you can learn about disasters.

1. Mukunda
Pr iya

6. A natural disaster is a major adverse event
resulting from natural processes of the Earth.
Examples: floods, volcanic
eruptions, earthquakes, tsunamis, and other
geologic processes.
A natural disaster can cause loss of life or
property damage and typically leaves some
economic damage in its wake, the severity of
which depends on the affected
population's resilience or ability to recover.

7. An adverse event will not rise to the level
of a disaster if it occurs in an area without
vulnerable population. In a vulnerable
area, however, such as San Francisco,
an earthquake can have disastrous
consequences and leave lasting damage,
requiring years to repair.

8. Catastrophes Statistics for the year 2012
Total count 905
Meteorological (Storms) 45%
Hydrological (Floods), 36%
Climatological (Heat waves, cold
Waves, Droughts, Wildfires) 12%
Geophysical events (Earthquakes
and Volcanic eruptions). 7%
Catastrophes 93%
Cost in Billion US $ 170
Insured Losses US $ 70
93%
Between 1980 and 2011 geophysical events
accounted for 14% of all natural catastrophes.

12.  Droughts
 Hailstorms
 Heat waves
 Tornadoes
 Wild fires
 Health disasters
 Epidemics
 Space disasters
 Impact events
 Solar flare
 Gamma-ray burst
 Protection by international law

15. Avalanches

16. An avalanche (also called a snowslide or snowslip)
is a rapid flow of snow down a slope. Avalanches are
typically triggered in a starting zone from a
mechanical failure in the snowpack (slab avalanche)
when the forces on the snow exceed its strength but
sometimes only with gradually widening (loose snow
avalanche). After initiation, avalanches usually
accelerate rapidly and grow in mass and volume as
they entrain more snow. If the avalanche moves fast
enough some of the snow may mix with the air
forming a powder snow avalanche, which is a type
of gravity current.

17. Slides of rocks or debris, behaving in a similar way to snow,
are also referred to as avalanches (see rockslide). The
remainder of this article refers to snow avalanches.
The load on the snowpack may be only due to gravity, in
which case failure may result either from weakening in the
snowpack or increased load due to precipitation.
Avalanches that occur in this way are known as
spontaneous avalanches. Avalanches can also be triggered
by other loads such as skiers, snowmobilers, animals or
explosives. Seismic activity may also trigger failure in the
snowpack and avalanches.

18. Although primarily composed of flowing snow and
air, large avalanches have the capability to entrain
ice, rocks, trees, and other material on the slope,
and are distinct from mudslides, rock slides,
and serac collapses on an icefall. Avalanches are
not rare or random events and are endemic to any
mountain range that accumulates a standing
snowpack. Avalanches are most common during
winter or spring but glacier movements may cause
ice and snow avalanches at any time of year.

19. In mountainous terrain, avalanches are among the
most serious objective natural hazards to life and
property, with their destructive capability resulting
from their potential to carry enormous masses of
snow at high speeds.
There is no universally accepted classification of
avalanches—different classifications are useful for
different purposes. Avalanches can be described by
their size, their destructive potential, their
initiation mechanism, their composition and their
dynamics.

20.  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

21. Formation
and
classification
Most avalanches
occur spontaneously
during storms under
increased load due to
snowfall. The second
largest cause of
natural avalanches is
metamorphic
changes in the
snowpack such as
melting due to solar
radiation.

22. Loose snow
avalanches
Loose snow
avalanches(far left)
and slab avalanches
(near center)
nearMount
Shuksan in
the North Cascades
mountains.
Fracture
propagation is
relatively limited.

23. Slab
avalanches
A crown fracture
from a slab
avalanche near
the Neve Glacier
in the North
Cascades
mountains.
Extensive fracture
propagation is
evident.

24. Powder
snow
avalanches
A powder snow
avalanche in
the Himalayas
nearMount
Everest.

25. Dry snow
avalanches
Dry snow
avalanche
with a
powder
cloud

26. Terrain,
snowpack,
weather
Doug Fesler and
Jill Fredston
developed a
conceptual
model of the
three primary
elements of
avalanches:
terrain, weather,
and snowpack.

27. Terrain
In steep
avalanche-prone
terrain,
traveling on
ridges is
generally safer
than traversing
the slopes.

29. Drought is an extended period when a region notes a deficiency in its water
supply whether surface or underground water. A drought can last for months or
years, or may be declared after as few as 15 days. Generally, this occurs when a
region receives consistently below average precipitation. It can have a substantial
impact on the ecosystem and agriculture of the affected region
Many plant species, such as cacti, have adaptations such as reduced leaf area and
waxy cuticles to enhance their ability to tolerate drought. Some others survive
dry periods as buried seeds. Semi-permanent drought produces arid biomes such
as deserts and grasslands. Most arid ecosystems have inherently low
productivity.
This global phenomenon has a widespread impact on agriculture. Lengthy periods
of drought have long been a key trigger for mass migration and played a key role
in a number of ongoing migrations and other humanitarian crises in the Horn of
Africa and the Sahel.
According to F. Bagouls and Henri Gaussen's definition, a month is dry when the
mean monthly precipitation in millimeters is equal to or lower than twice the mean
monthly temperature in °C.

30. •Consequences
•Globally
. Regions affected
•Causes
•Types
•Protection and relief

31. Consequences
A Mongolian gazelle dead due to drought. Periods of droughts can have
significant environmental, agricultural, health, economic and social
consequences. The effect varies according to vulnerability. For example,
subsistence farmers are more likely to migrate during drought because they
do not have alternative food sources. Areas with populations that depend on as
a major food source are more vulnerable to famine.
Drought can also reduce water quality, because lower water flows reduce
dilution of pollutants and increase contamination of remaining water sources.
Common consequences of drought include:
•Diminished crop growth or yield productions and carrying capacity for
livestock
•Dust bowls, themselves a sign of erosion, which further erode the landscape
•Dust storms, when drought hits an area suffering from desertification and
erosion

32. •Famine due to lack of water for irrigation
•Habitat damage, affecting both terrestrial and aquatic wildlife
•Hunger, drought provides too little water to support food crops.
•Malnutrition, dehydration and related diseases
•Mass migration, resulting in internal displacement and international refugees
•Reduced electricity production due to reduced water flow through
hydroelectric dams
•Shortages of water for industrial users
•Snake migration and increases in snakebites
•Social unrest
•War over natural resources, including water and food

33. Globally
Drought is a normal, recurring feature of the climate in most parts of the
world. It is among the earliest documented climatic events, present in the Epic
of Gilgamesh and tied to the biblical story of Joseph's arrival in and the later
Exodus from Ancient Egypt.
Hunter-gatherer migrations in 9,500 BC Chile have been linked to the
phenomenon, as has the exodus of early humans out of Africa and into the rest
of the world around 135,000 years ago.
Modern people can effectively mitigate much of the impact of drought through
irrigation and crop rotation. Failure to develop adequate drought mitigation
strategies carries a grave human cost in the modern era, exacerbated by ever-increasing
population densities.
Regions affected
Recurring droughts leading to desertification in the Horn of Africa have
created grave ecological catastrophes, prompting massive food shortages, still
recurring. To the north-west of the Horn, the Darfur conflict in neighbouring

34. Sudan, also affecting Chad, was fueled by decades of drought and overpopulation
are among the causes of the Darfur conflict, because the Arab Baggara nomads
searching for water have to take their livestock further south, to land mainly
occupied by non-Arab farming peoples.
Approximately 2.4 billion people live in the drainage basin of the Himalayan
rivers. India, China, Pakistan, Bangladesh, Nepal and Myanmar could experience
floods followed by droughts in coming decades. Drought in India affecting the
Ganges is of particular concern, as it provides drinking water and agricultural
irrigation 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 Rocky
Mountains 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 showing
that the forest in its present form could survive only three years of drought.
Scientists at the Brazilian National Institute of Amazonian Research argue in
the article that this drought response, coupled with the effects of
deforestation on regional climate, are pushing the rainforest towards a "tipping
point" where it would irreversibly start to die.

35. It concludes that the rainforest is on the brink of being turned into savanna or
desert, with catastrophic consequences for the world's climate. According to
the WWF, the combination of climate change and deforestation increases the
drying effect of dead trees that fuels forest fires.
By far the largest part of Australia is desert or semi-arid lands commonly known
as the outback. A 2005 study by Australian and American researchers
investigated the desertification of the interior, and suggested that one
explanation was related to human settlers who arrived about 50,000 years ago.
Regular burning by these settlers could have prevented monsoons from reaching
interior 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 basin
if it does not receive sufficient water by October. Australia could experience
more severe droughts and they could become more frequent in the future, a
government-commissioned report said on July 6, 2008. Australian
environmentalist Tim Flannery, predicted that unless it made drastic changes,
Perth in Western Australia could become the world’s first ghost metropolis, an
abandoned city with no more water to sustain its population

36. Causes
Generally, rainfall is related to the amount and dew point [determined by air
temperature] of water vapour carried by regional atmosphere, combined with
the upward forcing of the air mass containing that water vapour. If these
combined factors do not support precipitation volumes sufficient to reach the
surface, the result is a drought. This can be triggered by high level of reflected
sunlight, [high albedo], and above average prevalence of high pressure systems,
winds carrying continental, rather than oceanic air masses (i.e. reduced water
content), and ridges of high pressure areas from behaviors which prevent or
restrict the developing of thunderstorm activity or rainfall over one certain
region. Oceanic and atmospheric weather cycles such as the El Niño-Southern
Oscillation (ENSO) make drought a regular recurring feature of the Americas
along the Midwest and Australia. Guns, Germs, and Steel author Jared Diamond
sees the stark impact of the multi-year ENSO cycles on Australian weather
patterns as a key reason that Australian aborigines remained a hunter-gatherer
society rather than adopting agriculture. Another climate oscillation known as
the North Atlantic Oscillation has been tied to droughts in northeast Spain.
Human activity can directly trigger exacerbating factors such as

37. farming, excessive irrigation, deforestation, and erosion adversely impact the
ability of the land to capture and hold water. While these tend to be relatively
isolated in their scope, activities resulting in global climate change are expected
to trigger droughts with a substantial impact on agriculture throughout the
world, and especially in developing nations. Overall, global warming will result in
increased world rainfall. Along with drought in some areas, flooding and erosion
will increase in others. Paradoxically, some proposed solutions to global warming
that focus on more active techniques, solar radiation management through the
use of a space sunshade for one, may also carry with them increased chances of
drought.

38. Types
As a drought persists, the conditions surrounding it gradually worsen and its
impact on the local population gradually increases. People tend to define
droughts in three main ways:
Meteorological drought is brought about when there is a prolonged period
with less than average precipitation. Meteorological drought usually precedes
the other kinds of drought.
Agricultural droughts are droughts that affect crop production or the ecology
of the range. This condition can also arise independently from any change in
precipitation levels when soil conditions and erosion triggered by poorly planned
agricultural endeavors cause a shortfall in water available to the crops. However,
in a traditional drought, it is caused by an extended period of below average
precipitation.
Hydrological drought is brought about when the water reserves available in
sources such as aquifers, lakes and reservoirs fall below the statistical average.
Hydrological drought tends to show up more slowly because it involves stored

39. water that is used but not replenished. Like an agricultural drought, this can be
triggered by more than just a loss of rainfall. For instance, Kazakhstan was
recently awarded a large amount of money by the World Bank to restore water
that had been diverted to other nations from the Aral Sea under Soviet rule.
Similar circumstances also place their largest lake, Balkhash, at risk of
completely drying out.

40. Protection and Relief
Strategies for drought protection, mitigation or relief include:
 Dams - many dams and their associated reservoirs supply additional water in
times 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 and
comparisons with current usage levels can help prevent man-made drought. For
instance, 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 Khat
crop. Careful monitoring of moisture levels can also help predict increased risk
for wildfires, using such metrics as the Keetch-Byram Drought Index
 Land use - Carefully planned crop rotation can help to minimize erosion and
allow farmers to plant less water-dependent crops in drier years.

41.  Outdoor water- use restriction - Regulating the use of sprinklers, hoses or
buckets on outdoor plants, filling pools, and other water-intensive home
maintenance tasks.
 Rainwater harvesting - Collection and storage of rainwater from roofs or
other suitable catchments.
 Recycled water - Former wastewater (sewage) that has been treated and
purified for reuse.
 Transvasement - Building canals or redirecting rivers as massive attempts at
irrigation in drought-prone areas.

43. Hail Storms is a form of solid precipitation. It consists of balls or irregular
lumps of ice, each of which is called a hailstone. Unlike graupel, which is made
of rime, and ice pellets, which are smaller and translucent, hailstones consist
mostly of water ice and measure between 5 millimetres (0.20 in) and 15
centimetres (6 in) in diameter. The METAR reporting code for hail 5 mm (0.20
in) or greater is GR, while smaller hailstones and graupel are coded GS. Hail is
possible within most thunderstorms as it is produced by cumulonimbi, and
within 2 nautical miles (3.7 km) of the parent storm. Hail formation requires
environments 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, it
tends to be confined to high elevations.
There are methods available to detect hail-producing thunderstorms using
weather satellites and weather radar imagery. Hailstones generally fall at
higher speeds as they grow in size, though complicating factors such as
melting, friction with air, wind, and interaction with rain and other hailstones
can slow their descent through Earth's atmosphere. Severe weather warnings
are issued for hail when the stones reach a damaging size, as it can cause
serious damage to human-made structures and, most commonly, farmers' crops.

44. • 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

45. Definition
Any thunderstorm which produces hail that reaches the ground is known as a
hailstorm. Hail has a diameter of 5 millimetres (0.20 in) or more. Hailstones can
grow 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 clumped
together. Hail is composed of transparent ice or alternating layers of
transparent and translucent ice at least 1 millimetre (0.039 in) thick, which are
deposited upon the hailstone as it travels through the cloud, suspended aloft by
air with strong upward motion until its weight overcomes the updraft and falls
to the ground. Although the diameter of hail is varied, in the United States, the
average 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 cause
damage. The Meteorological Service of Canada will issue severe thunderstorm
warnings when hail that size or above is expected. The US National Weather
Service has a 2.5 cm (1 in) or greater in diameter threshold, effective January
2010, an increase over the previous threshold of ¾-inch hail.

46. Formation
Hail forms in strong thunderstorm clouds, particularly those with intense
updrafts, 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. The
growth rate is maximized where air is near a temperature of −13 °C (9 °F).
Layer nature of the hailstones
Like other precipitation in cumulonimbus clouds hail begins as water droplets. As
the droplets rise and the temperature goes below freezing, they become
supercooled water and will freeze on contact with condensation nuclei. A cross-section
through a large hailstone shows an onion-like structure. This means the
hailstone is made of thick and translucent layers, alternating with layers that
are thin, white and opaque. Former theory suggested that hailstones were
subjected to multiple descents and ascents, falling into a zone of humidity and
refreezing as they were uplifted. This up and down motion was thought to be
responsible for the successive layers of the hailstone. New research, based on
theory as well as field study, has shown this is not necessarily true.

47. The storm's updraft, with upwardly directed wind speeds as high as 110 miles
per hour (180 km/h), blow the forming hailstones up the cloud. As the hailstone
ascends it passes into areas of the cloud where the concentration of humidity
and supercooled water droplets varies. The hailstone’s growth rate changes
depending on the variation in humidity and supercooled water droplets that it
encounters. The accretion rate of these water droplets is another factor in
the hailstone’s growth.
Furthermore, the hailstone’s speed depends on its position in the cloud’s
updraft and its mass. This determines the varying thicknesses of the layers of
the hailstone. The accretion rate of supercooled water droplets onto the
hailstone depends on the relative velocities between these water droplets and
the hailstone itself. This means that generally the larger hailstones will form
some distance from the stronger updraft where they can pass more time
growing. As the hailstone grows it releases latent heat, which keeps its
exterior in a liquid phase.

48. The hailstone will keep rising in the thunderstorm until its mass can no longer
be supported by the updraft. This may take at least 30 minutes based on the
force of the updrafts in the hail-producing thunderstorm, whose top is usually
greater than 10 km high. It then falls toward the ground while continuing to
grow, based on the same processes, until it leaves the cloud. It will later begin
to melt as it passes into air above freezing temperature.
Thus, a unique trajectory in the thunderstorm is sufficient to explain the
layer-like structure of the hailstone. The only case in which multiple
trajectories can be discussed is in a multicellular thunderstorm where the
hailstone may be ejected from the top of the "mother" cell and captured in the
updraft of a more intense "daughter cell". This however is an exceptional case.
Factors favoring hail
Hail is most common within continental interiors of the mid-latitudes, as hail
formation is considerably more likely when the freezing level is below the
altitude of 11,000 feet (3,400 m). Movement of dry air into strong
thunderstorms over continents can increase the frequency of hail by promoting
evaporational cooling which lowers the freezing level of thunderstorm clouds
giving hail a larger volume to grow in. Accordingly, hail is less common in

49. the tropics despite a much higher frequency of thunderstorms than in the mid-latitudes
because the atmosphere over the tropics tends to be warmer over a
much 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 above
20,000 feet (6,100 m). Between 10,000 feet (3,000 m) and 20,000 feet (6,100
m), 60 percent of hail is still within the thunderstorm, though 40 percent now
lies within the clear air under the anvil. Below 10,000 feet (3,000 m), hail is
equally distributed in and around a thunderstorm to a distance of 2 nautical
miles (3.7 km).
Climatology
Hail occurs most frequently within continental interiors at mid-latitudes and is
less common in the tropics, despite a much higher frequency of thunderstorms
than in the midlatitudes. Hail is also much more common along mountain ranges
because mountains force horizontal winds upwards (known as orographic
lifting), thereby intensifying the updrafts within thunderstorms and making

50. hail more likely. The higher elevations also result in there being less time
available for hail to melt before reaching the ground. One of the more common
regions for large hail is across mountainous northern India, which reported one
of the highest hail-related death tolls on record in 1888. China also
experiences significant hailstorms. Central Europe and southern Australia also
experience a lot of hailstorms. Popular regions for hailstorms are southern and
western Germany, northern and eastern France and southern and eastern
Benelux. In south-eastern Europe, Croatia and Serbia experience frequent
occurrences 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 between
the months of March and October during the afternoon and evening hours, with
the bulk of the occurrences from May through September. Cheyenne, Wyoming
is North America's most hail-prone city with an average of nine to ten
hailstorms per season.

51. Certain patterns of reflectivity are important clues for the meteorologist as
well. The three body scatter spike is an example. This is the result of energy
from the radar hitting hail and being deflected to the ground, where they
deflect back to the hail and then to the radar. The energy took more time to
go from the hail to the ground and back, as opposed to the energy that went
direct from the hail to the radar, and the echo is further away from the radar
than the actual location of the hail on the same radial path, forming a cone of
weaker reflectivities.
More recently, the polarization properties of weather radar returns have been
analyzed to differentiate between hail and heavy rain. The use of differential
reflectivity (Z_{dr}), in combination with horizontal reflectivity (Z_{h}) has led
to a variety of hail classification algorithms. Visible satellite imagery is
beginning to be used to detect hail, but false alarm rates remain high using this
method.
Size and terminal velocity
The size of hailstones is best determined by measuring their diameter with a
ruler. In the absence of a ruler, hailstone size is often visually estimated by
comparing its size to that of known objects, such as coins. Using the

52. objects such as hen's eggs, peas, and marbles for comparing hailstone sizes is
often 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 the
size of the hailstone. Within METAR code, GR is used to indicate larger hail, of
a diameter of at least 0.25 inches (6.4 mm). GR is derived from the French
word grêle. Smaller-sized hail, as well as snow pellets, use the coding of GS,
which is short for the French word grésil.
Hail records
Megacryometeors, large rocks of ice that are not associated with
thunderstorms, are not officially recognized by the World Meteorological
Organization as "hail," which are aggregations of ice associated with
thunderstorms, and therefore records of extreme characteristics of
megacryometers 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

53. •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 the
ground, 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), while
stones the size of 8 centimetres (3.1 in) in diameter fall at a rate of 48 metres
per 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 fall
through a warmer atmosphere.
Hazards
Hail can cause serious damage, notably to automobiles, aircraft, skylights, glass-roofed
structures, livestock, and most commonly, farmers' crops. Hail damage
to roofs often goes unnoticed until further structural damage is seen, such as
leaks or cracks. It is hardest to recognize hail damage on shingled roofs and

54. flat roofs, but all roofs have their own hail damage detection problems. Metal
roofs are fairly resistant to hail damage, but may accumulate cosmetic damage in
the form of dents and damaged coatings.
Hail is one of the most significant thunderstorm hazards to aircraft. When
hailstones exceed 0.5 inches in diameter, planes can be seriously damaged within
seconds. The hailstones accumulating on the ground can also be hazardous to
landing aircraft. Hail is also a common nuisance to drivers of automobiles,
severely denting the vehicle and cracking or even shattering windshields and
windows. Wheat, corn, soybeans, and tobacco are the most sensitive crops to hail
damage. Hail is one of Canada's most expensive hazards. Rarely, massive
hailstones 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 in
Roopkund, Uttarakand, India. The largest hailstone in terms of diameter and
weight ever recorded in the United States fell on July 23, 2010 in Vivian, South
Dakota; 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 a
hailstone 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 for
weight, set by a hailstone of 1.67 pounds that fell in Coffeyville, Kansas in 1970.

55. Accumulations
Narrow zones where hail accumulates on the ground in association with
thunderstorm activity are known as hail streaks or hail swaths, which can be
detectable by satellite after the storms pass by. Hailstorms normally last from a
few minutes up to 15 minutes in duration. Accumulating hail storms can blanket
the 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 steep
terrain can be a concern with accumulating hail.
On somewhat rare occasions, a thunderstorm can become stationary or nearly so
while 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 a
foot of hail accumulation in Boulder County, Colorado. Depths of up to a metre
have been reported. A landscape covered in accumulated hail generally resembles
one covered in accumulated snow and any significant accumulation of hail has the
same restrictive effects as snow accumulation, albeit over a smaller area, on
transport and infrastructure. Accumulated hail can also cause flooding by
blocking drains, and hail can be carried in the floodwater, turning into a snow like
slush which is deposited at lower elevations.

56. Suppression and prevention
During the Middle Ages, people in Europe used to ring church bells and fire
cannons to try to prevent hail, and the subsequent damage to crops. Updated
versions of this approach are available as modern hail cannons. Cloud seeding
after World War II was done to eliminate the hail threat, particularly across
Russia.

58. A heat wave is a prolonged period of excessively hot weather, which may be
accompanied by high humidity. While definitions vary, a heat wave is measured
relative to the usual weather in the area and relative to normal temperatures
for the season. Temperatures that people from a hotter climate consider normal
can be termed a heat wave in a cooler area if they are outside the normal
climate pattern for that area. The term is applied both to routine weather
variations and to extraordinary spells of heat which may occur only once a
century. Severe heat waves have caused catastrophic crop failures, thousands of
deaths from hyperthermia, and widespread power outages due to increased use
of air conditioning.
• Definitions
• How they occur
• Health effects
. Mortality
. Psychological and sociological effects
. Wildfires

60. Definitions
The definition recommended by the World Meteorological Organization is when
the daily maximum temperature of more than five consecutive days exceeds the
average maximum temperature by 5 °C (9 °F), the normal period being 1961–
1990.
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 conventionally
it lasts from several days to several weeks. In 1900, A. T. Burrows more rigidly
defined a “hot wave” as a spell of three or more days on each of which the
maximum shade temperature reaches or exceeds 90 °F (32.2 °C). Temperature
anomalies, March to May, 2007 In the Netherlands, a heat wave is defined as
period of at least 5 consecutive days in which the maximum temperature in De
Bilt exceeds 25 °C (77 °F), provided that on at least 3 days in this period the
maximum temperature in De Bilt exceeds 30 °C (86 °F). This definition of a heat
wave is also used in Belgium and Luxembourg.

61. 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 Watch
system which places each Local Authority region into one of four levels.
Heatwave conditions are defined by the maximum daytime temperature and
minimum nighttime temperature rising above the threshold for a particular
region. The length of time spent above that threshold determines the particular
level. Level 1 is normal summer conditions. Level 2 is reached when there is a
60% or higher risk that the temperature will be above the threshold levels for
two days and the intervening night.

62. How they occur
Heat waves form when high pressure aloft (from 10,000–25,000 feet (3,000–
7,600 metres)) strengthens and remains over a region for several days up to
several weeks. This is common in summer (in both Northern and Southern
Hemispheres) as the jet stream 'follows the sun'. On the equator side of the
jet stream, in the middle layers of the atmosphere, is the high pressure area.
Summertime weather patterns are generally slower to change than in winter. As
a 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 dome
capping the atmosphere.
This cap helps to trap heat instead of allowing it to lift. Without the lift there
is little or no convection and therefore little or no convective clouds (cumulus
clouds) with minimal chances for rain. The end result is a continual build-up of
heat at the surface that we experience as a heat wave.
Global warming boosts the probability of extreme weather events, like heat
waves, far more than it boosts more moderate events

63. Health effects

64. The heat index (as shown in the table above) is a measure of how hot it feels when
relative humidity is factored with the actual air temperature. Hyperthermia, also
known as heat stroke, becomes commonplace during periods of sustained high
temperature 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 a
higher risk for heat-related illness. The chronically ill and elderly are often taking
prescription medications (e.g., diuretics, anticholinergics, antipsychotics, and
antihypertensives) that interfere with the body's ability to dissipate heat.
Heat edema presents as a transient swelling of the hands, feet, and ankles and is
generally secondary to increased aldosterone secretion, which enhances water
retention. When combined with peripheral vasodilation and venous stasis, the
excess fluid accumulates in the dependent areas of the extremities. The heat
edema usually resolves within several days after the patient becomes acclimated
to the warmer environment. No treatment is required, although wearing support
stocking and elevating the affected legs with help minimize the edema.
Heat rash, also known as prickly heat, is a maculopapular rash accompanied by
acute inflammation and blocked sweat ducts. The sweat ducts may become dilated
and may eventually rupture, producing small pruritic vesicles on an erythematous
base. Heat rash affects areas of the body covered by tight clothing.

65. Underreporting and "Harvesting" effect
The number of heat fatalities is likely highly underreported due to lack of
reports 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 a
short-term forward mortality displacement. It has been observed that for some
heat waves, there is a compensatory decrease in overall mortality during the
subsequent weeks after a heat wave. Such compensatory reduction in mortality
suggests that heat affects especially those so ill that they "would have died in
the short term anyway“.
Psychological and sociological effects
In addition to physical stress, excessive heat causes psychological stress, to a
degree which affects performance, and is also associated with an increase in
violent crime.
Power outages
Abnormally hot temperatures cause electricity demand to increase during the
peak summertime hours of 4 to 7 p.m. when air conditioners are straining to

66. 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 and
buildings retains the heat from previous days. This heat build-up causes air
conditioners to turn on earlier and to stay on later in the day. As a result,
available electricity supplies are challenged during a higher, wider, peak
electricity consumption period.
Wildfires
If a heat wave occurs during a drought, which dries out vegetation, it can
contribute to bushfires and wildfires. During the disastrous heat wave that
struck Europe in 2003, fires raged through Portugal, destroying over 3,010
square kilometres (1,160 sq mi) or 301,000 hectares (740,000 acres) of forest
and 440 square kilometres (170 sq mi) or 44,000 hectares (110,000 acres) of
agricultural land and causing an estimated €1 billion worth of damage. High end
farmlands have irrigation systems to back up crops with.
Physical damage
Heat waves can and do cause roads and highways to buckle and melt,[31] water
lines to burst, and power transformers to detonate, causing fires. See the 2006
North American heat wave article about heat waves causing physical damage.

68. A tornado is a violently rotating column of air that is in contact with both the
surface of the earth and a cumulonimbus cloud or, in rare cases, the base of a
cumulus cloud. They are often referred to as twisters or cyclones, although the
word cyclone is used in meteorology, in a wider sense, to name any closed low
pressure circulation. Tornadoes come in many shapes and sizes, but they are
typically in the form of a visible condensation funnel, whose narrow end touches
the earth and is often encircled by a cloud of debris and dust. Most tornadoes
have 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 per
hour (483 km/h), stretch more than two miles (3.2 km) across, and stay on the
ground for dozens of miles (more than 100 km).
Various types of tornadoes include the landspout, multiple vortex tornado, and
waterspout. Waterspouts are characterized by a spiraling funnel-shaped wind
current, connecting to a large cumulus or cumulonimbus cloud. They are
generally classified as non-supercellular tornadoes that develop over bodies of
water, but there is disagreement over whether to classify them as true
tornadoes. These spiraling columns of air frequently develop in tropical areas
close to the equator, and are less common at high latitudes. Other

69. 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 United
States, although they can occur nearly anywhere in North America. They also
occasionally occur in south-central and eastern Asia, northern and east-central
South America, Southern Africa, northwestern and southeast Europe, western
and southeastern Australia, and New Zealand. Tornadoes can be detected
before or as they occur through the use of Pulse-Doppler radar by recognizing
patterns in velocity and reflectivity data, such as hook echoes or debris balls, as
well as by the efforts of storm spotters.
There are several scales for rating the strength of tornadoes. The Fujita scale
rates tornadoes by damage caused and has been replaced in some countries by
the updated Enhanced Fujita Scale. An F0 or EF0 tornado, the weakest
category, damages trees, but not substantial structures. An F5 or EF5 tornado,
the strongest category, rips buildings off their foundations and can deform
large skyscrapers. The similar TORRO scale ranges from a T0 for extremely
weak tornadoes to T11 for the most powerful known tornadoes.

70. • Etymology
• Definitions
• Characteristics
• Life cycle
• Types
• Intensity and damage
• Climatology
• Detection
• Extremes
• Safety

71. Etymology
The word tornado is an altered form of the Spanish word tronada, which means
"thunderstorm". This in turn was taken from the Latin tonare, meaning "to
thunder". It most likely reached its present form through a combination of the
Spanish tronada and tornar ("to turn"); however, this may be a folk
etymology.[10][11] A tornado is also commonly referred to as a "twister", and is
also 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 1939
film The Wizard of Oz. The term "twister" is also used in that film, along with
being the title of the 1996 tornado-related film Twister.
Definitions
A tornado is "a violently rotating column of air, in contact with the ground,
either pendant from a cumuliform cloud or underneath a cumuliform cloud, and
often (but not always) visible as a funnel cloud". For a vortex to be classified as
a 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 funnel
constitute separate tornadoes. Tornado refers to the vortex of wind, not the
condensation cloud.

72. Size and shape
A wedge tornado, nearly a mile wide, which hit Binger, Oklahoma in 1981
Most 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 be
obscured completely by rain or dust. These tornadoes are especially dangerous,
as even experienced meteorologists might not see them. Tornadoes can appear in
many shapes and sizes.
Small, relatively weak landspouts may be visible only as a small swirl of dust on
the ground. Although the condensation funnel may not extend all the way to the
ground, if associated surface winds are greater than 40 mph (64 km/h), the
circulation is considered a tornado. A tornado with a nearly cylindrical profile
and relative low height is sometimes referred to as a "stovepipe" tornado. Large
single-vortex tornadoes can look like large wedges stuck into the ground, and so
are known as "wedge tornadoes" or "wedges". The "stovepipe" classification is
also used for this type of tornado, if it otherwise fits that profile. A wedge can
be so wide that it appears to be a block of dark clouds, wider than the distance
from the cloud base to the ground. Even experienced storm observers may not
be able to tell the difference between a low-hanging cloud and a wedge.

74. Appearance
Tornadoes can have a wide range of colors, depending on the environment in which
they form. Those that form in dry environments can be nearly invisible, marked
only by swirling debris at the base of the funnel. Condensation funnels that pick
up 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-moving
funnels, 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 because
of the reddish tint of the soil, and tornadoes in mountainous areas can travel over
snow-covered ground, turning white.
Rotation
Tornadoes normally rotate cyclonically (when viewed from above, this is
counterclockwise 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 Coriolis
effect is unimportant, as indicated by their large Rossby numbers. Supercells and
tornadoes rotate cyclonically in numerical simulations even when the Coriolis
effect is neglected.[34][35] Low-level mesocyclones and tornadoes owe their
rotation to complex processes within the supercell and ambient environment.

75. Sound and seismology
Tornadoes emit widely on the acoustics spectrum and the sounds are caused by
multiple mechanisms. Various sounds of tornadoes have been reported, mostly
related to familiar sounds for the witness and generally some variation of a
whooshing roar. Popularly reported sounds include a freight train, rushing
rapids or waterfall, a nearby jet engine, or combinations of these. Many
tornadoes are not audible from much distance; the nature and propagation
distance of the audible sound depends on atmospheric conditions and
topography.
The winds of the tornado vortex and of constituent turbulent eddies, as well as
airflow interaction with the surface and debris, contribute to the sounds.
Funnel clouds also produce sounds. Funnel clouds and small tornadoes are
reported as whistling, whining, humming, or the buzzing of innumerable bees or
electricity, or more or less harmonic, whereas many tornadoes are reported as
a continuous, deep rumbling, or an irregular sound of "noise".
Tornadoes also produce identifiable inaudible infrasonic signatures.

76. Lifecycle
• Supercell relationship
• Formation
• Maturity
• Dissipation
As the tornado enters the dissipating stage, its associated mesocyclone often
weakens as well, as the rear flank downdraft cuts off the inflow powering it.
Sometimes, in intense supercells, tornadoes can develop cyclically. As the first
mesocyclone and associated tornado dissipate, the storm's inflow may be
concentrated into a new area closer to the center of the storm. If a new
mesocyclone develops, the cycle may start again, producing one or more new
tornadoes. Occasionally, the old (occluded) mesocyclone and the new
mesocyclone 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 as
landspouts, long-lived tornadoes, or tornadoes with multiple vortices. These
each have different mechanisms which influence their development—however,
most tornadoes follow a pattern similar to this one.

77. Types
• Multiple vortex
• Waterspout
• Landspout
• Gustnado
• Dust devil
• Fire whirls and steam devils
Small-scale, tornado-like circulations can occur near any intense surface heat
source. Those that occur near intense wildfires are called fire whirls. They are
not considered tornadoes, except in the rare case where they connect to
a pyrocumulus or other cumuliform cloud above. Fire whirls usually are not as
strong as tornadoes associated with thunderstorms. They can, however,
produce significant damage. A steam devil is a rotating updraft that involves
steam or smoke. Steam devils are very rare. They most often form from smoke
issuing from a power plant's smokestack. Hot springs and deserts may also be
suitable locations for a steam devil to form. The phenomenon can occur over
water, when cold arctic air passes over relatively warm water.

78. Intensity and damage
The Fujita scale and the Enhanced Fujita Scale rate tornadoes by damage
caused. The Enhanced Fujita (EF) Scale was an upgrade to the older Fujita scale,
by expert elicitation, using engineered wind estimates and better damage
descriptions. The EF Scale was designed so that a tornado rated on the Fujita
scale would receive the same numerical rating, and was implemented starting in
the United States in 2007. An EF0 tornado will probably damage trees but not
substantial structures, whereas an EF5 tornado can rip buildings off their
foundations leaving them bare and even deform large skyscrapers.
Climatology
The United States has the most tornadoes of any country, nearly four times
more than estimated in all of Europe, excluding waterspouts. This is mostly due
to the unique geography of the continent. North America is a large continent
that extends from the tropics north into arctic areas, and has no major east-west
mountain range to block air flow between these two areas. In the middle
latitudes, where most tornadoes of the world occur, the Rocky Mountains block
moisture and buckle the atmospheric flow, forcing drier air at mid-levels of the
troposphere due to downsloped winds, and causing the formation of a low.

79. Detection
Rigorous 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 by
someone seeing it on the ground. Often, news of a tornado would reach a local
weather office after the storm. However, with the advent of weather radar,
areas near a local office could get advance warning of severe weather. The first
public tornado warnings were issued in 1950 and the first tornado watches and
convective outlooks in 1952.
• Radar
• Storm spotting
• Visual evidence

80. Extremes
The most record-breaking tornado in recorded history was the Tri-State
Tornado, which roared through parts of Missouri, Illinois, and Indiana on March
18, 1925. It was likely an F5, though tornadoes were not ranked on any scale in
that era. It holds records for longest path length (219 miles, 352 km), longest
duration (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 single
tornado in United States history (695 dead). The tornado was also the second
costliest tornado in history at the time.
Safety
Though tornadoes can strike in an instant, there are precautions and preventative
measures that people can take to increase the chances of surviving a tornado.
Authorities such as the Storm Prediction Center advise having a pre-determined
plan should a tornado warning be issued. When a warning is issued, going to a
basement or an interior first-floor room of a sturdy building greatly increases
chances of survival. In tornado-prone areas, many buildings have storm cellars on
the property. These underground refuges have saved thousands of lives.

82. •Epidemics
• The A H5N1 virus, which causes Avian influenza
• An epidemic is an outbreak of a contractible disease that spreads through a
human population. A pandemic is an epidemic whose spread is global. There have
been many epidemics throughout history, such as the Black Death. In the last
hundred years, significant pandemics include:
• The 1918 Spanish flu pandemic, killing an estimated 50 million people
worldwide
• 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 global
health emergencies by the WHO, include:
• XDR TB, a strain of tuberculosis that is extensively resistant to drug
treatments
• Malaria, which kills an estimated 1.6 million people each year
• Ebola hemorrhagic fever, which has claimed hundreds of victims in Africa in
several outbreaks

84. Space disasters
Impact events
One of the largest impact events in modern times was the Tunguska event in
June 1908.
Solar flare
A solar flare is a phenomenon where the sun suddenly releases a great amount
of solar radiation, much more than normal. Some known solar flares include:
•An X20 event on August 16, 1989
•A similar flare on April 2, 2001
•The most powerful flare ever recorded, on November 4, 2003, estimated at
between X40 and X45
•The most powerful flare in the past 500 years is believed to have occurred in
September 1859

85. Gamma-ray burst
Gamma-ray bursts (GRBs) are flashes of gamma rays associated with extremely
energetic explosions that have been observed in distant galaxies. They are the
brightest electromagnetic events known to occur in the universe. Bursts can last
from ten milliseconds to several minutes. The initial burst is usually followed by a
longer-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 distant
galaxies and have been harmless to Earth, but if one occurred within our galaxy
and were aimed straight at us, the effects could be devastating. Currently
orbiting satellites detect an average of about one gamma-ray burst per day. The
closest known GRB so far was GRB 031203.

86. Protection by international law
International law, for example Geneva Conventions defines International Red
Cross and Red Crescent Movement the Convention on the Rights of Persons
with Disabilities, requires that "States shall take, in accordance with their
obligations under international law, including international humanitarian law and
international human rights law, all necessary measures to ensure the
protection and safety of persons with disabilities in situations of risk,
including the occurrence of natural disaster." And further United Nations
Office for the Coordination of Humanitarian Affairs is formed by General
Assembly Resolution 44/182. People displaced due to natural disasters are
currently protected under international law (Guiding Principles of
International Displacement, Campala Convention of 2009)