Hail is a form of solid precipitation which consists of balls or irregular lumps of ice, that are individually called hail stones. Hail stones on Earth consist mostly of water ice and measure between and in diameter, with the larger stones coming from severe thunderstorms. The METAR reporting code for hail or greater in diameter is GR, while smaller hailstones and graupel are coded GS. Hail is possible within most thunderstorms as it is produced by cumulonimbi (thunderclouds), and within 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. Hail is most frequently formed in the interior of continents within the mid-latitudes of Earth, with hail generally confined to higher elevations within the tropics.

There are methods available to detect hail-producing thunderstorms using weather satellites and weather radar imagery. Hail stones 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 hail stones 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 man-made structures and, most commonly, farmers' crops.

Definition

Any thunderstorm which produces hail that reaches the ground is known as a hailstorm. Hail has a diameter of or more. Hail stones can grow to and weigh more than .

Unlike ice pellets, hail stones 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 thick, which are deposited upon the hail stone as it cycles 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 (0.75 in) are usually considered large enough to cause damages. 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. Other countries will have different thresholds according local sensitivity to hail, for instance grape growing areas could be adversely impacted by smaller hailstones.

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 . These type of strong updrafts can also indicate the presence of a tornado. The growth rate is maximized where air is near a temperature of .

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 and field study) has shown this is not necessarily true.

The storm's updraft, with upwardly directed wind speeds as high as , 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. When the hailstone moves into an area with a high concentration of water droplets, it captures the latter and acquires a translucent layer. Should the hailstone move into an area where mostly water vapour is available, it acquires a layer of opaque white ice.

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. Undergoing 'wet growth', the outer layer is ''sticky'', or more adhesive, so a single hailstone may grow by collision with other smaller hailstones, forming a larger entity with an irregular shape.

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 we can discuss mutiple trajectories 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 . 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 actually less common in 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 depth. Hail in the tropics occurs mainly at higher elevations.

Hail growth becomes vanishingly small when air temperatures fall below as supercooled water droplets become rare at these temperatures. Around thunderstorms, hail is most likely within the cloud at elevations above . Between and , 60 percent of hail is still within the thunderstorm, though 40 percent now lies within the clear air under the anvil. Below , hail is equally distributed in and around a thunderstorm to a distance of .

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 hail more likely. 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 experiences also 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.

Short-term detection

Weather radar is a very useful tool to detect the presence of hail producing thunderstorms. However, radar data has to be complemented by a knowledge of current atmospheric conditions which can allow one to determine if the current atmosphere is conducive to hail development.

Modern radar scans many angles around the site. Reflectivity values at multiple angles above ground level in a storm are proportional to the precipitation rate at those levels. Summing reflectivities in the Vertically Integrated Liquid or VIL, gives the liquid water content in the cloud. Research shows that hail development in the upper levels of the storm is related to the evolution of VIL. VIL divided by the vertical extent of the storm, called VIL density, has a relationship with hail size, although this varies with atmospheric conditions and therefore is not highly accurate. Traditionally, hail size and probability can be estimated from radar data by computer using algorithms based on this research. Some algorithms include the height of the freezing level to estimate the melting of the hailstone and what would be left on the ground.

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 hail stones is best determined by measuring their diameter with a ruler. In the absence of a ruler, hail stone size is often visually estimated by comparing its size to that of known objects, such as coins. Below is a table of commonly used objects for this purpose. Note that using the objects such as hen's eggs, peas, and marbles for comparing hailstone sizes is often inaccurate, 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 hail stone. Within METAR code, GR is used to indicate larger hail, of a diameter of at least . 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.

Terminal velocity of hail, or the speed at which hail is falling when it strikes the ground, varies by the diameter of the hail stones. A hail stone of in diameter falls at a rate of , while stones the size of in diameter fall at a rate of . Hail stone 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 hail stones, and melting as the stones fall through a warmer atmosphere.

+ Common coin sizes
!	United States Mint coin sizes>United States	Coins of the Canadian dollar>Canada
Dime
Cent (or "Penny")
Five cents (Nickel)
Twenty-five cents (Quarter dollar)
Dollar (Loonie)
50 Cents/Half Dollar
Two Dollars (Toonie)

+ Other Objects	! Object	! Diameter
Pea
Mothball
Grape (small)
Olive (large)
Golf ball
Tennis ball
Cricket ball
Teacup
Grapefruit
Melon (small)
Cantaloupe
Volleyball
Bowling Ball

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 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 hail stones exceed 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, Uttarakhand, 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 in diameter and in circumference, weighing in at . 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 that fell in Coffeyville, Kansas in 1970.

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 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 whilst 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. Depths of up to a metre have been reported.

Suppression and prevention

During the Middle Ages, people in Europe used to ring church bells and fire cannons to try to prevent hail. After World War II, cloud seeding was done to eliminate the hail threat, particularly across Russia. Russia claimed a 50 to 80 percent reduction in crop damage from hail storms by deploying silver iodide in clouds using rockets and artillery shells. Their results have not been able to be verified. Hail suppression programs have been undertaken by 15 countries between 1965 and 2005. To this day, no hail prevention method has been proven to work.

See also

Ice pellets, also called Sleet.

Graupel

Snow grains

Hail cannon

Crop insurance

Megacryometeor, a very large (as much as tens of kilograms) chunk of atmospheric ice originating under different conditions from hail.

References

Further reading

External links

Hail by income and population (Realtime)

Hail Factsheet

The Economic Costs of Hail Storm Damage NOAA Economics

; Images

Hail and hailstorms

Major hail event in Brazil

NOAA Hail Reports on Google map (non commercial)

; Video

Major Hail Storm in Calgary, Alberta, Canada, Aug.22,2010

Category:Precipitation Category:Storm Category:Weather hazards

ar:برد (هطول) an:Piedra (meteorolochía) gn:Amandáu ay:Chhijchhi az:Dolu (yağıntı) be:Град be-x-old:Град bs:Grad (padavina) bg:Градушка ca:Calamarsa cs:Kroupy (meteorologie) da:Hagl (nedbør) de:Hagel et:Rahe el:Χαλάζι es:Granizo eo:Hajlo eu:Txingor fa:تگرگ fr:Grêle gv:Sniaghtey garroo gl:Sarabia ko:우박 hr:Tuča io:Grelo id:Hujan es iu:ᓇᑕᖅᑯᕐᓇᐃᑦ/nataqqurnait it:Grandine he:ברד ka:სეტყვა kk:Бұршақ (жауын-шашын) sw:Mvua ya mawe ku:Zîpik la:Grando lv:Krusa lt:Kruša mk:Град (метеорологија) mg:Havandra ml:ആലിപ്പഴം mr:गार ms:Hujan air batu nl:Hagel (neerslag) ja:雹 no:Hagl nn:Hagl oc:Granissa mhr:Шолем pl:Grad pt:Granizo ro:Grindină qu:Chikchi ru:Град sah:Тобурах sco:Hail sq:Breshëri scn:Gragnola simple:Hail sk:Krúpa (ľadovec) sl:Toča sr:Град (падавина) fi:Rae sv:Hagel tl:Hail te:వడగళ్ళు th:ลูกเห็บ tg:Жола tr:Dolu uk:Град vi:Mưa đá wa:Gurzea wuu:雹 zh-yue:雹 bat-smg:Kroša zh:冰雹

Coordinates	3°8′51″N101°41′36″N
Name	Mary Mallon
Birth date	September 23, 1869
Birth place	County Tyrone, Northern Ireland
Death date	November 11, 1938
Residence	United States
Nationality	Irish
Other names	Mary Brown
Known for	Healthy carrier of typhoid fever
Footnotes	}}

Mary Mallon (September 23, 1869 – November 11, 1938), also known as Typhoid Mary, was the first person in the United States identified as an Asymptomatic carrier of the pathogen associated with typhoid fever. She was presumed to have infected some 53 people, three of whom died, over the course of her career as a cook. She was forcibly quarantined twice by public health authorities and died after nearly three decades altogether in quarantine.

Cook

Mary Mallon was born in 1869 in Cookstown, County Tyrone, Ireland, UK (now Northern Ireland). She emigrated to the United States in 1884. From 1900 to 1907 she worked as a cook in the New York City area.

In 1900, she had been working in a house in Mamaroneck, New York, for under two weeks when the residents developed typhoid fever. She moved to Manhattan in 1901, and members of the family for whom she worked developed fevers and diarrhea and the laundress died. She then went to work for a lawyer until seven of the eight household members developed typhoid; Mary spent months helping to care for the people she made sick, but her care further spread the disease through the household. In 1906, she took a position in Oyster Bay, Long Island. Within two weeks, six of eleven family members were hospitalised with typhoid. She changed employment again, and similar occurrences happened in three more households.

When typhoid researcher George Soper approached Mallon about her possible role spreading typhoid, she adamantly rejected his request for urine and stool samples. Soper left and later published the report in June, 1907, in the ''Journal of the American Medical Association''. On his next contact with her, he brought a doctor with him, but was turned away again. Mallon's denials that she was a carrier were based in part on the diagnosis of a reputable chemist who had found her to not harbor the bacteria. Moreover, when Soper first told her she was a carrier, the concept of a healthy carrier of a pathogen was not commonly known. Further, class prejudice and prejudice towards the Irish were strong in the period, as was the belief that slum-dwelling immigrants were a major cause of epidemics. During a later encounter in the hospital, he told Mary that he would write a book about her and give her all the royalties; she angrily rejected his proposal and locked herself in the bathroom until he left.

Quarantine

The New York City Health Department sent Dr. Sara Josephine Baker to talk to Mary, but "by that time she was convinced that the law was only persecuting her when she had done nothing wrong." A few days later, Baker arrived at Mary's workplace with several police officers who took her into custody. The New York City health inspector determined her to be a carrier. Under sections 1169 and 1170 of the Greater New York Charter, Mallon was held in isolation for three years at a clinic located on North Brother Island.

It is believed that individuals can develop typhoid fever after ingesting food or water contaminated during handling by a human carrier. The human carrier is usually a healthy person who has survived a previous episode of typhoid fever yet who continues to shed the associated bacteria, Salmonella typhi, in feces and urine. It takes vigorous scrubbing and thorough disinfection with soap and hot water to remove the bacteria from the hands.

Eventually, the New York State Commissioner of Health, Eugene H. Porter, M.D., decided that disease carriers would no longer be held in isolation. Mallon could be freed if she agreed to abandon working as a cook and to take reasonable steps to prevent transmitting typhoid to others. On February 19, 1910, Mallon agreed that she "[was] prepared to change her occupation (that of a cook), and would give assurance by affidavit that she would upon her release take such hygienic precautions as would protect those with whom she came in contact, from infection". She was released from quarantine and returned to the mainland.

After being given a job as a laundress, which paid lower wages, however, Mallon adopted the pseudonym ''Mary Brown'', returned to her previous occupation as a cook, and in 1915 was believed to have infected 25 people, resulting in one death, while working as a cook at New York's Sloane Hospital for Women. Public-health authorities again found and arrested Mallon, returned to quarantine on the island on March 27, 1915. Mallon was confined there for the remainder of her life. She became something of a minor celebrity, and was interviewed by journalists, who were forbidden to accept even a glass of water from her. Later, she was allowed to work as a technician in the island's laboratory.

Death

Mallon spent the rest of her life in quarantine. Six years before her death, she was paralysed by a stroke. On November 11, 1938, aged 69, she died of pneumonia. An autopsy found evidence of live typhoid bacteria in her gallbladder. It is possible that she was born with the infection, as her mother had typhoid fever during her pregnancy. Her body was cremated, and the ashes were buried at Saint Raymond's Cemetery in the Bronx.

Legacy

Mallon was the first healthy typhoid carrier to be identified by medical science, and there was no policy providing guidelines for handling the situation. Some difficulties surrounding her case stemmed from Mallon's vehement denial of her possible role, as she refused to acknowledge any connection between her working as a cook and the typhoid cases. Mallon maintained that she was perfectly healthy, had never had typhoid fever, and could not be the source. Public-health authorities determined that permanent quarantine was the only way to prevent Mallon from causing significant future typhoid outbreaks.

Other healthy typhoid carriers identified in the first quarter of the 20th century include Tony Labella, an Italian immigrant, presumed to have caused over 100 cases (with five deaths); an Adirondack guide dubbed ''Typhoid John'', presumed to have infected 36 people (with two deaths); and Alphonse Cotils, a restaurateur and bakery owner.

Today, ''Typhoid Mary'' is a generic term for a healthy carrier of a noted pathogen, especially one who refuses to cooperate with health authorities to minimize the risk of infection. The term is also applied to computer users who spread malicious computer software by their naïveté and neglect (or outright refusal) to use security software against the malware. See also "Typhoid adware".

Mary Mallon is also referenced in the name of hip-hop group Hail Mary Mallon.

References

Further reading

Bourdain, Anthony. ''Typhoid Mary: An Urban Historical''. New York: Bloomsbury, 2001. Hardcover, 148 pages, ISBN 1-58234-133-8

Leavitt, Judith Walzer. ''Typhoid Mary, Captive to the Public's Health''. Boston: Beacon Press, 1996. Hardcover, 331 pages, ISBN 0-8070-2102-4

Baker, Josephine Sarah. ''Fighting for Life''. New York: Macmillan Press, 1939. ISBN 0-405-05945-0 (1974 ed), ISBN 0-88275-611-7 (1980 ed)

Federspiel, Jürg. '' The Ballad of Typhoid Mary'' [translated by Joel Agee]. New York: Ballantine Press, 1985.

External links

A more detailed profile of Typhoid Mary

PBS NOVA site: "The Most Dangerous Woman in America"

www.snopes.com about Typhoid Mary

Photo of Mary Mallon's grave

Category:1869 births Category:1938 deaths Category:Deaths from pneumonia Category:Irish emigrants to the United States (before 1923) Category:People from Cookstown Category:People from New York City Category:People in public health Category:Infectious disease deaths in New York Category:People from Oyster Bay, New York Category:Women and death

cs:Tyfová Mary de:Mary Mallon es:María la Tifosa fr:Mary Mallon gd:Mary Mallon it:Mary Mallon he:מרי טיפוס nl:Mary Mallon ja:メアリー・マローン pl:Mary Mallon pt:Mary Mallon ru:Тифозная Мэри fi:Mary Mallon sv:Mary Mallon tr:Mary Mallon vi:Mary Mallon zh:傷寒瑪莉