Health Effects of Heat Wave:

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. If this continues for a duration of time it can lead to the development of chronic dermatitis or a secondary bacterial infection. Prevention is the best therapy. It is also advised to wear loose-fitting clothing in the heat. However, once heat rash has developed, the initial treatment involves the application of chlorhexidine lotion to remove any desquamated skin. The associated itching may be treated with topical or systemic antihistamines. If infection occurs a regimen of antibiotics is required.

Heat cramps are painful, often severe, involuntary spasms of the large muscle groups used in strenuous exercise. Heat cramps tend to occur after intense exertion. They usually develop in people performing heavy exercise while sweating profusely and replenishing fluid loss with non-electrolyte containing water. This is believed to lead to hyponatremia that induces cramping in stressed muscles. Rehydration with salt-containing fluids provides rapid relief. Patients with mild cramps can be given oral .2% salt solutions, while those with severe cramps require IV isotonic fluids. The many sport drinks on the market are a good source of electrolytes and are readily accessible.

Heat syncope is related to heat exposure that produces orthostatic hypotension. This hypotension can precipitate a near-syncopal episode. Heat syncope is believed to result from intense sweating, which leads to dehydration, followed by peripheral vasodilation and reduced venous blood return in the face of decreased vasomotor control. Management of heat syncope consists of cooling and rehydration of the patient using oral rehydration therapy (sport drinks) or isotonic IV fluids. People who experience heat syncope should avoid standing in the heat for long periods of time. They should move to a cooler environment and lie down if they recognize the initial symptoms. Wearing support stockings and engaging in deep knee-bending movements can help promote venous blood return.

Heat exhaustion is considered by experts to be the forerunner of heat stroke (hyperthermia). It may even resemble heat stroke, with the difference being that the neurologic function remains intact. Heat exhaustion is marked by excessive dehydration and electrolyte depletion. Symptoms may include headache, nausea, and vomiting, dizziness, tachycardia, malaise, and myalgia. Definitive therapy includes removing patients from the heat and replenishing their fluids. Most patients will require fluid replacement with IV isotonic fluids at first. The salt content is adjusted as necessary once the electrolyte levels are known. After discharge from the hospital, patients are instructed to rest, drink plenty of fluids for 2–3 hours, and avoid the heat for several days. If this advice is not followed it may then lead to heat stroke.

One public health measure taken during heat waves is the setting-up of air-conditioned public cooling centers.

Mortality

Heat waves are the most lethal type of weather phenomenon, overall. Between 1992 and 2001, deaths from excessive heat in the United States numbered 2,190, compared with 880 deaths from floods and 150 from hurricanes. The average annual number of fatalities directly attributed to heat in the United States is about 400. The 1995 Chicago heat wave, one of the worst in US history, led to approximately 600 heat-related deaths over a period of five days. Eric Klinenberg has noted that in the United States, the loss of human life in hot spells in summer exceeds that caused by all other weather events combined, including lightning, rain, floods, hurricanes, and tornadoes. Despite the dangers, Scott Sheridan, professor of geography at Kent State University, found that less than half of people 65 and older abide by heat-emergency recommendations like drinking lots of water. In his study of heat-wave behavior, focusing particularly on seniors in Philadelphia, Phoenix, Toronto, and Dayton, Ohio, he found that people over 65 "don't consider themselves seniors." "Heat doesn't bother me much, but I worry about my neighbors," said one of his older respondents. According to the Agency for Health care Research and Quality, about 6,200 Americans are hospitalized each summer due to excessive heat, and those at highest risk are poor, uninsured or elderly.

Underreporting and "Harvesting" effect

The number of heat fatalities is likely highly underreported due to lack of reports and misreports. 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 outage

Heat waves often lead to electricity spikes due to increased air conditioning use, which can create power outages, exacerbating the problem. During the 2006 North American heat wave, thousands of homes and businesses went without power, especially in California. In Los Angeles, electrical transformers failed, leaving thousands without power for as long as five days. The 2009 South Eastern Australia Heat Wave caused the city of Melbourne, Australia to experience some major power disruptions which left over half a million people without power as the heat wave blew transformers and overloaded the power grid.

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 (740,000 acres) of forest and 440 square kilometres (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, water lines to burst, power transformers to detonate, causing fires. See the 2006 North American heat wave article about heat waves causing physical damage.

In the summer in warm climates, an area of high pressure with little or no rain or clouds, the air and ground easily heats to excess. A static high pressure area can impose a very persistent heat wave.

The position of the jet stream allows air on one side to be considerably warmer than the other side. Heat waves are far more common and more severe on the warm side and at times an unusual position of the jet stream places unusual warmth in an unusual place for hot weather, and imposes a heat wave. El Niño and La Niña (opposite reaction to El Niño) can severely disrupt the positions of the jet streams.

Large desert zones and dry areas are more likely to get extreme heat because there is rarely any high cloud cover with very low humidity.

Winds from hot deserts typically push hot, dry air towards areas normally cooler than during a heat wave. During the summer an area that has no geographic features that might cool winds that originate in the hot deserts get little mitigation, especially near the summer solstice when long days and a high sun would create warm conditions even without the transport of hot air from other locations. Should such a hot air mass travel above a large body of water, as a sirocco of Saharan origin crossing the Mediterranean sea, it likely picks up much water vapor with a reduction in temperature but far greater humidity that makes the original desert air little less moderate as demonstrated in a high heat index. Heat waves can also come from air originating over tropical seas penetrating far into the middle latitudes heating further overland, as often occurs in the eastern United States and southeastern Canada. The heat island created by dense urbanization of large cities only exacerbate heat waves because of the weakness of night-time cooling.

A heat wave is a prolonged period of excessively hot weather, which may be accompanied by high humidity. There is no universal definition of a heat wave; the term is relative to the usual weather in the area. 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 of Heat Wave

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 Celsius degrees (9 Fahrenheit degrees), 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 °C). More realistically, the comfort criteria for any one region are dependent upon the normal conditions of that region.

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.

In Denmark a heat wave is defined as a period of at least 3 consecutive days of which period the average maximum temperature across more than fifty percent of the country exceeds 28 °C.

In the United States, definitions also vary by region; however, a heat wave is usually defined as a period of at least two or more days of excessively hot weather. In the Northeast, a heat wave is typically defined as three consecutive days where the temperature reaches or exceeds 90 °F (32 °C), but not always as this is ties in with humidity levels to determine a heat index threshold. The same does not apply to drier climates. A heat storm is a Californian term for an extended heat wave. Heat storms occur when the temperature reaches 100 °F (38 °C) for three or more consecutive days over a wide area (tens of thousands of square miles). The National Weather Service issues heat advisories and excessive heat warnings when unusual periods of hot weather are expected.

In Adelaide, Australia, 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).

This is a list of the costliest or deadliest hailstorms on record.

Date	Location	Incident
Circa 9th century	Roopkund, Uttarakhand, India	Several hundred pilgrims were killed by a massive hailstorm in Roopkund.
May 24 1937	Belgrade, Yugoslavia
January 1 1947	Sydney, Australia	1947 Sydney hailstorm
December 1967	Los Angeles County, California, USA	A hailstorm hit the county, blanketing the region much like a snowstorm. The storm also produced lightning, and one bolt struck an oil tank in Manhattan Beach, causing an explosion that covered much of the South Bay with the oil. The next hailstorm to hit the area was in 1979.
July 1979	Fort Collins, Colorado, USA	A violent forty-minute hailstorm bombed Fort Collins, CO, with hail up to grapefruit size. Two thousand homes and 2500 automobiles were severely damaged, and about 25 persons were injured, mainly when hit on the head by the huge stones. A three month old baby died of a fractured skull, struck by a large hailstone while being carried by his mother, who was running with him to seek cover. (The Weather Channel)
12 July 1984	Munich, Bavaria, Germany	Tennis ball sized hail fell on Munich and surrounding areas on this date. It was the greatest loss event in the history of the German insurance industry: 200,000 cars were damaged and the storm cost an estimated 166 million Deutschmark. For years afterwards people jokingly referred to those cars whose bodywork was not repaired as 'Munich Design'.
18 January 1985	Brisbane, Queensland, Australia	Late in the afternoon, a supercell thunderstorm swept in from the west dropping hailstones as large as 6 cm over parts of the city. A wind gust of over 180 km/h was recorded at Brisbane airport and 57mm of rain recorded in 15 minutes at the same location. The 30 minutes of destruction caused $AU300 million ($AU1.7 billion in 2007 adjusted dollars) damage to vehicles and buildings. This rates as the 5th most costly insured event in Australia since 1968.
11 July 1990	Denver, Colorado, USA	Softball-sized hail destroyed roofs and cars, causing $625 million in total damage ($1 billion in damage adjusted to 2007 dollars).
7 September 1991	Calgary, Alberta, Canada	A Labour Day thunderstorm caused $342 million worth of insurable damage in Calgary. Thirteen additional hailstorms between 1981 and 1998 caused an estimated $600 million in damage in the Calgary area alone.
June 19, 1992	Wichita, Kansas, USA	Two batches of severe thunderstorms, occurring within 6 hours of each other, dumped hailstones up to 4.5 inches in diameter across Sedgwick, including the city of Wichita, and surrounding counties in south-central Kansas. Over 10,000 homes were damaged. The hail left wheat fields in a near total loss. Estimated property damage totaled $500 million with crop damage at $100 million. The thunderstorm episode ranks as one of the worst ever to hit Kansas.
5 May 1995	Dallas and Fort Worth, Texas, USA	The 1995 Mayfest Storm produced $1.1 billion insured losses, and total storm damage was reported at around $2 billion. The storms produced hail about the size of softballs.
14 April 1999	Sydney, New South Wales, Australia	1999 Sydney hailstorm: 20,000 properties and 40,000 vehicles were damaged during the storm with more than 25 aircraft damaged at Sydney Airport. One person was killed while fishing after getting struck by lightning and several other people were injured. At $AU1.5 billion ($AU3.3 billion in 2007 adjusted dollars), it was the costliest hailstorm to hit an Australian populated city. Largest stone measured was 9.5 cm.
29 March 2000	Lake Worth, Texas, USA	The last known hail fatality in the United States occurs. The victim was Juan Oseguera, a nineteen-year-old man who died from head injuries after being hit by a softball sized hailstone in Lake Worth.
18 May 2000	McHenry, Lake, northern Kane, and northern Cook County, Illinois, USA	$572 million. Golfball-, baseball-, and softball-sized hail damaged roofs, cars, patio furniture, skylights, and windows in the area's worst and most widespread hailstorm in 30 years. Around 100,000 homes lost power. Hail was 3 inches (76 mm) deep in many areas. There were 100 canceled flights, and train service was disrupted.
10 April 2001	St. Louis, Missouri, USA	$2.0 billion+. The costliest hailstorm in US history struck the I-70 corridor of eastern Kansas, across Missouri, into southwestern Illinois.
19 July 2002	Henan Province, the People's Republic of China	25 dead and hundreds injured.
22 June 2003	Aurora, Nebraska, USA	The largest hailstone on record at the time, it had a 7-inch (180 mm) diameter and a circumference of 18.75 inches (476 mm).
20–21 April 2006	San Marcos, Texas, USA	Hail of sizes up to 10 cm results in 10,000 auto claims, 7,000 homeowner and commercial property claims ranging between $100–160 million insured losses. One woman was hospitalized. The storm was especially costly at the San Marcos Outlet Malls and a nearby Toyota dealership.
28-29 Jun 2006	Villingen-Schwenningen and suburbs, Baden-Württemberg, Germany	Supercell thunderstorms, severe damage by grapefruit-sized hailstones, causing € 150 million damage, more than 100 injuries.
9 December 2007	Sydney, Australia	Supercell thunderstorms caused immense damage in the North and Western Suburbs of Sydney. Worst hit were the suburbs of Blacktown, Castle Hill and neighbouring Baulkham Hills. Hail stones the size of golf balls damaged cars, windows and homes.
22 March 2010	Perth, Australia	The hailstorm that struck after a long dry spell lasted only a few minutes but caused damage to many houses, a number of hospitals and schools, and an airport terminal. The storm damaged thousands of cars, cut power lines and left 150,000 houses and many traffic lights without power. The storm also caused a landslip at King's Park.
10,16 May 2010	Oklahoma City, Oklahoma, USA	Two major hailstorms separated by less than a week caused damage to large portions of the Oklahoma City metropolitan area. Damages from the first storm caused an estimated $595 million (USD) damage, and the second storm was expected to be around the same, totaling around $1 billion in damages.
12 July 2010	Calgary, Alberta, Canada	The hailstorm that pounded Calgary on July 12 eclipsed the record originally set in 1991, with insurable damages totalling at least $400 million, excluding agricultural crop damage. The storm brought hail as wide as four centimetres (over 1.5 inches).

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 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.

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 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). 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	Canada
Dime	0.705 inches (17.9 mm)	18.03 millimetres (0.710 in)
Cent (or "Penny")	0.75 inches (19 mm)	19.05 millimetres (0.750 in)
Five cents (Nickel)	0.88 inches (22 mm)	21.2 millimetres (0.83 in)
Twenty-five cents (Quarter dollar)	1.00 inch (25 mm)	23.88 millimetres (0.940 in)
Dollar (Loonie)	1.043 inches (26.5 mm)	26.5 millimetres (1.04 in)
50 Cents/Half Dollar	1.25 inches (32 mm)	27.13 millimetres (1.068 in)
Two Dollars (Toonie)		28 millimetres (1.1 in)

Other Objects

Object	Diameter
Pea	0.25 inches (6.4 mm)
Marble (small)	0.50 inches (13 mm)
Mothball	0.50 inches (13 mm)
Grape (small)	0.62 inches (16 mm)
Olive (large)	0.75 inches (19 mm)
Shooter Marble	0.75 inches (19 mm)
Walnut/Ping-pong ball	1.50 inches (38 mm)
Ping-pong ball	1.60 inches (41 mm)
Squash ball	1.65 inches (42 mm)
Golf ball	1.75 inches (44 mm)
Hen egg	2.00 inches (51 mm)
Billiards (Pool) Ball	2.25 inches (57 mm)
Orange (Valencia/sweet)	2.38 inches (60 mm)
Tennis ball	2.50 inches (64 mm)
Baseball	2.75 inches (70 mm)
Cricket ball	2.80 inches (71 mm)
Teacup	3.00 inches (76 mm)
Grapefruit	4.00 inches (102 mm)
Softball	4.50 inches (114 mm)
Melon (small)	4.75 inches (121 mm)
Computer CD	5.00 inches (127 mm)
Cantaloupe	6.50 inches (165 mm)
45 RPM Phonograph Record	7.00 inches (178 mm)
Volleyball	8.00 inches (203 mm)
Bowling Ball	8.25 inches (210 mm)

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.

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.

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

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. 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 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 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 −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).

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 5 millimetres (0.20 in) and 200 millimetres (7.9 in) in diameter, with the larger stones coming from severe thunderstorms. The METAR reporting code for hail 5 millimetres (0.20 in) 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 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. 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 5 millimetres (0.20 in) or more. Hail stones can grow to 15 centimetres (6 in) and weigh more than 0.5 kilograms (1.1 lb).

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 1 millimetre (0.039 in) 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.