In meteorology, a cloud is an aerosol consisting of a visible mass of minute liquid droplets, frozen crystals, or other particles suspended in the atmosphere of a planetary body or similar space. Water or various other chemicals may compose the crystals. On Earth, clouds are formed as a result of saturation of the air when it is cooled to its dew point, or when it gains sufficient moisture from an adjacent source to raise the dew point to the ambient temperature, they are seen in the Earth's homosphere. Nephology is the science of clouds, undertaken in the cloud physics branch of meteorology. There are two methods of naming clouds in their respective layers of the atmosphere. Cloud types in the troposphere, the atmospheric layer closest to Earth's surface, have Latin names due to the universal adaptation of Luke Howard's nomenclature. Formally proposed in 1802, it became the basis of a modern international system that divides clouds into five physical forms that appear in any or all of three altitude levels.
These physical types, in approximate ascending order of convective activity, include stratiform sheets, cirriform wisps and patches, stratocumuliform layers, cumuliform heaps, large cumulonimbiform heaps that show complex structure. The physical forms are divided by altitude level into ten basic genus-types; the Latin names for applicable high-level genera carry a cirro- prefix, an alto- prefix is added to the names of the mid-level genus-types. Most of the genera can be further subdivided into varieties. Low stratiform clouds that extend down to the Earth's surface are given the common names fog and mist, but have no Latin names. Several clouds that form higher up in the stratosphere and mesosphere have common names for their main types, they are seen infrequently in the polar regions of Earth. Clouds have been observed in the atmospheres of other planets and moons in the Solar System and beyond. However, due to their different temperature characteristics, they are composed of other substances such as methane and sulfuric acid as well as water.
Taken as a whole, homospheric clouds can be cross-classified by form and level to derive the ten tropospheric genera, the fog and mist that forms at surface level, several additional major types above the troposphere. The cumulus genus includes three species. Clouds with sufficient vertical extent to occupy more than one altitude level are classified as low- or mid-level according to the altitude range at which each forms; however they are more informally classified as multi-level or vertical. The origin of the term cloud can be found in the old English clud or clod, meaning a hill or a mass of rock. Around the beginning of the 13th century, the word came to be used as a metaphor for rain clouds, because of the similarity in appearance between a mass of rock and cumulus heap cloud. Over time, the metaphoric usage of the word supplanted the old English weolcan, the literal term for clouds in general. Ancient cloud studies were not made in isolation, but were observed in combination with other weather elements and other natural sciences.
In about 340 BC the Greek philosopher Aristotle wrote Meteorologica, a work which represented the sum of knowledge of the time about natural science, including weather and climate. For the first time and the clouds from which precipitation fell were called meteors, which originate from the Greek word meteoros, meaning'high in the sky'. From that word came the modern term meteorology, the study of clouds and weather. Meteorologica was based on intuition and simple observation, but not on what is now considered the scientific method, it was the first known work that attempted to treat a broad range of meteorological topics. After centuries of speculative theories about the formation and behavior of clouds, the first scientific studies were undertaken by Luke Howard in England and Jean-Baptiste Lamarck in France. Howard was a methodical observer with a strong grounding in the Latin language and used his background to classify the various tropospheric cloud types during 1802, he believed. Lamarck had worked independently on cloud classification the same year and had come up with a different naming scheme that failed to make an impression in his home country of France because it used unusual French names for cloud types.
His system of nomenclature included twelve categories of clouds, with such names as hazy clouds, dappled clouds and broom-like clouds. By contrast, Howard used universally accepted Latin, which caught on after it was published in 1803; as a sign of the popularity of the naming scheme, the German dramatist and poet Johann Wolfgang von Goethe composed four poems about clouds, dedicating them to Howard. An elaboration of Howard's system was formally adopted by the International Meteorological Conference in 1891; this system covered only the tropospheric cloud types, but the discovery of clouds above the troposphere during the late 19th century led to the creation separate classification schemes for these high clouds. Terrestrial clouds can be found throughout most of the homosphere, which includes the troposphere and mesosphere. Within these layers of the atmosphere, air can become saturated as a result of being cooled to its dew point or by having moisture added from an adjacent source. In the latter case, saturation occurs when the dew po
Nature, in the broadest sense, is the natural, physical, or material world or universe. "Nature" can refer to the phenomena of the physical world, to life in general. The study of nature is a large, part of science. Although humans are part of nature, human activity is understood as a separate category from other natural phenomena; the word nature is derived from the Latin word natura, or "essential qualities, innate disposition", in ancient times meant "birth". Natura is a Latin translation of the Greek word physis, which related to the intrinsic characteristics that plants and other features of the world develop of their own accord; the concept of nature as a whole, the physical universe, is one of several expansions of the original notion. This usage continued during the advent of modern scientific method in the last several centuries. Within the various uses of the word today, "nature" refers to geology and wildlife. Nature can refer to the general realm of living plants and animals, in some cases to the processes associated with inanimate objects—the way that particular types of things exist and change of their own accord, such as the weather and geology of the Earth.
It is taken to mean the "natural environment" or wilderness—wild animals, forest, in general those things that have not been altered by human intervention, or which persist despite human intervention. For example, manufactured objects and human interaction are not considered part of nature, unless qualified as, for example, "human nature" or "the whole of nature"; this more traditional concept of natural things which can still be found today implies a distinction between the natural and the artificial, with the artificial being understood as that, brought into being by a human consciousness or a human mind. Depending on the particular context, the term "natural" might be distinguished from the unnatural or the supernatural. Earth is the only planet known to support life, its natural features are the subject of many fields of scientific research. Within the solar system, it is third closest to the sun, its most prominent climatic features are its two large polar regions, two narrow temperate zones, a wide equatorial tropical to subtropical region.
Precipitation varies with location, from several metres of water per year to less than a millimetre. 71 percent of the Earth's surface is covered by salt-water oceans. The remainder consists of continents and islands, with most of the inhabited land in the Northern Hemisphere. Earth has evolved through geological and biological processes that have left traces of the original conditions; the outer surface is divided into several migrating tectonic plates. The interior remains active, with a thick layer of plastic mantle and an iron-filled core that generates a magnetic field; this iron core is composed of a solid inner phase, a fluid outer phase. Convective motion in the core generates electric currents through dynamo action, these, in turn, generate the geomagnetic field; the atmospheric conditions have been altered from the original conditions by the presence of life-forms, which create an ecological balance that stabilizes the surface conditions. Despite the wide regional variations in climate by latitude and other geographic factors, the long-term average global climate is quite stable during interglacial periods, variations of a degree or two of average global temperature have had major effects on the ecological balance, on the actual geography of the Earth.
Geology is the study of the solid and liquid matter that constitutes the Earth. The field of geology encompasses the study of the composition, physical properties and history of Earth materials, the processes by which they are formed and changed; the field is a major academic discipline, is important for mineral and hydrocarbon extraction, knowledge about and mitigation of natural hazards, some Geotechnical engineering fields, understanding past climates and environments. The geology of an area evolves through time as rock units are deposited and inserted and deformational processes change their shapes and locations. Rock units are first emplaced either by deposition onto the surface or intrude into the overlying rock. Deposition can occur when sediments settle onto the surface of the Earth and lithify into sedimentary rock, or when as volcanic material such as volcanic ash or lava flows, blanket the surface. Igneous intrusions such as batholiths, laccoliths and sills, push upwards into the overlying rock, crystallize as they intrude.
After the initial sequence of rocks has been deposited, the rock units can be deformed and/or metamorphosed. Deformation occurs as a result of horizontal shortening, horizontal extension, or side-to-side motion; these structural regimes broadly relate to convergent boundaries, divergent boundaries, transform boundaries between tectonic plates. Earth is estimated to have formed 4.54 billion years ago from the solar nebula, along with the Sun and other planets. The moon formed 20 million years later. Molten, the outer layer of the Earth cooled, resulting in the solid crust. Outgassing and volcanic activity produced the primordial atmosphere. Condensing water vapor, most or all of which came from ice delivered by comets, produced the oceans and other water sources; the energetic chemistry is believed to have produced a self-replicat
An ice storm is a type of winter storm characterized by freezing rain known as a glaze event or, in some parts of the United States, as a silver thaw. The U. S. National Weather Service defines an ice storm as a storm which results in the accumulation of at least 0.25-inch of ice on exposed surfaces. From 1982 to 1994, ice storms were more common than blizzards in the U. S. averaging 16 per year. They are not violent storms but instead are perceived as gentle rains occurring at temperatures just below freezing; the formation of ice begins with a layer of above-freezing air above a layer of sub-freezing temperatures closer to the surface. Frozen precipitation melts to rain while falling into the warm air layer, begins to refreeze in the cold layer below. If the precipitate refreezes while still in the air, it will land on the ground as sleet. Alternatively, the liquid droplets can continue to fall without freezing, passing through the cold air just above the surface; this thin layer of air cools the rain to a temperature below freezing.
However, the drops themselves do not freeze, a phenomenon called supercooling. When the supercooled drops strike ground or anything else below 0 °C, a layer of ice accumulates as the cold water drips off, forming a thickening film of ice, hence freezing rain. While meteorologists can predict when and where an ice storm will occur, some storms still occur with little or no warning. In the United States, most ice storms are in the northeastern part of the country, but damaging storms have occurred farther south. An ice storm in February 1994 resulted in tremendous ice accumulation as far south as Mississippi, caused reported damage in nine states. More timber was damaged than that caused by Hurricane Camille. An ice storm in eastern Washington in November 1996 directly followed heavy snowfall; the combined weight of the snow and 25 to 37 millimeters of ice caused widespread damage and was considered the most severe ice storm in the Spokane area since 1940. The freezing rain from an ice storm covers everything with smooth glaze ice.
In addition to hazardous driving or walking conditions, branches or whole trees may break from the weight of ice. Falling branches can block roads, tear down power and telephone lines, cause other damage. Without falling trees and tree branches, the weight of the ice itself can snap power lines and break and bring down power/utility poles; this can leave people without power for anywhere from several days to a month. According to most meteorologists, just one quarter of an inch of ice accumulation can add about 500 pounds of weight per line span. Damage from ice storms is capable of shutting down entire metropolitan areas. Additionally, the loss of power during ice storms has indirectly caused numerous illnesses and deaths due to unintentional carbon monoxide poisoning. At lower levels, CO poisoning causes symptoms such as nausea, dizziness and headache, but high levels can cause unconsciousness, heart failure, death; the high incidence of CO poisoning during ice storms occurs due to the use of alternative methods of heating and cooking during prolonged power outages, common after severe ice storms.
Gas generators and propane barbecues, kerosene heaters contribute to CO poisoning when they operate in confined locations. CO is produced when appliances burn fuel without enough oxygen present, such as basements and other indoor locations. Loss of electricity during ice storms can indirectly lead to hypothermia and death from hypothermia, it can lead to ruptured pipes due to water freezing inside the pipes. November 26-29, 1921. Worst ice storm in known New England history. 4" of ice hit eastern and central Massachusetts. A ice storm had ice up to 6" thick in northwestern Texas during January 22-24, 1940. A ice storm had ice up to 6" thick in upstate New York during December 29-30, 1942. An ice storm which struck Northern Idaho January 1961 set a record for thickest recorded ice accumulation from a single storm in the United States, at 8 inches. In March 1991, a major ice storm in the area of Rochester, New York caused $375 million in damages, placing it among the worst natural disasters in New York State history.
In February 1994, a severe ice storm caused over $1 billion in damage in the Southern United States in Mississippi and Alabama. The North American ice storm of 1998 occurred during January 5–10, 1998, it was one of the most devastating and costly ice storms in North American history and one of the most devastating ice storms in modern history. Reported ice accumulations of 4" in some areas; the storm caused massive power failures in several large cities on the East Coast of the United States. The most affected area was eastern Ontario and southwestern Quebec in Canada, where over 3 million people were without power for up to a month and a half. Whole trees snapped and electrical pylons were flattened under the weight of the accumulated ice, it caused $340 million dollars of damage in Maine and became one of Canada's costliest natural disasters. The Northeastern United States was impacted by a major ice storm on December 11–12, 2008, which left about 1.25 million homes and businesses without power.
Areas impacted with 3⁄4 to 1 in of ice accumulation included eastern New York in the Albany area and western Massachusetts, southern New Hampshire and south-central Maine, Pennsylvania in the Pocono Mountains region, northwestern Connecticut, southern Vermont. Southern New Hampshire and northernmost Massachusetts got hit the worst with the s
A microburst is an intense small-scale downdraft produced by a thunderstorm or rain shower. There are two types of microbursts: dry microbursts, they go through three stages in their cycle, the downburst and cushion stages called "Suriano's Stroke". A microburst can be dangerous to aircraft during landing, due to the wind shear caused by its gust front. Several fatal and historic crashes have been attributed to the phenomenon over the past several decades, flight crew training goes to great lengths on how to properly recover from a microburst/wind shear event. A microburst has high winds that can knock over grown trees, they last for seconds to minutes. The term was defined by mesoscale meteorology expert Ted Fujita as affecting an area 4 km in diameter or less, distinguishing them as a type of downburst and apart from common wind shear which can encompass greater areas. Fujita coined the term macroburst for downbursts larger than 4 km. A distinction can be made between a wet microburst which consists of precipitation and a dry microburst which consists of virga.
They are formed by precipitation-cooled air rushing to the surface, but they also could be powered by strong winds aloft being deflected toward the surface by dynamical processes in a thunderstorm. When rain falls below the cloud base or is mixed with dry air, it begins to evaporate and this evaporation process cools the air; the cool air accelerates as it approaches the ground. When the cool air approaches the ground, it spreads out in all directions. High winds spread out in this type of pattern showing little or no curvature are known as straight-line winds. Dry microbursts produced by high based thunderstorms that generate little to no surface rainfall, occur in environments characterized by a thermodynamic profile exhibiting an inverted-V at thermal and moisture profile, as viewed on a Skew-T log-P thermodynamic diagram. Wakimoto developed a conceptual model of a dry microburst environment that comprised three important variables: mid-level moisture, a deep and dry adiabatic lapse rate in the sub-cloud layer, low surface relative humidity.
Wet microbursts are downbursts accompanied by significant precipitation at the surface. These downbursts rely more on the drag of precipitation for downward acceleration of parcels as well as the negative buoyancy which tend to drive "dry" microbursts; as a result, higher mixing ratios are necessary for these downbursts to form. Melting of ice hail, appears to play an important role in downburst formation in the lowest 1 km above ground level; these factors, among others, make forecasting wet microbursts difficult. The evolution of microbursts is broken down into three stages: the contact stage, the outburst stage, the cushion stage. Start by using the vertical momentum equation: d w d t = − 1 ρ ∂ p ∂ z − g By decomposing the variables into a basic state and a perturbation, defining the basic states, using the ideal gas law the equation can be written in the form B ≡ − ρ ′ ρ ¯ g = g T v ′ − T ¯ v T ¯ v where B is buoyancy; the virtual temperature correction is rather small and to a good approximation.
The effects of precipitation loading on the vertical motion are parametrized by including a term that decreases buoyancy as the liquid water mixing ratio increases, leading to the final form of the parcel's momentum equation: d w ′ d t = 1 ρ ¯ ∂ p ′ ∂ z + B − g ℓ The first term is the effect of perturbation pressure gradients on vertical motion. In some storms this term has a large effect on updrafts but there is not much reason to believe it has much of an impact on downdrafts and therefore will be ignored; the second term is the effect of buoyancy on vertical motion. In the case of microbursts, one expects to find that B is negative meaning the parcel is cooler than its environment; this cooling takes place as a result of phase changes. Precipitation particles that are small, but are in great quantity, promote a maximum contribution to cooling and, hence, to creation of negative buoyancy; the major contribution to this process is from evaporation. The last term is the effect of water loading.
Whereas evaporation is promoted by large numbers of small droplets, it only requires a few large drops to c
Autumn known as fall in American English and sometimes in Canadian English, is one of the four temperate seasons. Autumn marks the transition from summer to winter, in September or March, when the duration of daylight becomes noticeably shorter and the temperature cools considerably. One of its main features in temperate climates is the shedding of leaves from deciduous trees; some cultures regard the autumnal equinox as "mid-autumn", while others with a longer temperature lag treat it as the start of autumn. Meteorologists use a definition based on Gregorian calendar months, with autumn being September and November in the northern hemisphere, March and May in the southern hemisphere. In North America, autumn traditionally starts on September 21 and ends on December 21, it is considered to end with the winter solstice. Popular culture in the United States associates Labor Day, the first Monday in September, as the end of summer and the start of autumn; as daytime and nighttime temperatures decrease, trees shed their leaves.
In traditional East Asian solar term, autumn starts on or around 8 August and ends on or about 7 November. In Ireland, the autumn months according to the national meteorological service, Met Éireann, are September and November. However, according to the Irish Calendar, based on ancient Gaelic traditions, autumn lasts throughout the months of August and October, or a few days depending on tradition; the names of the months in Manx Gaelic are based on autumn covering August and October. In Argentina and New Zealand, autumn begins on 1 March and ends on 31 May; the word autumn comes from the ancient Etruscan root autu- and has within it connotations of the passing of the year. It was borrowed by the neighbouring Romans, became the Latin word autumnus. After the Roman era, the word continued to be used as the Old French word autompne or autumpne in Middle English, was normalised to the original Latin. In the Medieval period, there are rare examples of its use as early as the 12th century, but by the 16th century, it was in common use.
Before the 16th century, harvest was the term used to refer to the season, as it is common in other West Germanic languages to this day. However, as more people moved from working the land to living in towns, the word harvest lost its reference to the time of year and came to refer only to the actual activity of reaping, autumn, as well as fall, began to replace it as a reference to the season; the alternative word fall for the season traces its origins to old Germanic languages. The exact derivation is unclear, with the Old English fiæll or feallan and the Old Norse fall all being possible candidates. However, these words all have the meaning "to fall from a height" and are derived either from a common root or from each other; the term came to denote the season in 16th-century England, a contraction of Middle English expressions like "fall of the leaf" and "fall of the year". During the 17th century, English emigration to the British colonies in North America was at its peak, the new settlers took the English language with them.
While the term fall became obsolete in Britain, it became the more common term in North America. The name backend, a once common name for the season in Northern England, has today been replaced by the name autumn. Association with the transition from warm to cold weather, its related status as the season of the primary harvest, has dominated its themes and popular images. In Western cultures, personifications of autumn are pretty, well-fed females adorned with fruits and grains that ripen at this time. Many cultures feature autumnal harvest festivals the most important on their calendars. Still extant echoes of these celebrations are found in the autumn Thanksgiving holiday of the United States and Canada, the Jewish Sukkot holiday with its roots as a full-moon harvest festival of "tabernacles". There are the many North American Indian festivals tied to harvest of ripe foods gathered in the wild, the Chinese Mid-Autumn or Moon festival, many others; the predominant mood of these autumnal celebrations is a gladness for the fruits of the earth mixed with a certain melancholy linked to the imminent arrival of harsh weather.
This view is presented in English poet John Keats' poem To Autumn, where he describes the season as a time of bounteous fecundity, a time of'mellow fruitfulness'. In North America, while most foods are harvested during the autumn, foods associated with the season include pumpkins and apples, which are used to make the seasonal beverage apple cider. Autumn in poetry, has been associated with melancholia; the possibilities and opportunities of summer are gone, the chill of winter is on the horizon. Skies turn grey, the amount of usable daylight drops and many people turn inward, both physically and mentally, it has been referred to as an unhealthy season. Similar examples may be found in Irish poet William Butler Yeats' poem The Wild Swans at Coole where the maturing season that the poet observes symbolically represents his own ageing self. Like the natural world that he observes, he too has reached his prime and now must look forward to the inevitability of old age and death. French p
Hail is a form of solid precipitation. It is distinct from ice pellets, though the two are confused, it consists of balls or irregular lumps of ice, each of, called a hailstone. Ice pellets fall in cold weather while hail growth is inhibited during cold surface temperatures. Unlike other forms of water ice such as graupel, made of rime, ice pellets, which are smaller and translucent, hailstones measure between 5 mm and 15 cm in diameter; the METAR reporting code for hail 5 mm or greater is GR, while smaller hailstones and graupel are coded GS. Hail is possible within most thunderstorms as it is produced by cumulonimbus, within 2 nmi of the parent storm. Hail formation requires environments of strong, upward motion of air with the parent thunderstorm 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 fall at higher speeds as they grow in size, though complicating factors such as melting, friction with air 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 farmers' crops. Any thunderstorm which produces hail that reaches the ground is known as a hailstorm. Hail has a diameter of 5 millimetres or more. Hailstones can weigh more than 0.5 kilograms. Unlike ice pellets, hailstones can be irregular and clumped together. Hail is composed of transparent ice or alternating layers of transparent and translucent ice at least 1 millimetre 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 and golf ball-sized.
Stones larger than 2 cm are considered large enough to cause damage. The Meteorological Service of Canada issues severe thunderstorm warnings when hail that size or above is expected; the US National Weather Service has a 2.5 cm or greater in diameter threshold, effective January 2010, an increase over the previous threshold of ¾-inch hail. Other countries have different thresholds according to local sensitivity to hail. Hailstones can be large or small, depending on how strong the updraft is: weaker hailstorms produce smaller hailstones than stronger hailstorms. Hail forms in strong thunderstorm clouds those with intense updrafts, high liquid water content, great vertical extent, large water droplets, where a good portion of the cloud layer is below freezing 0 °C; these types of strong updrafts can indicate the presence of a tornado. The growth rate of hailstones is impacted by factors such as higher elevation, lower freezing zones, wind shear. 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 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 true; the storm's updraft, with upwardly directed wind speeds as high as 110 miles per hour, blows 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 water vapour is available, it acquires a layer of opaque white ice. Furthermore, the hailstone's speed depends on its position in 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 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; because it undergoes'wet growth', the outer layer is sticky, so a single hailstone may grow by collision with other smaller hailstones, forming a larger entity with an irregular shape. Hail can undergo'dry growth' in which the latent heat release through freezing is not enough to keep the outer layer in a liquid state.
Hail forming in this manner
A firestorm is a conflagration which attains such intensity that it creates and sustains its own wind system. It is most a natural phenomenon, created during some of the largest bushfires and wildfires. Although the term has been used to describe certain large fires, the phenomenon's determining characteristic is a fire with its own storm-force winds from every point of the compass; the Black Saturday bushfires and the Great Peshtigo Fire are possible examples of forest fires with some portion of combustion due to a firestorm, as is the Great Hinckley Fire. Firestorms have occurred in cities as a deliberate effect of targeted explosives, such as occurred as a result of the aerial firebombings of Hamburg, firebombing of Tokyo and the atomic bombing of Hiroshima. A firestorm is created as a result of the stack effect as the heat of the original fire draws in more and more of the surrounding air; this draft can be increased if a low-level jet stream exists over or near the fire. As the updraft mushrooms, strong inwardly-directed gusty winds develop around the fire, supplying it with additional air.
This would seem to prevent the firestorm from spreading on the wind, but the tremendous turbulence created may cause the strong surface inflow winds to change direction erratically. Firestorms resulting from the bombardment of urban areas in the Second World War were confined to the areas seeded with incendiary devices, the firestorm did not appreciably spread outward. A firestorm may develop into a mesocyclone and induce true tornadoes/fire whirls; this occurred with the 2002 Durango fire, with the much greater Peshtigo Fire. The greater draft of a firestorm draws in greater quantities of oxygen, which increases combustion, thereby substantially increasing the production of heat; the intense heat of a firestorm manifests as radiated heat, which may ignite flammable material at a distance ahead of the fire itself. This serves to expand the area and the intensity of the firestorm. Violent, erratic wind drafts suck movables into the fire and as is observed with all intense conflagrations, radiated heat from the fire can melt asphalt, some metals, glass, turn street tarmac into flammable hot liquid.
The high temperatures ignite anything that might burn, until the firestorm runs low on fuel. A firestorm does not appreciably ignite material at a distance ahead of itself. During the formation of a firestorm many fires merge to form a single convective column of hot gases rising from the burning area and strong, fire-induced, radial winds are associated with the convective column, thus the fire front is stationary and the outward spread of fire is prevented by the in-rushing wind. A firestorm is characterized by strong to gale-force winds blowing toward the fire, everywhere around the fire perimeter, an effect, caused by the buoyancy of the rising column of hot gases over the intense mass fire, drawing in cool air from the periphery; these winds from the perimeter blow the fire brands into the burning area and tend to cool the unignited fuel outside the fire area so that ignition of material outside the periphery by radiated heat and fire embers is more difficult, thus limiting fire spread.
At Hiroshima, this inrushing to feed the fire is said to have prevented the firestorm perimeter from expanding, thus the firestorm was confined to the area of the city damaged by the blast. Large wildfire conflagrations are distinct from firestorms if they have moving fire fronts which are driven by the ambient wind and do not develop their own wind system like true firestorms. Furthermore, non-firestorm conflagrations can develop from a single ignition, whereas firestorms have only been observed where large numbers of fires are burning over a large area, with the important caveat that the density of burning fires needs to be above a critical threshold for a firestorm to form; the high temperatures within the firestorm zone ignite most everything that might burn, until a tipping point is reached, that is, upon running low on fuel, which occurs after the firestorm has consumed so much of the available fuel within the firestorm zone that the necessary fuel density required to keep the firestorm's wind system active drops below the threshold level, at which time the firestorm breaks up into isolated conflagrations.
In Australia, the prevalence of eucalyptus trees that have oil in their leaves results in forest fires that are noted for their tall and intense flame front. Hence the bush fires appear more as a firestorm than a simple forest fire. Sometimes, emission of combustible gases from swamps has a similar effect. For instance, methane explosions enforced the Peshtigo Fire. Firestorms will produce hot buoyant smoke clouds of water vapor that will form condensation clouds as it enters the cooler upper atmosphere, generating what is known as pyrocumulus clouds or, if large enough, pyrocumulonimbus clouds. For example, the black rain that began to fall 20 minutes after the atomic bombing of Hiroshima produced in total 5–10