drought. This can be triggered by high level of reflectedsunlight, [high albedo], and above average prevalence of high pressure systems,winds carrying continental, rather than oceanic air masses (i.e. reduced watercontent), and ridges of high pressure areas from behaviors which prevent orrestrict the developing of thunderstorm activity or rainfall over one certainregion. Oceanic and atmospheric weather cycles such as the El Nio-SouthernOscillation (ENSO) make drought a regular recurring feature of the Americasalong the Midwest and Australia. Guns, Germs, and Steel author Jared Diamondsees the stark impact of the multi-year ENSO cycles on Australian weatherpatterns as a key reason that Australian aborigines remained a hunter-gatherersociety rather than adopting agriculture. Another climate oscillation known asthe North Atlantic Oscillation has been tied to droughts in northeast Spain.Human activity can directly trigger exacerbating factors such as 27. farming, excessive irrigation, deforestation, and erosion adversely impact theability of the land to capture and hold water. While these tend to be relativelyisolated in their scope, activities resulting in global climate change are expectedto trigger droughts with a substantial impact on agriculture throughout theworld, and especially in developing nations. Overall, global warming will result inincreased world rainfall. Along with drought in some areas, flooding and erosionwill increase in others. Paradoxically, some proposed solutions to global warmingthat focus on more active techniques, solar radiation management through theuse of a space sunshade for one, may also carry with them increased chances ofdrought. 28. TypesAs a drought persists, the conditions surrounding it gradually worsen and itsimpact on the local population gradually increases. People tend to definedroughts in three main ways:Meteorological drought is brought about when there is a prolonged periodwith less than average precipitation. Meteorological drought usually precedesthe other kinds of drought.Agricultural droughts are droughts that affect crop production or the ecologyof the range. This condition can also arise independently from any change inprecipitation levels when soil conditions and erosion triggered by poorly plannedagricultural endeavors cause a shortfall in water available to the crops. However,in a traditional drought, it is caused by an extended period of below averageprecipitation.Hydrological drought is brought about when the water reserves available insources such as aquifers, lakes and reservoirs fall below the statistical average.Hydrological drought tends to show up more slowly because it involves stored 29. water that is used but not replenished. Like an agricultural drought, this can betriggered by more than just a loss of rainfall. For instance, Kazakhstan wasrecently awarded a large amount of money by the World Bank to restore waterthat had been diverted to other nations from the Aral Sea under Soviet rule.Similar circumstances also place their largest lake, Balkhash, at risk ofcompletely drying out. 30. Protection and ReliefStrategies for drought protection, mitigation or relief include: Dams - many dams and their associated reservoirs supply additional water intimes of drought. Cloud seeding - a form of intentional weather modification to induce rainfall. Desalination - of sea water for irrigation or consumption. Drought monitoring - Continuous observation of rainfall levels andcomparisons with current usage levels can help prevent man-made drought. Forinstance, analysis of water usage in Yemen has revealed that their water table(underground water level) is put at grave risk by over-use to fertilize their Khatcrop. Careful monitoring of moisture levels can also help predict increased riskfor wildfires, using such metrics as the Keetch-Byram Drought Index Land use - Carefully planned crop rotation can help to minimize erosion andallow farmers to plant less water-dependent crops in drier years. 31. Outdoor water- use restriction - Regulating the use of sprinklers, hoses orbuckets on outdoor plants, filling pools, and other water-intensive homemaintenance tasks. Rainwater harvesting - Collection and storage of rainwater from roofs orother suitable catchments. Recycled water - Former wastewater (sewage) that has been treated andpurified for reuse. Transvasement - Building canals or redirecting rivers as massive attempts atirrigation in drought-prone areas. 32. Hail Storms is a form of solid precipitation. It consists of balls or irregularlumps of ice, each of which is called a hailstone. Unlike graupel, which is madeof rime, and ice pellets, which are smaller and translucent, hailstones consistmostly of water ice and measure between 5 millimetres (0.20 in) and 15centimetres (6 in) in diameter. The METAR reporting code for hail 5 mm (0.20in) or greater is GR, while smaller hailstones and graupel are coded GS. Hail ispossible within most thunderstorms as it is produced by cumulonimbi, andwithin 2 nautical miles (3.7 km) of the parent storm. Hail formation requiresenvironments of strong, upward motion of air with the parent thunderstorm(similar to tornadoes) and lowered heights of the freezing level. In the mid-latitudes,hail forms near the interiors of continents, while in the tropics, ittends to be confined to high elevations.There are methods available to detect hail-producing thunderstorms usingweather satellites and weather radar imagery. Hailstones generally fall athigher speeds as they grow in size, though complicating factors such asmelting, friction with air, wind, and interaction with rain and other hailstonescan slow their descent through Earth's atmosphere. Severe weather warningsare issued for hail when the stones reach a damaging size, as it can causeserious damage to human-made structures and, most commonly, farmers' crops. 33. Definition Formation. Layer nature of the hailstones. Factors favoring hail Climatology Short-term detection Size and terminal velocity. Hail records Hazards Accumulations Suppression and prevention 34. DefinitionAny thunderstorm which produces hail that reaches the ground is known as ahailstorm. Hail has a diameter of 5 millimetres (0.20 in) or more. Hailstones cangrow to 15 centimetres (6 in) and weigh more than 0.5 kilograms (1.1 lb).Unlike ice pellets, hailstones are layered and can be irregular and clumpedtogether. Hail is composed of transparent ice or alternating layers oftransparent and translucent ice at least 1 millimetre (0.039 in) thick, which aredeposited upon the hailstone as it travels through the cloud, suspended aloft byair with strong upward motion until its weight overcomes the updraft and fallsto the ground. Although the diameter of hail is varied, in the United States, theaverage observation of damaging hail is between 2.5 cm (1 in) and golf ball-sized(1.75 in).Stones larger than 2 cm (0.80 in) are usually considered large enough to causedamage. The Meteorological Service of Canada will issue severe thunderstormwarnings when hail that size or above is expected. The US National WeatherService has a 2.5 cm (1 in) or greater in diameter threshold, effective January2010, an increase over the previous threshold of -inch hail. 35. FormationHail forms in strong thunderstorm clouds, particularly those with intenseupdrafts, high liquid water content, great vertical extent, large water droplets,and where a good portion of the cloud layer is below freezing 0 C (32 F).These types of strong updrafts can also indicate the presence of a tornado. Thegrowth rate is maximized where air is near a temperature of 13 C (9 F).Layer nature of the hailstonesLike other precipitation in cumulonimbus clouds hail begins as water droplets. Asthe droplets rise and the temperature goes below freezing, they becomesupercooled water and will freeze on contact with condensation nuclei. A cross-sectionthrough a large hailstone shows an onion-like structure. This means thehailstone is made of thick and translucent layers, alternating with layers thatare thin, white and opaque. Former theory suggested that hailstones weresubjected to multiple descents and ascents, falling into a zone of humidity andrefreezing as they were uplifted. This up and down motion was thought to beresponsible for the successive layers of the hailstone. New research, based ontheory as well as field study, has shown this is not necessarily true. 36. The storm's updraft, with upwardly directed wind speeds as high as 110 milesper hour (180 km/h), blow the forming hailstones up the cloud. As the hailstoneascends it passes into areas of the cloud where the concentration of humidityand supercooled water droplets varies. The hailstones growth rate changesdepending on the variation in humidity and supercooled water droplets that itencounters. The accretion rate of these water droplets is another factor inthe hailstones growth.Furthermore, the hailstones speed depends on its position in the cloudsupdraft and its mass. This determines the varying thicknesses of the layers ofthe hailstone. The accretion rate of supercooled water droplets onto thehailstone depends on the relative velocities between these water droplets andthe hailstone itself. This means that generally the larger hailstones will formsome distance from the stronger updraft where they can pass more timegrowing. As the hailstone grows it releases latent heat, which keeps itsexterior in a liquid phase. 37. The hailstone will keep rising in the thunderstorm until its mass can no longerbe supported by the updraft. This may take at least 30 minutes based on theforce of the updrafts in the hail-producing thunderstorm, whose top is usuallygreater than 10 km high. It then falls toward the ground while continuing togrow, based on the same processes, until it leaves the cloud. It will later beginto melt as it passes into air above freezing temperature.Thus, a unique trajectory in the thunderstorm is sufficient to explain thelayer-like structure of the hailstone. The only case in which multipletrajectories can be discussed is in a multicellular thunderstorm where thehailstone may be ejected from the top of the "mother" cell and captured in theupdraft of a more intense "daughter cell". This however is an exceptional case.Factors favoring hailHail is most common within continental interiors of the mid-latitudes, as hailformation is considerably more likely when the freezing level is below thealtitude of 11,000 feet (3,400 m). Movement of dry air into strongthunderstorms over continents can increase the frequency of hail by promotingevaporational cooling which lowers the freezing level of thunderstorm cloudsgiving hail a larger volume to grow in. Accordingly, hail is less common in 38. the tropics despite a much higher frequency of thunderstorms than in the mid-latitudesbecause the atmosphere over the tropics tends to be warmer over amuch greater altitude. Hail in the tropics occurs mainly at higher elevations.Hail growth becomes vanishingly small when air temperatures fall below 30 C(22 F) as supercooled water droplets become rare at these temperatures.Around thunderstorms, hail is most likely within the cloud at elevations above20,000 feet (6,100 m). Between 10,000 feet (3,000 m) and 20,000 feet (6,100m), 60 percent of hail is still within the thunderstorm, though 40 percent nowlies within the clear air under the anvil. Below 10,000 feet (3,000 m), hail isequally distributed in and around a thunderstorm to a distance of 2 nauticalmiles (3.7 km).ClimatologyHail occurs most frequently within continental interiors at mid-latitudes and isless common in the tropics, despite a much higher frequency of thunderstormsthan in the midlatitudes. Hail is also much more common along mountain rangesbecause mountains force horizontal winds upwards (known as orographiclifting), thereby intensifying the updrafts within thunderstorms and making 39. hail more likely. The higher elevations also result in there being less timeavailable for hail to melt before reaching the ground. One of the more commonregions for large hail is across mountainous northern India, which reported oneof the highest hail-related death tolls on record in 1888. China alsoexperiences significant hailstorms. Central Europe and southern Australia alsoexperience a lot of hailstorms. Popular regions for hailstorms are southern andwestern Germany, northern and eastern France and southern and easternBenelux. In south-eastern Europe, Croatia and Serbia experience frequentoccurrences of hail.In North America, hail is most common in the area where Colorado, Nebraska,and Wyoming meet, known as "Hail Alley". Hail in this region occurs betweenthe months of March and October during the afternoon and evening hours, withthe bulk of the occurrences from May through September. Cheyenne, Wyomingis North America's most hail-prone city with an average of nine to tenhailstorms per season. 40. Certain patterns of reflectivity are important clues for the meteorologist aswell. The three body scatter spike is an example. This is the result of energyfrom the radar hitting hail and being deflected to the ground, where theydeflect back to the hail and then to the radar. The energy took more time togo from the hail to the ground and back, as opposed to the energy that wentdirect from the hail to the radar, and the echo is further away from the radarthan the actual location of the hail on the same radial path, forming a cone ofweaker reflectivities.More recently, the polarization properties of weather radar returns have beenanalyzed to differentiate between hail and heavy rain. The use of differentialreflectivity (Z_{dr}), in combination with horizontal reflectivity (Z_{h}) has ledto a variety of hail classification algorithms. Visible satellite imagery isbeginning to be used to detect hail, but false alarm rates remain high using thismethod.Size and terminal velocityThe size of hailstones is best determined by measuring their diameter with aruler. In the absence of a ruler, hailstone size is often visually estimated bycomparing its size to that of known objects, such as coins. Using the 41. objects such as hen's eggs, peas, and marbles for comparing hailstone sizes isoften imprecise, due to their varied dimensions. The UK organisation, TORRO,also scales for both hailstones and hailstorms. When observed at an airport,METAR code is used within a surface weather observation which relates to thesize of the hailstone. Within METAR code, GR is used to indicate larger hail, ofa diameter of at least 0.25 inches (6.4 mm). GR is derived from the Frenchword grle. Smaller-sized hail, as well as snow pellets, use the coding of GS,which is short for the French word grsil.Hail recordsMegacryometeors, large rocks of ice that are not associated withthunderstorms, are not officially recognized by the World MeteorologicalOrganization as "hail," which are aggregations of ice associated withthunderstorms, and therefore records of extreme characteristics ofmegacryometers are not given as hail records. Heaviest: 1.0 kg (2.25 lb); Gopalganj District, Bangladesh, 14 April 1986. Largest diameter officially measured: 8.0 inches (20 cm) diameter, 18.625 42. inches (47.3 cm) circumference; Vivian, South Dakota, 23 July 2010. Largest circumference officially measured: 18.75 inches (47.6 cm)circumference, 7.0 inches (18 cm) diameter; Aurora, Nebraska, 22 June 2003.Terminal velocity of hail, or the speed at which hail is falling when it strikes theground, varies by the diameter of the hailstones. A hailstone of 1 centimetre(0.39 in) in diameter falls at a rate of 9 metres per second (20 mph), whilestones the size of 8 centimetres (3.1 in) in diameter fall at a rate of 48 metresper second (110 mph). Hailstone velocity is dependent on the size of the stone,friction with air it is falling through, the motion of wind it is falling through,collisions with raindrops or other hailstones, and melting as the stones fallthrough a warmer atmosphere.HazardsHail can cause serious damage, notably to automobiles, aircraft, skylights, glass-roofedstructures, livestock, and most commonly, farmers' crops. Hail damageto roofs often goes unnoticed until further structural damage is seen, such asleaks or cracks. It is hardest to recognize hail damage on shingled roofs and 43. flat roofs, but all roofs have their own hail damage detection problems. Metalroofs are fairly resistant to hail damage, but may accumulate cosmetic damage inthe form of dents and damaged coatings.Hail is one of the most significant thunderstorm hazards to aircraft. Whenhailstones exceed 0.5 inches in diameter, planes can be seriously damaged withinseconds. The hailstones accumulating on the ground can also be hazardous tolanding aircraft. Hail is also a common nuisance to drivers of automobiles,severely denting the vehicle and cracking or even shattering windshields andwindows. Wheat, corn, soybeans, and tobacco are the most sensitive crops to haildamage. Hail is one of Canada's most expensive hazards. Rarely, massivehailstones have been known to cause concussions or fatal head trauma.Hailstorms have been the cause of costly and deadly events throughout history.One of the earliest recorded incidents occurred around the 9th century inRoopkund, Uttarakand, India. The largest hailstone in terms of diameter andweight ever recorded in the United States fell on July 23, 2010 in Vivian, SouthDakota; it measured 8 inches in diameter and 18.62 inches in circumference,weighing in at 1.93 pounds. This broke the previous record for diameter set by ahailstone 7 inches diameter and 18.75 inches circumference which fell in Aurora,Nebraska in the United States on June 22, 2003, as well as the record forweight, set by a hailstone of 1.67 pounds that fell in Coffeyville, Kansas in 1970. 44. AccumulationsNarrow zones where hail accumulates on the ground in association withthunderstorm activity are known as hail streaks or hail swaths, which can bedetectable by satellite after the storms pass by. Hailstorms normally last from afew minutes up to 15 minutes in duration. Accumulating hail storms can blanketthe ground with over 2 inches (5.1 cm) of hail, cause thousands to lose power,and bring down many trees. Flash flooding and mudslides within areas of steepterrain can be a concern with accumulating hail.On somewhat rare occasions, a thunderstorm can become stationary or nearly sowhile prolifically producing hail and significant depths of accumulation do occur;this tends to happen in mountainous areas, such as the July 29, 2010 case of afoot of hail accumulation in Boulder County, Colorado. Depths of up to a metrehave been reported. A landscape covered in accumulated hail generally resemblesone covered in accumulated snow and any significant accumulation of hail has thesame restrictive effects as snow accumulation, albeit over a smaller area, ontransport and infrastructure. Accumulated hail can also cause flooding byblocking drains, and hail can be carried in the floodwater, turning into a snow likeslush which is deposited at lower elevations. 45. Suppression and preventionDuring the Middle Ages, people in Europe used to ring church bells and firecannons to try to prevent hail, and the subsequent damage to crops. Updatedversions of this approach are available as modern hail cannons. Cloud seedingafter World War II was done to eliminate the hail threat, particularly acrossRussia. 46. A heat wave is a prolonged period of excessively hot weather, which may beaccompanied by high humidity. While definitions vary, a heat wave is measuredrelative to the usual weather in the area and relative to normal temperaturesfor the season. Temperatures that people from a hotter climate consider normalcan be termed a heat wave in a cooler area if they are outside the normalclimate pattern for that area. The term is applied both to routine weathervariations and to extraordinary spells of heat which may occur only once acentury. Severe heat waves have caused catastrophic crop failures, thousands ofdeaths from hyperthermia, and widespread power outages due to increased useof air conditioning. Definitions How they occur Health effects. Mortality. Psychological and sociological effects. Wildfires 47. DefinitionsThe definition recommended by the World Meteorological Organization is whenthe daily maximum temperature of more than five consecutive days exceeds theaverage maximum temperature by 5 C (9 F), the normal period being 19611990.A formal, peer-reviewed definition from the Glossary of Meteorology is:A period of abnormally and uncomfortably hot and usually humid weather.To be a heat wave such a period should last at least one day, but conventionallyit lasts from several days to several weeks. In 1900, A. T. Burrows more rigidlydefined a hot wave as a spell of three or more days on each of which themaximum shade temperature reaches or exceeds 90 F (32.2 C). Temperatureanomalies, March to May, 2007 In the Netherlands, a heat wave is defined asperiod of at least 5 consecutive days in which the maximum temperature in DeBilt exceeds 25 C (77 F), provided that on at least 3 days in this period themaximum temperature in De Bilt exceeds 30 C (86 F). This definition of a heatwave is also used in Belgium and Luxembourg. 48. In Adelaide, a heat wave is defined as five consecutive days at or above 35 C(95 F), or three consecutive days at or over 40 C (104 F).In the England and Wales, the Met Office operates a Heat Health Watchsystem which places each Local Authority region into one of four levels.Heatwave conditions are defined by the maximum daytime temperature andminimum nighttime temperature rising above the threshold for a particularregion. The length of time spent above that threshold determines the particularlevel. Level 1 is normal summer conditions. Level 2 is reached when there is a60% or higher risk that the temperature will be above the threshold levels fortwo days and the intervening night. 49. How they occurHeat waves form when high pressure aloft (from 10,00025,000 feet (3,0007,600 metres)) strengthens and remains over a region for several days up toseveral weeks. This is common in summer (in both Northern and SouthernHemispheres) as the jet stream 'follows the sun'. On the equator side of thejet stream, in the middle layers of the atmosphere, is the high pressure area.Summertime weather patterns are generally slower to change than in winter. Asa result, this mid-level high pressure also moves slowly. Under high pressure,the air subsides (sinks) toward the surface. This sinking air acts as a domecapping the atmosphere.This cap helps to trap heat instead of allowing it to lift. Without the lift thereis little or no convection and therefore little or no convective clouds (cumulusclouds) with minimal chances for rain. The end result is a continual build-up ofheat at the surface that we experience as a heat wave.Global warming boosts the probability of extreme weather events, like heatwaves, far more than it boosts more moderate events 50. Health effects 51. The heat index (as shown in the table above) is a measure of how hot it feels whenrelative humidity is factored with the actual air temperature. Hyperthermia, alsoknown as heat stroke, becomes commonplace during periods of sustained hightemperature and humidity. Sweating is absent from 84%100% of those affected.Older adults, very young children, and those who are sick or overweight are at ahigher risk for heat-related illness. The chronically ill and elderly are often takingprescription medications (e.g., diuretics, anticholinergics, antipsychotics, andantihypertensives) that interfere with the body's ability to dissipate heat.Heat edema presents as a transient swelling of the hands, feet, and ankles and isgenerally secondary to increased aldosterone secretion, which enhances waterretention. When combined with peripheral vasodilation and venous stasis, theexcess fluid accumulates in the dependent areas of the extremities. The heatedema usually resolves within several days after the patient becomes acclimatedto the warmer environment. No treatment is required, although wearing supportstocking and elevating the affected legs with help minimize the edema.Heat rash, also known as prickly heat, is a maculopapular rash accompanied byacute inflammation and blocked sweat ducts. The sweat ducts may become dilatedand may eventually rupture, producing small pruritic vesicles on an erythematousbase. Heat rash affects areas of the body covered by tight clothing. 52. Underreporting and "Harvesting" effectThe number of heat fatalities is likely highly underreported due to lack ofreports and misreports.[20] Part of the mortality observed during a heat wave,however, can be attributed to a so-called "harvesting effect", a term for ashort-term forward mortality displacement. It has been observed that for someheat waves, there is a compensatory decrease in overall mortality during thesubsequent weeks after a heat wave. Such compensatory reduction in mortalitysuggests that heat affects especially those so ill that they "would have died inthe short term anyway.Psychological and sociological effectsIn addition to physical stress, excessive heat causes psychological stress, to adegree which affects performance, and is also associated with an increase inviolent crime.Power outagesAbnormally hot temperatures cause electricity demand to increase during thepeak summertime hours of 4 to 7 p.m. when air conditioners are straining to 53. overcome the heat. If a hot spell extends to three days or more, however,nighttime temperatures do not cool down, and the thermal mass in homes andbuildings retains the heat from previous days. This heat build-up causes airconditioners to turn on earlier and to stay on later in the day. As a result,available electricity supplies are challenged during a higher, wider, peakelectricity consumption period.WildfiresIf a heat wave occurs during a drought, which dries out vegetation, it cancontribute to bushfires and wildfires. During the disastrous heat wave thatstruck Europe in 2003, fires raged through Portugal, destroying over 3,010square kilometres (1,160 sq mi) or 301,000 hectares (740,000 acres) of forestand 440 square kilometres (170 sq mi) or 44,000 hectares (110,000 acres) ofagricultural land and causing an estimated 1 billion worth of damage. High endfarmlands have irrigation systems to back up crops with.Physical damageHeat waves can and do cause roads and highways to buckle and melt,[31] waterlines to burst, and power transformers to detonate, causing fires. See the 2006North American heat wave article about heat waves causing physical damage. 54. A tornado is a violently rotating column of air that is in contact with both thesurface of the earth and a cumulonimbus cloud or, in rare cases, the base of acumulus cloud. They are often referred to as twisters or cyclones, although theword cyclone is used in meteorology, in a wider sense, to name any closed lowpressure circulation. Tornadoes come in many shapes and sizes, but they aretypically in the form of a visible condensation funnel, whose narrow end touchesthe earth and is often encircled by a cloud of debris and dust. Most tornadoeshave wind speeds less than 110 miles per hour (177 km/h), are about 250 feet(76 m) across, and travel a few miles (several kilometers) before dissipating.The most extreme tornadoes can attain wind speeds of more than 300 miles perhour (483 km/h), stretch more than two miles (3.2 km) across, and stay on theground for dozens of miles (more than 100 km).Various types of tornadoes include the landspout, multiple vortex tornado, andwaterspout. Waterspouts are characterized by a spiraling funnel-shaped windcurrent, connecting to a large cumulus or cumulonimbus cloud. They aregenerally classified as non-supercellular tornadoes that develop over bodies ofwater, but there is disagreement over whether to classify them as truetornadoes. These spiraling columns of air frequently develop in tropical areasclose to the equator, and are less common at high latitudes. Other 55. tornado-like phenomena that exist in nature include the gustnado, dust devil,fire whirls, and steam devil; downbursts are frequently confused with tornadoes,though their action is dissimilar.Tornadoes have been observed on every continent except Antarctica. However,the vast majority of tornadoes occur in the Tornado Alley region of the UnitedStates, although they can occur nearly anywhere in North America. They alsooccasionally occur in south-central and eastern Asia, northern and east-centralSouth America, Southern Africa, northwestern and southeast Europe, westernand southeastern Australia, and New Zealand. Tornadoes can be detectedbefore or as they occur through the use of Pulse-Doppler radar by recognizingpatterns in velocity and reflectivity data, such as hook echoes or debris balls, aswell as by the efforts of storm spotters.There are several scales for rating the strength of tornadoes. The Fujita scalerates tornadoes by damage caused and has been replaced in some countries bythe updated Enhanced Fujita Scale. An F0 or EF0 tornado, the weakestcategory, damages trees, but not substantial structures. An F5 or EF5 tornado,the strongest category, rips buildings off their foundations and can deformlarge skyscrapers. The similar TORRO scale ranges from a T0 for extremelyweak tornadoes to T11 for the most powerful known tornadoes. 56. Etymology Definitions Characteristics Life cycle Types Intensity and damage Climatology Detection Extremes Safety 57. EtymologyThe word tornado is an altered form of the Spanish word tronada, which means"thunderstorm". This in turn was taken from the Latin tonare, meaning "tothunder". It most likely reached its present form through a combination of theSpanish tronada and tornar ("to turn"); however, this may be a folketymology.[10][11] A tornado is also commonly referred to as a "twister", and isalso sometimes referred to by the old-fashioned colloquial term cyclone.[12][13]The term "cyclone" is used as a synonym for "tornado" in the often-aired 1939film The Wizard of Oz. The term "twister" is also used in that film, along withbeing the title of the 1996 tornado-related film Twister.DefinitionsA tornado is "a violently rotating column of air, in contact with the ground,either pendant from a cumuliform cloud or underneath a cumuliform cloud, andoften (but not always) visible as a funnel cloud". For a vortex to be classified asa tornado, it must be in contact with both the ground and the cloud base.Scientists have not yet created a complete definition of the word; for example,there is disagreement as to whether separate touchdowns of the same funnelconstitute separate tornadoes. Tornado refers to the vortex of wind, not thecondensation cloud. 58. Size and shapeA wedge tornado, nearly a mile wide, which hit Binger, Oklahoma in 1981Most tornadoes take on the appearance of a narrow funnel, a few hundred yards(meters) across, with a small cloud of debris near the ground. Tornadoes may beobscured completely by rain or dust. These tornadoes are especially dangerous,as even experienced meteorologists might not see them. Tornadoes can appear inmany shapes and sizes.Small, relatively weak landspouts may be visible only as a small swirl of dust onthe ground. Although the condensation funnel may not extend all the way to theground, if associated surface winds are greater than 40 mph (64 km/h), thecirculation is considered a tornado. A tornado with a nearly cylindrical profileand relative low height is sometimes referred to as a "stovepipe" tornado. Largesingle-vortex tornadoes can look like large wedges stuck into the ground, and soare known as "wedge tornadoes" or "wedges". The "stovepipe" classification isalso used for this type of tornado, if it otherwise fits that profile. A wedge canbe so wide that it appears to be a block of dark clouds, wider than the distancefrom the cloud base to the ground. Even experienced storm observers may notbe able to tell the difference between a low-hanging cloud and a wedge. 59. AppearanceTornadoes can have a wide range of colors, depending on the environment in whichthey form. Those that form in dry environments can be nearly invisible, markedonly by swirling debris at the base of the funnel. Condensation funnels that pickup little or no debris can be gray to white. While traveling over a body of water(as a waterspout), tornadoes can turn very white or even blue. Slow-movingfunnels, which ingest a considerable amount of debris and dirt, are usually darker,taking on the color of debris. Tornadoes in the Great Plains can turn red becauseof the reddish tint of the soil, and tornadoes in mountainous areas can travel oversnow-covered ground, turning white.RotationTornadoes normally rotate cyclonically (when viewed from above, this iscounterclockwise in the northern hemisphere and clockwise in the southern).While large-scale storms always rotate cyclonically due to the Coriolis effect,thunderstorms and tornadoes are so small that the direct influence of the Corioliseffect is unimportant, as indicated by their large Rossby numbers. Supercells andtornadoes rotate cyclonically in numerical simulations even when the Corioliseffect is neglected.[34][35] Low-level mesocyclones and tornadoes owe theirrotation to complex processes within the supercell and ambient environment. 60. Sound and seismologyTornadoes emit widely on the acoustics spectrum and the sounds are caused bymultiple mechanisms. Various sounds of tornadoes have been reported, mostlyrelated to familiar sounds for the witness and generally some variation of awhooshing roar. Popularly reported sounds include a freight train, rushingrapids or waterfall, a nearby jet engine, or combinations of these. Manytornadoes are not audible from much distance; the nature and propagationdistance of the audible sound depends on atmospheric conditions andtopography.The winds of the tornado vortex and of constituent turbulent eddies, as well asairflow interaction with the surface and debris, contribute to the sounds.Funnel clouds also produce sounds. Funnel clouds and small tornadoes arereported as whistling, whining, humming, or the buzzing of innumerable bees orelectricity, or more or less harmonic, whereas many tornadoes are reported asa continuous, deep rumbling, or an irregular sound of "noise".Tornadoes also produce identifiable inaudible infrasonic signatures. 61. Lifecycle Supercell relationship Formation Maturity DissipationAs the tornado enters the dissipating stage, its associated mesocyclone oftenweakens as well, as the rear flank downdraft cuts off the inflow powering it.Sometimes, in intense supercells, tornadoes can develop cyclically. As the firstmesocyclone and associated tornado dissipate, the storm's inflow may beconcentrated into a new area closer to the center of the storm. If a newmesocyclone develops, the cycle may start again, producing one or more newtornadoes. Occasionally, the old (occluded) mesocyclone and the newmesocyclone produce a tornado at the same time.Although this is a widely accepted theory for how most tornadoes form, live,and die, it does not explain the formation of smaller tornadoes, such aslandspouts, long-lived tornadoes, or tornadoes with multiple vortices. Theseeach have different mechanisms which influence their developmenthowever,most tornadoes follow a pattern similar to this one. 62. Types Multiple vortex Waterspout Landspout Gustnado Dust devil Fire whirls and steam devilsSmall-scale, tornado-like circulations can occur near any intense surface heatsource. Those that occur near intense wildfires are called fire whirls. They arenot considered tornadoes, except in the rare case where they connect toa pyrocumulus or other cumuliform cloud above. Fire whirls usually are not asstrong as tornadoes associated with thunderstorms. They can, however,produce significant damage. A steam devil is a rotating updraft that involvessteam or smoke. Steam devils are very rare. They most often form from smokeissuing from a power plant's smokestack. Hot springs and deserts may also besuitable locations for a steam devil to form. The phenomenon can occur overwater, when cold arctic air passes over relatively warm water. 63. Intensity and damageThe Fujita scale and the Enhanced Fujita Scale rate tornadoes by damagecaused. The Enhanced Fujita (EF) Scale was an upgrade to the older Fujita scale,by expert elicitation, using engineered wind estimates and better damagedescriptions. The EF Scale was designed so that a tornado rated on the Fujitascale would receive the same numerical rating, and was implemented starting inthe United States in 2007. An EF0 tornado will probably damage trees but notsubstantial structures, whereas an EF5 tornado can rip buildings off theirfoundations leaving them bare and even deform large skyscrapers.ClimatologyThe United States has the most tornadoes of any country, nearly four timesmore than estimated in all of Europe, excluding waterspouts. This is mostly dueto the unique geography of the continent. North America is a large continentthat extends from the tropics north into arctic areas, and has no major east-westmountain range to block air flow between these two areas. In the middlelatitudes, where most tornadoes of the world occur, the Rocky Mountains blockmoisture and buckle the atmospheric flow, forcing drier air at mid-levels of thetroposphere due to downsloped winds, and causing the formation of a low. 64. DetectionRigorous attempts to warn of tornadoes began in the United States in the mid-20th century. Before the 1950s, the only method of detecting a tornado was bysomeone seeing it on the ground. Often, news of a tornado would reach a localweather office after the storm. However, with the advent of weather radar,areas near a local office could get advance warning of severe weather. The firstpublic tornado warnings were issued in 1950 and the first tornado watches andconvective outlooks in 1952. Radar Storm spotting Visual evidence 65. ExtremesThe most record-breaking tornado in recorded history was the Tri-StateTornado, which roared through parts of Missouri, Illinois, and Indiana on March18, 1925. It was likely an F5, though tornadoes were not ranked on any scale inthat era. It holds records for longest path length (219 miles, 352 km), longestduration (about 3.5 hours), and fastest forward speed for a significant tornado(73 mph, 117 km/h) anywhere on Earth. In addition, it is the deadliest singletornado in United States history (695 dead). The tornado was also the secondcostliest tornado in history at the time.SafetyThough tornadoes can strike in an instant, there are precautions and preventativemeasures that people can take to increase the chances of surviving a tornado.Authorities such as the Storm Prediction Center advise having a pre-determinedplan should a tornado warning be issued. When a warning is issued, going to abasement or an interior first-floor room of a sturdy building greatly increaseschances of survival. In tornado-prone areas, many buildings have storm cellars onthe property. These underground refuges have saved thousands of lives. 66. Epidemics The A H5N1 virus, which causes Avian influenza An epidemic is an outbreak of a contractible disease that spreads through ahuman population. A pandemic is an epidemic whose spread is global. There havebeen many epidemics throughout history, such as the Black Death. In the lasthundred years, significant pandemics include: The 1918 Spanish flu pandemic, killing an estimated 50 million peopleworldwide The 1957-58 Asian flu pandemic, which killed an estimated 1 million people The 1968-69 Hong Kong water flu pandemic The 2002-3 SARS pandemic The AIDS pandemic, beginning in 1959 The H1N1 Influenza (Swine Flu) Pandemic 2009-2010 Other diseases that spread more slowly, but are still considered to be globalhealth emergencies by the WHO, include: XDR TB, a strain of tuberculosis that is extensively resistant to drugtreatments Malaria, which kills an estimated 1.6 million people each year Ebola hemorrhagic fever, which has claimed hundreds of victims in Africa inseveral outbreaks 67. Space disastersImpact eventsOne of the largest impact events in modern times was the Tunguska event inJune 1908.Solar flareA solar flare is a phenomenon where the sun suddenly releases a great amountof solar radiation, much more than normal. Some known solar flares include:An X20 event on August 16, 1989A similar flare on April 2, 2001The most powerful flare ever recorded, on November 4, 2003, estimated atbetween X40 and X45The most powerful flare in the past 500 years is believed to have occurred inSeptember 1859 68. Gamma-ray burstGamma-ray bursts (GRBs) are flashes of gamma rays associated with extremelyenergetic explosions that have been observed in distant galaxies. They are thebrightest electromagnetic events known to occur in the universe. Bursts can lastfrom ten milliseconds to several minutes. The initial burst is usually followed by alonger-lived "afterglow" emitted at longer wavelengths (X-ray, ultraviolet,optical, infrared, microwave and radio).All the bursts astronomers have recorded so far have come from distantgalaxies and have been harmless to Earth, but if one occurred within our galaxyand were aimed straight at us, the effects could be devastating. Currentlyorbiting satellites detect an average of about one gamma-ray burst per day. Theclosest known GRB so far was GRB 031203. 69. Protection by international lawInternational law, for example Geneva Conventions defines International RedCross and Red Crescent Movement the Convention on the Rights of Personswith Disabilities, requires that "States shall take, in accordance with theirobligations under international law, including international humanitarian law andinternational human rights law, all necessary measures to ensure theprotection and safety of persons with disabilities in situations of risk,including the occurrence of natural disaster." And further United NationsOffice for the Coordination of Humanitarian Affairs is formed by GeneralAssembly Resolution 44/182. People displaced due to natural disasters arecurrently protected under international law (Guiding Principles ofInternational Displacement, Campala Convention of 2009)


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