Is it true that no two snowflakes are identical? (McGill OSS)

4 minute read

Snow crystals—better known as snowflakes—are intricate, delicate, tiny miracles of beauty. Their very existence seems unlikely, yet incomprehensible numbers of them fall every year to iteratively construct wintery wonderlands.

Every snowflake is formed of around 100,000 water droplets in a process that takes roughly 30-45 minutes. Even with this level of complexity contributing to each and every snow crystal, it seems nearly impossible that every single flake is truly singularly matchless. Yet, the scientific explanation of snow formation can explain how every snowflake tells its life story, and every story is unique.
The first known reference to snowflakes’ unique shapes was by a Scandinavian bishop, Olaus Magnus, in 1555, but he was a touch mistaken in some of his proposed designs.

Photo source: http://www.snowcrystals.com/history/history.html

Snowflakes’ six-fold symmetry was first identified in 1591 by English astronomer Thomas Harriot. Still, a scientific reasoning for this symmetry wasn’t proposed until 1611 when Johannes Kepler, a German astronomer, wrote The Six-Cornered Snowflake. Indeed, almost all snowflakes exhibit a six-fold symmetry—for reasons explained here—however, they rarely can be found with 3- or 12-fold symmetry.

The notion that no two snowflakes are alike was put forth by Wilson Bentley, a meteorologist from Vermont who took the first detailed photos of snowflakes between 1885 and 1931. He went on to photograph over 5000 snow crystals and, in the words of modern snowflake expert Kenneth Libbrecht, “did it so well that hardly anybody bothered to photograph snowflakes for almost 100 years.” Bentley’s assertion of snowflakes’ unique natures might be 100 years old, but it has held up to scientific scrutiny. Understanding how snow forms can help us understand precisely how nature continues to create novel snowflake patterns.

Snow crystals begin forming when warm moist air collides with another mass of air at a weather front. The warm air rises, cooling as it does, and water droplets condense out of it, just like when your shower deposits steam onto your bathroom mirror. Unlike in your bathroom, however, these water droplets don’t have a large surface to attach to and instead form tiny droplets around microscopic particles in the air like dust or even bacteria. Big aggregates of these drops are what form clouds.

If the air continues to cool, the water enters what’s called a supercooled state. This means that they are below 0˚C, the freezing point of pure water, but still a liquid. Ice crystals will start to grow within the drop only once given a nucleation point, a position from which ice crystals can begin to grow. If you’ve ever seen the frozen beer trick, it relies on the same mechanics.

Once a droplet is frozen, water vapour in the surrounding air will condense onto it, forming snow crystals, aka snowflakes. Not every droplet freezes but those that don’t will evaporate, providing more water vapour to condense onto the frozen ones. Once roughly 100,000 droplets have condensed onto the crystal, it’s heavy enough that it falls to earth.

The crystal patterns formed when the water vapour condenses onto a growing flake are highly dependent on temperature, and how saturated the air around it is. Below you can see a Nakaya diagram. Created in the 1930s and named for its creator, Japanese physicist Ukichiro Nakaya, it shows the typical shapes of snow crystals formed under different supersaturation and temperature conditions.

Photo source: http://www.snowcrystals.com/morphology/morphology.html

Above roughly -2˚C, thin plate-like crystals tend to form. Between -2˚C and -10˚C, the formations are slender columns. Colder still, -10˚C and -22˚C herald the production of the wider thin plates we’re most used to, and below -22˚C comes a rarely seen mix of small plates and columns. Snow crystals grow rapidly and form complex, highly branched designs when humidity is high and the air is supersaturated with water vapour. When humidity is low, the flakes grow more slowly, and the designs are simpler.

As a growing snowflake moves through the air, it encounters countless different microenvironments with slightly different humidity and temperature, each affecting its growth pattern. In this way, the shape of a snowflake tells its life story—the second-by-second conditions it encounters determine its final form. That’s where the unique nature of each snowflake comes from.

Kenneth Libbrecht is a snowflake scholar—a professor of physics at California Institute of Technology who has dedicated years of his career to uncovering the mysteries of snow crystals. He was even a consultant on the movie Frozen! He grows snowflakes in his laboratory using specialized chambers under highly controlled environmental conditions. Growing multiple snow crystals very closely together under essentially identical conditions, Libbrecht can create ostensibly identical snowflakes. But even still, he considers them more like identical twins. Can you visually see a difference between them? No, not really. But if you were to zoom in, and in, and in, on some level, you would be able to find differences.

Libbrecht thinks that the question of whether there have ever been identical snowflakes is just silly. “Anything that has any complexity is different than everything else,” even if you have to go down to the molecular level to find it.

This article was written for the McGill Office for Science and Society. View the original here: https://www.mcgill.ca/oss/article/environment-you-asked/it-true-no-two-snowflakes-are-identical

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The Little Ice Age That Made Christmas White Forever (McGill OSS)

3 minute read

Our collective vision of Christmas landscapes is so immersed in snow that the very phrase “It’s beginning to look a lot like Christmas” conjures up imagery that is nearly all frosted, sparkling and white. This even though a snow-covered Christmas is the exception rather than the rule for the majority of the world.

Despite what the song “White Christmas” would make you think, for more than half the continental U.S., there is less than a 50% chance of a white Christmas occurring. Snow on December 25^th is rare in the U.K. and not even as common in the Great White North of Canada as you may expect! So why do we pine for a pearly white holiday time?

Maybe Bing Crosby crooning, “I’m dreaming of a White Christmas, just like the ones I used to know,” has given you the impression that climate change is to blame for the seeming lack of modern-day snowy holidays. Global warming certainly has played a role in decreasing the chances of frosty festivities and will continue to do so. But the real reason behind our widespread association of Christmas and snow is less to do with changing weather patterns and more to do with our media.

Charles Dickens’ classic tale “A Christmas Carol” was written and published in England during the Victorian era. Where nowadays, you see far more fake snow than real, during Dickens’ early life, winters in the U.K. were snow-filled times of “piercing, searching, biting cold.” The 16^th to the 19^th century was a climatic period known as the Little Ice Age. As a result, most of Europe saw colder, longer, and more snowy winters than previously known. Winters cold enough to allow the River Thames frost fairs to occur on a frozen-solid Thames—something that hasn’t happened since 1814.

While familiar to us in much of Canada, the lasting snowy landscapes and beauty created by ice and frost were novelties to many artists, and Father Winter served as a muse for many. The Little Ice Age period gave birth to the vast majority of European depictions of winter in paintings and inspired numerous enduring works of art.

Charles Dickens has been called the man who invented Christmas—a definite exaggeration. But we can thank him, Jacob Marley, and Ebenezer Scrooge for helping to cement a Christmas aesthetic that has persisted with impressive consistency. Christmas is a time of nostalgia for many of us, and it was no different for Dickens. His stories contain references to the snowy cold winters of his childhood, making it ironic, in a sense, that we should now feel a sort of nostalgia for Dickens’ childhood winters too.

Our views that Christmases should be snowy don’t exclusively come from the England of yore. New media and art through the years have iterated upon Dickens’ Christmas setting and only further enshrined our association of Christmastime as snow filled. The United States have contributed their fair share to the frost-filled Christmas media. From “A Visit from St. Nicholas”—better known as “’Twas the night before Christmas”—discussing newly fallen snow to stories like “How the Grinch Stole Christmas” by Theodor “Dr. Seuss” Geisel, to the lithographic prints of Currier and Ives and the Christmas scenes of Norman Rockwell. The classic Christmas movie “It’s a Wonderful Life” even won an award for developing a new version of fake snow to replace the painted cornflakes used previously!

While Bing Crosby sings less about the white Christmases he personally knew and more about the ones we as a society used to know, the man who wrote the lyrics for “White Christmas,” Irving Berlin, was likely talking about both. a Jewish immigrant to the U.S., Berlin was born in Tyumen in modern-day Russia. With average daily December temperatures of -12.9 ˚C, he very well may have been referencing both his childhood Christmases and the historic Victorian ones enshrined in our holiday ideals.

This article was written for the McGill Office for Science and Society. View the original here: https://www.mcgill.ca/oss/article/history-environment/little-ice-age-made-christmas-white-forever

Peckers Get Smaller Where It Gets Colder (McGill OSS)

1 minute read

Charles Darwin postulated that Toucan’s massive beaks might be for sexual selection purposes. Other scientists have theorized that it could be for shows of intimidation, for actual defence or for peeling fruit. Given the beak’s serrated edge, it was once thought that toucans used it to catch and eat fish. We now know that toucans are almost entirely fructivorous, although they do opportunistically eat insects, lizards, and even small birds.

Another thing we now know is that the main function of a toucan’s beak is actually thermoregulation! Just like elephants do with their ears and dogs with their tongues, Toucans rely on their big beaks as heat sinks to maintain their homeostasis and save them from overheating.

Bird beaks across the globe follow a trend called Allen’s Rule, which proposes that the appendages of endotherms (warm-blooded animals) are smaller, relative to body size, in colder climates in order to reduce heat loss. A study of 214 bird species from every continent found strongly significant differences in their beak sizes according to latitude and local environmental temperatures. From penguins to parrots, the species that live in colder places have smaller peckers.

This article was written for the McGill Office for Science and Society. View the original here: https://www.mcgill.ca/oss/article/did-you-know/peckers-get-smaller-where-it-gets-colder

What rhythm does throbbing pain follow? (McGill OSS)

1 minute read

There are many kinds of pain—Piercing, burning, aching, shocking—but the type I want to focus on today is throbbing. Throbbing pain is often associated with toothaches, headaches, migraines, and pain in the extremities but can occur nearly anywhere in the body. Its pulsing nature can be incredibly annoying to those affected, but it also raises an interesting question: when pain throbs, what rhythm is it following?

Contrary to what you might think, throbbing pain is not beating to your heartbeat or pulse. A 2012 study looked at the throbbing rate of 29 dental patients’ pain, as recorded by patients pushing a button every time they felt a painful throb, compared to their arterial pulse measured in their earlobes. The mean arterial pulse rate was 73 beats per minute (bpm), compared to a throbbing pain rate of just 44 bpm. Researchers further analyzed the simultaneous recordings and found that the two rhythms weren’t synchronous in any way.

If throbbing pain isn’t paced against our heartbeat or pulse, then what determines its rhythm? Simply put, we don’t know! The study’s authors theorize that the pacemaker of throbbing pain is contained somewhere within the central nervous system, but we currently do not have any more specific theories. For now, we just have to accept that throbbing pain marches to the beat of its own drum.

This article was written for. the McGill Office for Science and Society. View the original here: https://www.mcgill.ca/oss/article/did-you-know/what-rhythm-does-throbbing-pain-follow

Why do we wake up feeling cold? (McGill OSS)

1 minute read

A few different bodily processes in humans follow a stable, roughly 24-hour cycle. For example, the cortisol and melatonin levels in our blood. Physical parameters like your blood pressure and heart rate too.

Also under a circadian cycle is our core body temperature. We reach our minimum temperature about halfway through our sleep cycle. By the time we wake up, our bodies have warmed up slightly, but often not yet to our typical body temp.

So, we wake up feeling cold because we are cold. From a normal body temperature of 36.4-37.2 °C (97.5-98.9 °F), normal circadian fluctuations can take us up or down about 1 ˚C. It might not feel like a lot, but remember that most doctors consider fevers to start at 38 ˚C.

Interestingly, there seems to be some variation in when we reach our minimum temperature during the night. A 2001 study measured the temperatures of 172 young men and women and sorted them according to their self-declared status of “morning person,” “evening person,” or “neither.” They found that morning people hit their minimum temps after an average of 3.5 hours, compared to 5.02 hours for neither types and 6.01 hours for evening types. Since individuals tend to feel more alert and perform better on cognitive tasks at higher body temperatures, these differences in the circadian rhythm of body temperature may be one reason some of us struggle to wake up and feel alert immediately.

Image source: https://www.mdpi.com/2079-7737/10/1/65

This article was written for the McGill Office for Science and Society. View the original here: https://www.mcgill.ca/oss/article/medical-you-asked/why-do-we-wake-feeling-cold

Squirrels can survive a fall from any height, at least hypothetically (McGill OSS)

1 minute read

Squirrels, in theory, can survive a fall from an object of any height due to two factors: their size and their mass. A force (such as the force of gravity) is calculated by multiplying mass and acceleration. The acceleration due to gravity on Earth is always roughly 9.81 m/s², regardless of what object it is acting on. Squirrels are not very heavy—a grey squirrel only weighs about 0.5 kg—meaning that the force acting on a falling squirrel just isn’t that big.

Force = mass*acceleration = 0.5 kg * 9.81 m/s² = 4.9 N

We measure forces in a unit called “Newtons”, named for Isaac Newton who gave us Newton’s three laws of motion.

Compare this to, for example, a falling 60 kg human, which would be pulled downward with a force of about 489 N. A factor of 100 higher!

On top of being small, squirrels are fluffy and intuitively spread their bodies out when falling. This allows them to experience as much wind resistance as possible, slowing down their rate of descent. Some squirrels even use this fact to glide through the air. While gliding is not the same as flight, we nonetheless call them flying squirrels.

For these two reasons, the terminal velocity (fastest speed while falling) of squirrels is slow enough that they will, at least in principle, never fall so hard that they hurt themselves.

This article was originally posted here: https://www.mcgill.ca/oss/article/did-you-know/squirrels-can-survive-fall-any-height-least-hypothetically

Birds Seem To Be Scared of Googly Eyes, and That’s a Good Thing (McGill OSS)

3 minute read

Every year upwards of 25 million birds are killed in Canada due to collisions with buildings, communication towers, wind turbines, and as a result of being tangled into marine gillnets. From window decals to flashing lights, humans have tried numerous preventative measures to stop these deaths. Their degree of success depends on the method, the location, and the types of birds in that ecosystem—amongst many other factors—and results are highly variable.

What may seem like benign interventions that—at worst—just won’t work, actually have the capacity to do harm. As an example, In Peru, bycatch (i.e., accidental catch) of Guanay Cormorants was reduced more than 80% after researchers attached green lights to gillnets. At the same time, bycatch of Peruvian Boobies increased. Possibly due to the boobie’s attraction to the lights.

Similarly, when researchers set out to the Baltic sea to compare the effects of attaching light panels, constant green lights, or flashing white lights to gillnets on sea birds (in particular the Long-tailed duck, a vulnerable species) they found that the nets with flashing white lights caught more ducks than the normal, non-illuminated ones.

One approach that is so far quite promising involves using giant looming googly eyes.

To continue reading for free, click here- https://www.mcgill.ca/oss/article/did-you-know-general-science/birds-seem-be-scared-googly-eyes-and-thats-good-thing

Does size matter when it comes to needles? (McGill OSS)

5 minute read

Shots, jabs, pricks—whatever you call it, having a needle inserted into your body is not most people’s idea of a fun afternoon activity. Even if you don’t have a specific needle phobia, injection reactions typically range from neutral at best to quite negative at worst. But what if needles didn’t have to hurt? Or, at least, what if they hurt less? It seems intuitively true that decreasing the size of a needle would make it hurt less, but is it really that simple?

The diameter of a needle (how big it is across) is measured in a unit called a gauge. Because the concept of a gauge pre-dates the 18^th century and has been defined in many different, inconsistent ways, it’s worth specifying that needle width is measured in the Birmingham gauge. The bigger the gauge, the smaller the needle. For example, the width of a 7-gauge needle is roughly 4.6 mm (0.18 inch), while the width of a 30-gauge needle is about 0.31 mm (0.012 inch). To give you some context, a typical spaghetti noodle is roughly 14-gauge, and a regular stud earring is about 19-gauge.

There are a few factors that determine what size of needle a practitioner needs to use, including the body size of the patient and the body part being pricked, but a critical factor is the amount of fluid being injected or drawn out of the patient. If you try to inject a large amount of fluid through a very thin needle, it will both take longer and hurt more due to the high pressure.

Image source: https://www.nms.ac.uk/explore-our-collections/stories/science-and-technology/syringes/

For blood collection, which is typically a few millilitres of blood, clinicians use needles of 21-22 gauge. Vaccines are often <1 mL and accordingly use needles that are slightly smaller, around 22-25 gauge. Delivering insulin to diabetic patients requires even less fluid and can use needles as small as 29-31 gauge.

Even with the limitations imposed by volume, there is some wiggle room in the gauge of needle used for a certain procedure. Medical practitioners can often use their own judgment, experience, and clinical guidelines to change the size of needle they use. Much like how artists may favour a certain size brush, some clinicians have personal preferences in the tools of their trade.

Luckily, it is actually relatively simple to study whether decreasing needle diameter decreases pain. Just find some volunteers who are willing to be stabbed for science (or who are already being treated with a needle-involved method), stick them with at least 2 needles of different gauges without telling them which is which, and ask them how much it hurt on a numeric scale. There are dozens of studies that take this form.

Regarding simple injections in the body, this study compared a 30-gauge needle with a 26-gauge one and found no significant difference in the reported pain. As did this study, which compared 27-gauge vs 23-gauge vs 21-gauge. For injecting Botox around patients’ eyes, this study found no difference in pain scores between a 32-gauge and a 30-gauge needle. These are by no means all of the studies on needle size and pain, but they are representative of the scientific literature on this topic. Again and again, trial participants seem to find no significant difference in their pain when comparing needle gauges.

For many people, the anesthetic injection is the worst part of any dentist visit. While it would be lovely to tell you that a quick swap to a thinner needle is all you need to decrease the pain of dental injections, there is a wealth of evidence to the contrary. For anesthetic injections in the mouth, smaller-width needles were not only ineffective at reducing pain; in one study, they actually increased it!

To continue reading, for free, click here- https://www.mcgill.ca/oss/article/medical/does-size-matter-when-it-comes-needles

When it comes to conservation, cat-fights only hurt our communication efforts (The Skeptic)

7 minute read

When non-native animals are introduced to an ecosystem, quite often, the very delicate balance of that environment is thrown off. Plants, animals, fungi, bacteria, and everything else in a biome are connected through the food web, meaning that small changes to any part of a habitat can have extensive consequences.

From zebra mussels in Canada to grey squirrels in the United Kingdom, invasive animals have become a massive problem with increases in global travel and shipping. We enact biosecurity laws and protocols, quarantine procedures and mandate pesticide treatments to try to limit their spread; but despite all our efforts to curb invasive invasions, there is one species that we tend to give a pass to: cats.

Domestic cats are not native to anywhere. While they are descended from Felis lybica, the African Wildcat, the domestic cat is a different species. They are even given a separate Latin species name: Felis catus.

Even when well fed at home, domestic cats often engage in predation and hunting behaviours. With some variance depending on location, cats tend to kill more birds and small mammals than anything else. Since domestic cats are an introduced species, they have tremendous potential to upset intricate ecological situations.

Some researchers strongly believe that domestic cats’ damaging influence on the environment has already been robustly demonstrated. They feel it is crucial to act immediately and decisively if we want to have any hope of counteracting the damage done by domestic felines. For example, in 2018, conservationists from Oklahoma State University and the Smithsonian Conservation Biology Institute published a paper wherein they denounced what they described as organised misinformation campaigns spreading junk science about domestic cats’ effects on ecosystems.

They invoke the Merchants of Doubt moniker—the name given to the “cabal of industry-beholden” contrarian scientists who denied evidence of harm by tobacco smoking, DDT and climate change for financial gain—and liken outdoor cat advocates to “cigarette and climate-change fact fighters” pushing “propaganda.”

Conversely, other researchers feel that many conservation scientists are fueling an unwarranted moral panic over outdoor cats with exaggerated claims and inadequate evidence. In response to the 2018 Merchants of Doubt publication, researchers from six universities around the world collaborated on a rebuttal. They wrote that:

equating the resources and power of global corporations and economic elites (e.g., Exxon Mobil) with the reach and advocacy of comparatively small non-profit organizations and university academics strains the [Merchants of Doubt simile] past the breaking point.

The authors take issue with conservationists concluding that cat advocates are acting with nefarious or bad faith motives and feel that calls for things such as “remov[ing cats] — once and for all — from the landscape” by “any means necessary” are sensationalist and premature. Instead, they call for better research to investigate the severity of the risks cats pose to habitats and the appropriate levels of interventions, and humane but effective alternatives to simply killing and banning outdoor cats.

A White-Hot Issue

If you’re not that familiar with the literary style research papers are usually written in, let me just say, it’s not usually quite like this. Usually, one side of an academic debate is not accusing the other of being corporate shills. The vast majority of the time, there are no mentions of “zombie apocalypse[s]” or calls to let things “weigh heavy on our shoulders.”

The rhetoric throughout the literature on outdoor cats is very inflammatory. The cats/birds issue isn’t just a problem to be solved. It is a fight; a conflict; a war. Solutions to this situation are needed urgently. Danger is imminent. “Drastic times call for drastic measures.” People “must ask themselves which animals should be saved but do so quickly because there is no time to [do both]… before extinctions occur”.

Clearly, the environmental impact of cats on birds, and the welfare of cats, are contentious and emotionally charged topics. It makes a lot of sense that they are. Environmental stewardship is an important role that humans are morally obligated to fulfill. Especially in the face of an existential threat. At the same time, cats also represent life that should be protected. Cats long ago transcended their status of just-another-animal. From their initial roles of pest control, they have become members of the family. Given as much, cat owners often take advice regarding their pets personally.

The thing is, this highly polarised landscape filled with provocative language and antagonistic interactions isn’t helping either side. And it isn’t helping the birds, or the cats, either.

Whether cats impact wildlife in a meaningful and long-lasting way is a question for the experts in this field. They do not seem to agree, which implies the need for more research on the matter. Either way, it doesn’t particularly matter who is “right” anymore.

What matters is how needlessly divided the debate has become.

A Birdy Binary

A false dichotomy has been created wherein one can either care about native wildlife or feline welfare, but never both. Either cats are the enemies — the representations of humans’ entitlement and disdain for the earth — or the most perfect companions, too often neglected and maligned, who are just following their natural instincts.

We do ourselves a massive disservice by reducing this complex and multifaceted issue to one side versus another, or ‘us versus them’. People are lumped into supposedly either loving birds and hating cats or vice-versa, when in truth, most conservationists and pet owners are motivated by similar loves of nature, flora, and fauna.

This artificial divide encourages more polarising solutions, more extreme takes and leads to fearmongering and moral panics. It not only creates this illusion of a lack of a middle ground, it eliminates any of the methods or solutions that would originate from there.

We can become so hyper-focused on advocating for one position that we become blinded to other parts of the issue. Habitat loss is displacing bird populations and climate change is affecting their ability to find food and water. As cities sprawl outward, they remove homelands for birds and disrupt migration routes. In Canada, around 100 million birds are estimated to die every year due to collisions with buildings, power lines and cars.

Such black-and-white thinking discourages the peer review process. With little room for nuance, any criticism of a study’s methods can be seen as dissent. Scientists need to feel free to question how research is performed and how it draws its conclusions without fear of being labelled as agents of misinformation.

It’s Getting Mean in Here

Outside of academic discussions, the binary division between perceived “bird lovers/cat haters” and “cat lovers/bird haters” is even wider. This pattern is seen to varying levels across social media, traditional media, and interpersonal relationships. Expressing the wrong opinion on Twitter about indoor/outdoor cats can lead to harassment and ostracisation.

We should all know that an anecdote is not good evidence for anything on its own. Nonetheless, let me tell you a short one.

I have written on a variety of “controversial” topics in the past — menstruation, copycat suicides, female ejaculation, transgender children, border walls — but only once have I been kicked out of a science-themed social media group. I was removed after sharing my (then) most recent article on whether bells on cat collars work to reduce the amount of prey that domestic cats kill. For the record, three studies (one published in 2005, one in 2006, and one in 2010) have shown that cats brought home less prey when they wore bells. But very quickly, the thread of responses devolved into name calling and insinuations of nefarious or financially motivated intentions.

Empathy works, not… whatever that is

What should be a logical debate on policies and practices has turned ugly. The cats and birds issue has become a hotbed for sensationalism and hyperbole, no matter your stance. And the worst part about it is that we know it won’t work as well as collaborative and kind approaches would.

We know that when trying to change somebody’s mind, what tends to work is empathy and ongoing dialogue. We want to avoid judgment, disdain, or anger. Scientists need to be transparent about how they draw their conclusions and accept legitimate criticisms. Science is not perfect or magic but just a tool to help us understand the world around us. Trust is crucial for effective communication of knowledge, and trust cannot be built on anything but honesty and openness.

Actually helping wildlife and domestic pets alike requires engaging with all stakeholders. Especially the ones that oppose your stance. As much as we may want to rant and kick and scream at the people who disagree with us, it’s pointless. Not only that, it’s actively detrimental to their understanding and your ability to communicate with them. Like with so many things, in science communication, kindness is key.

Article originally posted here- https://www.skeptic.org.uk/2022/09/when-it-comes-to-conservation-cat-fights-only-hurt-our-communication-efforts/

Problematic Perceptions of Probability of Precipitation (Skeptical Inquirer: But What Do I Know?)

5 minute read

As Benjamin Franklin wrote in 1789, “In this world, nothing can be said to be certain, except death and taxes.” For everything else, there is an inherent degree of uncertainty. We don’t often come face to face with quantitative probabilities in our everyday life, save for one: probability of precipitation (PoP).

A seemingly simple concept, PoP is present in most of our daily routines as we check the weather before getting dressed. You may not explicitly realize it, but we all have personalized thresholds for the PoP at which we will choose to bring an umbrella or cancel an outing. For some, a 60 percent PoP warrants carrying an umbrella; for others, only 80 percent or higher. Unfortunately, most of us don’t have an accurate idea of what PoP truly means, even though most of us are certain we do!

A 60 percent PoP does not mean that 60 percent of an area will receive precipitation. It also does not mean that it will rain for 60 percent of the time period, and it does not mean that a forecaster is 60 percent confident that it will rain. So, what does it mean?

As defined by the National Weather Service, a probability of precipitation (PoP) is the “chance that at least 0.01 [inch] of rain will fall at the point for which that forecast is valid over the period of time given.”

So, a 60 percent PoP means that when these meteorological conditions occur in this area, 6 times out of 10, there will be at least some rain. That’s what PoP is. But how is it calculated?

PoP = Confidence x Coverage

To find the PoP for a given area over a given time period, we take the confidence of the forecaster and multiply it by the area that will be affected by the precipitation. Say I was 100 percent confident that 50 percent of Cleveland would receive at least 0.01″ of precipitation tomorrow, the PoP would be (1 * 0.5 = 0.5) 50 percent. Now, if I was only 80 percent confident in my prediction that 50 percent of Cleveland is getting wet tomorrow, the PoP would be (0.8 * 0.5 = 0.4) 40 percent!

If you stay in the same spot all day (like I did writing this article), then a 40 percent PoP means you have a 4 in 10 chance of being rained on. But, if you move around within an area, or between areas, your probability of encountering rain increases. As Brad Panovich, Chief Meteorologist at WCNC Charlotte put it, “It’s like buying more raffle tickets. Each one you buy increases your chances of winning.”

If you were mistaken about PoP until today, count yourself among good company. Weather forecasts have been available to the general public in the United States since the late 1960s, but in studies, between 35 percent and 73.8 percent of respondents defined PoP wrong, even when they were meteorologists! A viral Tik-Tok from 2019 that incorrectly taught people the untrue percent-of-land PoP definition certainly hasn’t helped things.

It turns out that even those sort of wishy-washy terms meteorologists use to describe weather such as “scattered flurries” or “isolated showers” have fairly strict definitions too. The National Weather Service uses certain expressions to communicate the degree of certainty in a forecast: “slight chance,” “chance,” and “likely.” There are also particular qualifiers to convey the portion of the area that will be affected: “isolated/few”; “widely scattered”; “scattered”; “numerous”; or “occasional/periods of.”

Image source: https://www.weather.gov/bgm/forecast_terms

At least in Canada, the term “risk” as in a “risk of thunderstorms” indicates a 30–40 percent chance of said weather occurring. Fun fact, also in Canada, a PoP of 50 percent is never permitted, because it seems too indecisive.

So, to those who previously found themselves cursing the local weather forecaster for never getting it right, hopefully, this article helps explain that your own lack of knowledge was more likely at fault than theirs. Believe it or not, weather forecasts have actually been getting more and more accurate with time. In 1972, a National Weather Service forecast made three days before was off by an average of six degrees. Forty years later, it was down to three degrees. In the late 1980s, when trying to predict where hurricanes would make landfall three days in advance, the National Hurricane Center missed by an average of 350 miles. Now the average miss is only about 100 miles. 

Now, that’s not to say that meteorologists can’t be biased. Many weather agencies previously biased their forecasting toward more precipitation than will actually occur. This so-called “wet bias” meant that for years when the Weather Channel predicted a 20 percent PoP, it actually rained only roughly 5 percent of the time. It’s unclear if the wet bias is still influencing forecasts today.

Take a look at the chart below. It shows the percent confidence on the top and the percent area on the side. By multiplying these together we get the PoP at the intersection of the two. The chart is symmetric. For example, if your confidence is 90 percent and the area affected is 50 percent, the PoP equals 45 percent. If the values were opposite, the PoP would still be 45 percent. This means that even though there are multiple ways to arrive at each PoP, in the end, they mean similar things.

A 40 percent PoP can be arrived at by multiplying 100 percent and 40 percent or 50 percent and 80 percent. Therefore, a 40 percent PoP could mean that it is:

• Absolute certainty (100 percent) that some (40 percent) of the area will receive rain, or
• Quite likely (80 percent) that half (50 percent) of the area will receive rain, or
• Somewhat possible (40 percent) that all (100 percent) of the area will receive rain, or
• Possible (50 percent) that most (80 percent) of the area will receive rain.

In the end, all these basically mean “you probably won’t need an umbrella, but it’s not a bad idea.”

Similarly, you’ll notice that to get a PoP of 70 percent or above, one of either the confidence or area must be greater than 70 percent. Regardless of whether that’s the result of being very sure it’ll rain over more than half the area, or being fairly sure it’ll rain over the entire area, what matters is that you remember to close your bedroom window.

Original article posted here- https://skepticalinquirer.org/exclusive/new-column/