Malevus

Tag: fire

  • Why the Shadow of Candle Flame Looks Odd

    Why the Shadow of Candle Flame Looks Odd

    When a candle is lit, its shadow is cast on the window or wall in front of it. This shadow not only captures the outline of the wax body but also includes the image of the actual flame. It might seem like this is just the shadow of the light itself. You can better understand this phenomenon by observing a small object through the flame. This is because the flame’s transparency will vary in different spots.

    An object positioned behind the central, brightly lit area of the candle flame seems nearly impossible to see. This part of the flame is the least transparent and plays a major role in creating the shaded edge on its surface. On the other hand, the outer edges and the portion of the flame surrounding the wick are highly transparent, resulting in minimal discoloration on the wall.

    A candle emits a soft glow when exposed to sunlight. This glow casts a shadow on a white wall, forming two distinct bright bands of light – one above and one below the shadow of the flame. This creates a captivating visual effect. A similar radiant glow can also be observed above the wick.
    A candle emits a soft glow when exposed to sunlight. This glow casts a shadow on a white wall, forming two distinct bright bands of light – one above and one below the shadow of the flame. This creates a captivating visual effect. A similar radiant glow can also be observed above the wick.

    However, the projection offers more than just areas of darkness and light. It showcases bright patches that appear to shine even more intensely than the blazing sun. As a result, two parallel, brilliantly illuminated stripes flank each side of the flame, and a similarly radiant area stands out within the wick’s shadow. These heightened areas show that we’re not just seeing a candle’s shadow; rather, there’s a complex interaction between the flame’s light and the heated environment.

    Numerous interconnected systems facilitate the intricate physical and chemical processes occurring within a burning candle. At the base of the wick, wax melts and transforms into long-chain hydrocarbon molecules as it rises. As these molecules move toward the center of the dark flame, they become hotter and break down into smaller fragments. They only ignite when they reach the outermost layer of the flame and come into contact with the oxygen in the air. This is where temperatures tend to rise significantly. The products of combustion are expelled in an upward plume due to the strong buoyancy effect.

    The flame region consists of different parts, including the exhaust plume, luminous zone, reaction zone, and flame core.
    The flame region consists of different parts, including the exhaust plume, luminous zone, reaction zone, and flame core. (Credit: A loose necktie, CC BY-SA 4.0)

    When outside air enters the exhaust gas plume, a significant change in temperature occurs. This change causes the bright lines we observe, and this phenomenon is a result of light penetrating the boundary layer. Changes in the gas’s temperature and density have an impact on this boundary layer, which causes refraction. Think of mirages on hot asphalt roads or flickering over a fire—both are examples of this effect.

    The way light is refracted depends on the refractive index strength within the candle flame and the exhaust plume. Imagine tracing its path over the exhaust plume and the flame. Once, it travels on a plane right in the center of the luminous zone, and another time, it’s directly above the wick.

    Now, this holds true for both planes: when approaching the flame from outside the exhaust plume, the refractive index drops extremely sharply within a radius of about 0.40–0.16 inches (10–4 mm) from the axis of symmetry. After that, the drop becomes less steep until it levels out. But from this point onward, the behavior of the two planes diverges.

    As you move closer to the luminous zone, the refractive index starts to rise again, eventually leveling out in the center. However, things are quite different on the wick plane. Here, after hitting the minimum, the refractive index rises dramatically and even surpasses the value of the surrounding air.

    This behavior can be expected due to the rapid temperature changes along the symmetry axis. Yet, the temperature profile doesn’t completely explain the high refractive index in the wick-to-flame-core area. The composition of the gas also has an impact on this phenomenon. The pure wax vapor present refracts light much more intensely compared to the air or combustion gases in the exhaust plume.

    Gases having an optic impact

    Light gets bent from its straight path in various ways due to changes in the refractive index. This happens at the meeting point of the surrounding air and the rising exhaust plume, impacting the area’s rays. The curved paths of these deflected rays eventually cross paths with the straight-traveling rays coming from a distance. As a result, the projection surface becomes adorned with striking bands of light.

    To create a bright spot on the screen around the evaporating wax near the wick, light rays are directed towards the center and focused. On either side of the flame, the light rays are refracted at the edge, forming two stripes due to the plume's refraction from inside to outside.
    To create a bright spot on the screen around the evaporating wax near the wick, light rays are directed toward the center and focused. On either side of the flame, the light rays are refracted at the edge, forming two stripes due to the plume’s refraction from inside to outside. (Credit: Mike Zeitz)

    Furthermore, the rapidly increasing refractive index within the core of the flame causes light to be strongly deflected inward. To maintain symmetry, there’s some overlap to the right and left of the center. Consequently, the light intensity increases with distance in a specific region behind the candle. This explains how we’ve identified where the wick’s shadow is cast.

    This indicates that the two stripes and central spot observed behind a backlit candle flame are connected to the refractive index profile. This is because of the high refractive index of the wax vapor near the wick and the existing temperature gradient.

    For instance, the area with soot particles is where the candle flame is less visible. As a result, a shadow appears in the projection, although it’s notably brighter than the shadow from the candle’s central core. Interestingly, the shadow isn’t entirely gray; it holds a faint brownish tint.

    The soot particles, which are about 20 nanometers in size and more than ten times smaller than the wavelengths of visible light, also have an impact on coloration. This leads to predominantly Rayleigh scattering, affecting the shorter-wavelength blue components of light. Consequently, the projection allows the significant long-wavelength reds to pass through. This creates a similar effect to early morning or late afternoon lighting.

    References

    1. Can a fire have a shadow? – Wtamu.edu
    2. The Philosophy of Shadows – Oxford Academic (oup.com)
  • Why Does Wood Crackle and Pop When Burned?

    Why Does Wood Crackle and Pop When Burned?

    When the weather starts to get colder, it’s time to crack out the fireplaces and stoves. We love the calming warmth of the fire and the crackling sound of burning logs, even more, when it is chilly and dark outside. But why, when it burns, does wood crackle in the first place? What produces the sparking mini-explosions that sometimes radiate from the logs, and how do they do it?

    Tension, heat, and contraction

    The cracks happen because tensions in the wood eventually cause a fracture to form. The wood, in turn, tries to contract as a result of the heat, and this is what causes the tension.

    The fractured beams in an Alpine Hut originate from the same concept. The moisture content of the wood fibers in the beam progressively evaporates, causing them to conform more closely to their surroundings. 

    As a direct consequence of this, the beam will progressively shrink as it dries; however, the contraction will occur more across the wood than in the longitudinal direction. Because solid wood does not have sufficient elasticity, it rips, and over time, apparent fractures emerge in the wood.

    The situation is the same with fire, except that it develops more rapidly. The crackling and popping sounds are always produced when the wood structure tries to shrink but is unable to due to the strength of the wood itself. After then, it gives in to the strains and fractures. The noise is reminiscent of a branch being snapped in two.

    The flying sparks are caused by the resin

    However, there are moments when the crackles in the wood are very audible. Sparks often fly, much as they would in a smaller version of the explosion.

    There is another explanation for the loud crackling sounds in the burning wood. The noise may be traced to the bursting of resin pockets. These are the cavities in the wood that hold the liquid plaster that the wood produces. The oily resin protects the tree from microbial infections because it has chemicals that kill microorganisms and seal wounds.

    The heat of the fire causes the oils found in the resin to evaporate, which then causes the oils to expand. The surrounding hardwood structure is unable to absorb the oil vapors, which causes the material to crackle. This in turn causes the resin galls to rupture explosively. Because the oils that fly out are combustible, they often catch fire when they come into contact with flames, which results in sparks and small explosions or pops.

    Cracking is less common in hardwoods and fir

    But why do different types of wood crackle at different rates? One cause has to do with the varying amounts of the resin contained in the wood. Pine, for example, has a high resin content and, as a result, produces a lot of crackling noise as it burns. On the other hand, hardwoods rarely experience these kinds of explosions or pops since the hardwoods themselves do not contain any resin.

    The sound produced by fir and spruce combustion is noticeably different from one another. The reason for this is that fir wood does not contain any resin, but spruce wood has a significant amount of oil-rich compounds.

    The structure and form of the wood

    However, the structure and form of the wood can still play a role in causing crackles and pops. For example, light woods like spruce are more likely to crackle than heavier woods. Their wood is not as sturdy, and it is more prone to shatter when subjected to tension. Heavy hardwoods like oak or beech, on the other hand, are less likely to be damaged due to the much denser and more robust structure of these hardwoods.

    Crackling may also be affected by the form of the log and the method used to cut it from the trunk. The wood is less likely to crack if it is cut into smaller pieces. This is because the wood warps less in smaller pieces, and the tensions that arise from this are lower, making it less likely that the wood would crackle as a result of these forces.

  • What Are the Megafires? What Causes Them and How to Prevent Them?

    What Are the Megafires? What Causes Them and How to Prevent Them?

    This summer has seen the world set ablaze on all sides. Even the Arctic, a region that has been virtually untouched until now, is facing a growing risk of uncontrolled fires, say United Nations experts. Unusual fires and climate change are mutually aggravating. Governments are being called upon to shift their investments to prevention and readiness to deal with the risk of megafires. But what exactly is a megafire? When can a fire be considered a megafire? What methods do we have to fight it?

    Megafires represent 3% of all fires, but are responsible for more than 50% of the world’s burned areas. In the United States, burned areas have increased by 1200% over the last four decades. This year, these fires are occurring in a sustained and extreme way almost everywhere in Europe. Recent data on forest fires confirm what we have long feared: Forest fires are spreading, burning nearly twice as much tree cover today as they did 20 years ago. In a pessimistic scenario of greenhouse gas emissions between 2081 and 2100, it is estimated that the fire season in the south of many European countries will increase from one month, as it is today, to two or even three months.

    Moreover, according to data from the European Forest Fire Information System (EFFIS), from January 1 to July 30, 2022, 600,000 hectares (1,500,000 acres) went up in smoke in the European Union – more than half of them in the last two months alone. Not to mention that forest fires now ravage an additional 7 million acres (3 million hectares) each year, an area the size of Belgium, according to satellite data compiled by Global Forest Watch (GFW), the World Resources Institute (WRI) and the University of Maryland.

    The situation is dramatic and the loop we have entered seems to have no solution, climate change favors megafires, which themselves accelerate climate change. Understanding the underlying mechanics of these fires, their origin and evolution, could help us fight and prevent them.

    A Lack of Scientific Definition, but Clear Criteria

    wildfire

    The notion of megafires is a new case in the typology of forest fires in Europe, because of its emergence linked to climate change. Covering different realities in different countries, its scientific meaning remains to be defined.

    According to NASA, between climate change and nearly a century of fire exclusion, wildfires have become more extreme in terms of size, severity, the complexity of behavior, and resistance to suppression. These fires are commonly referred to as megafires and are at the extremes of historical variation.

    A few criteria are used to define these extraordinary fires, such as the speed of propagation, the intensity of propagation, and the area of burned the surface. The duration (several weeks or even months) and the uncontrollable dimension are the main indicators specific to megafires which have more meaning in countries with large areas like the United States.

    Meteorologists are not yet able to predict wildfire outbreaks, but there are three conditions that must be present for a wildfire to occur. Firefighters call it the fire triangle: fuel, oxygen, and heat source. 90% of wildfires are started by people, but dry weather, drought and high winds can create a recipe for the absolute disaster, which can turn a spark into a fire that lasts weeks or months and consumes tens of thousands of acres.

    Lightning is another possible cause of forest fires. Scientists have found that a 1 degree Celsius (1.8 degrees Fahrenheit) increase in global warming triggers a 12% increase in lightning activity. Since 1975, the number of fires started by lightning has increased by 2 to 5%.

    The Pyrocene Era

    wildfire

    Climate change is likely a major factor in the increase in fire activity. Extreme heat waves are already 5 times more likely today than they were 150 years ago and are expected to become even more frequent as the planet continues to warm. Warmer temperatures dry out the landscape and help create the ideal environment for larger and more frequent wildfires. This in turn leads to higher emissions from wildfires, further exacerbating climate change and contributing to more fires in a fire-climate loop. This is called the Pyrocene era we have entered.

    Fires like megafires are so large and intense that they create their own environment, making them all the more uncontrollable, going off in all directions without firefighters being able to anticipate. Indeed, the force released by large fires like megafires creates clouds called pyrocumulus. Like all cumulus clouds, pyrocumulus forms over sources of intense heat. They can be seen at the beginning of a fire, when the temperature of the ground surface increases. Hot air is quickly released and as it rises into the atmosphere, it cools. The water contained in this air pocket, coming from the burned plants, condenses.


    This creates massive smoke columns filled with heat and ash.

    Depending on the stability and humidity conditions available, there could be pyrocumulus humilis, mediocris, congestus or even pyrocumulonimbus formed over large fires. Pyrocumulus humilis and mediocris cut off the solar radiation.


    This may reduce local heating initially and help fight a forest fire, but by creating a cloudy area next to a sunny one, they increase the winds at the fire location which does harm. The pyrocumulus congestus, on the other hand, can give rain. Finally, the pyrocumulonimbus can reach the tropopause and generate sooted hail, lightning, downbursts, and even tornadoes in addition to rain.

    The megafires have a frightening capacity of propagation, because the pines are especially flammable. It is important to note that a usual fire in a pine forest during warmer weather in winter, spring or fall, burns the vegetation and plant litter on the surface, but does not normally burn the tree canopy. This reduces the risk of fire in summer since some of the flammable surface vegetation has been burned. But the same forest burns almost completely when the fire occurs in summer.

    When the thorns or the resin of the pine catch fire, it releases a gas that in contact with the air burns again. And in parallel, when the flames meet the colder air at ground level, the air mass is carried upwards, producing an internal depression which is the wind created by the fire itself. A fire like this can then spread at more than 4 mi/h (7km/h).

    Dramatic Consequences on the Planet

    Forest fires actually have the ability to warm the entire planet as revealed by a 2016 NASA study. In ecosystems like boreal forests, which store more carbon than any other terrestrial ecosystem on the planet, the effects of climate change are felt twice as more.

    In fact, fires ravaged boreal forests in the vast expanses of northern Canada in May 2016 and continued for months, consuming millions of acres of trees and burning the rich organic soil of the forest floor, which serves as a great store of carbon. For every degree our planet warms, the forest needs a 15% increase in precipitation to offset the increase in drought.

    When the forest burns the ashes of these megafires are deposited on the glaciers and precipitate the melting of ice: This is what happened in New Zealand, the glaciers have taken a caramel color with the ash from the Australian fires, and as a result reflected less light and melt faster.

    Not to mention that mortality is higher than in a typical fire, and the environmental damage is often long-lasting, especially if such fires are allowed to recur year after year, bringing some plant and animal species closer to extinction. The fires that affected Australia from September 2019 through early 2020, caused the death of approximately 500 million animals.

    How to Prevent Large Fires Like Megafires?

    Many European countries have been confronted with an increase in the number, size or intensity of fires in the last decades. They have often opted for an enhanced fire suppression strategy. However, this strategy is not sustainable in the long term, as climate change and landscape modification are already leading to an increase in large fires.

    If the risk of fire increases with global warming, the question is how to limit fire starts, but also the size of fires. For the forest, the maintenance and intensification of wood harvesting is the priority component of fire prevention, just like agricultural activities. These areas serve as firebreaks that firefighters can rely on in case a fire occurs because they are much less dense than forests and the soil is cleaner, not littered with dry vegetation, a perfect fuel for fires.

    According to a recent report by the United Nations Environment Programme (UNEP) and the GRID-Arendal, climate change and changing land use are expected to make uncontrolled fires more frequent and intense. Experts predict a global increase in extreme fires of up to 14% by 2030, 30% by 2050 and 50% by the end of the century. The paper calls on governments to radically change the way public spending on wildfires is allocated, by directing investments towards prevention and preparedness, rather than reaction and response.

    The publication calls on governments to adopt a new “fire preparedness formula” in which two-thirds of spending would be on planning, prevention, preparedness, and recovery, and one-third on response. Currently, direct responses to wildfires typically receive more than half of the corresponding spending, while planning and prevention receive less than 1%. The authors call for combining data and science-based monitoring systems with indigenous knowledge, as well as strengthening regional and international cooperation to prevent fires.

    That is why in 2019, the European Commission created “rescEU” to improve disaster risk management. Member countries pool their resources and build a pan-European pool of equipment. For example, in the event of an emergency, Croatia, France, Greece, Italy, Spain, and Sweden offer a fleet of 12 airplanes and six water bomber helicopters. In exchange, the less equipped states pay a financial contribution. The system also includes Iceland, Macedonia, Montenegro, Norway, Serbia, and Turkey, which are not members of the European Union.

    Humans Are the Main Reason for Forest Fires

    Whether it is through global warming or a simple act, in nine cases out of ten forest fires are of human origin – criminal or accidental. In other words, they are preventable. Preventive forest fire fighting and public planning policies can and should play a role in the management and evolution of these events.


    Furthermore, forest maintenance and brush clearing are pointed out as one of the causes of the increase in the frequency of megafires. Individuals, and sometimes even municipalities, do not carry out the necessary work to clear the undergrowth of dead vegetation and elements that promote the spread of fire. Improved forest management, like flood protection, is needed to combat wildfires.