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.”

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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-


Is hard water dangerous to drink?

3 minute read
Originally posted here:

What is Hard Water?

Hard water is water containing high concentrations of dissolved minerals, usually calcium or magnesium carbonates (CaCO­or MgCO3), chlorides (CaCl2or MgCl2) or sulphates (CaSO4or MgSO4). The hardness of water depends on its source. Groundwater that has been in contact with porous rocks containing deposits of minerals like limestone or dolomite will be very hard, while water from glaciers or flowing through igneous rocks is much softer.

The hardness of water is determined by the milligrams of calcium carbonateper litre and is reported it in parts-per-million (ppm). In general, water with less than 60 ppm can be considered soft, water with 60-120 ppm moderately hard, and water with greater than 120 ppm hard. For reference, Montreal’s water is typically around 116 ppm, or moderately hard, and sea water’s hardness is approximately 6,630 ppm since it contains many dissolved salts (depending on the sea, of course).

Hard water can interfere with the action of soaps and detergents and can result in deposits of calcium carbonate, calcium sulphate and magnesium hydroxide (Mg(OH)2) inside pipes and boilers, causing lower water flows and making for less efficient heating. The ions in hard water can also corrode metal pipes through galvanic corrosion. Water softening filters can circumvent these problems through the use of ion-exchange resins that replace calcium and magnesium ions with sodium and potassium ions. But if one consumes water with higher-than-average concentrations of calcium and magnesium. Is that OK?

The Health Effects of Hard Water

Studies have generally found hard water to have positive effects on the health of its drinkers. Several studies have reported that calcium and magnesium in drinking water have a dose-dependent protective effect when it comes to cardiovascular disease. There is also some evidence that calcium and magnesium in drinking water may help protect against gastric, colon, rectal cancer, and pancreatic cancer, and that magnesium may help protect against esophageal and ovarian cancer. Hard water may also serve a protective role against atherosclerosis in children and teens.

Some studies have shown a relationship between the mineral content of water and eczema or dermatitis in children. However, a 2011 study from the University of Nottingham involving 336 children aged 6 months to 16 years with eczema put that relationship to the test. The researchers installed water softening units in half of the participants’ homes and monitored the children’s eczema over a period of 3 months. Using a standard scoring system, the group that received softened water showed a 20% improvement, while the group that continued with hard water showed a 22% improvement, making it unlikely that hard water is contributing to worsening eczema symptoms.

Likewise, while some studies have shown correlations between water hardness and kidney stone formation, the majority of studies have found no such relationship.

It is estimated that individuals living in hard water areas who drink 2 litres of water a day ingest about 52 mg of magnesium from their water. Considering the daily recommended intake of magnesium is 420 mg, water can account for about 12% of that.

Individuals with type 2 diabetes often experience hypomagnesemia (low magnesium) as insulin regulation requires magnesium to function. In these people, the extra intake of magnesium through drinking water could be beneficial. The heightened magnesium concentration in hard water can also benefit people experiencing chronic constipation, as magnesium salts act as laxatives. One study noted that vegetables cooked in hard water often show an increase in their calcium concentration, as opposed to the decrease seen when they’re cooked in soft water.

It is fairly difficult for humans with healthy kidneys to experience hypercalcemia (too much calcium), as any excess calcium is excreted through the kidneys. Similarly, hypermagnesemia is fairly rare, and usually just results in short episodes of diarrhea.

The Physical Effects of Hard Water

There are, however, some non-medical reasons hard water isn’t always preferable. Hard water can appear cloudy if the solubility of mineral salts is exceeded. Furthermore, if the calcium concentration surpasses 100 ppm, the water will taste “funny.” Neither of these presents a risk, but consumers prefer a “clean” appearance and taste.

Basically, while hard water can be hard on appliances and pipes, it is not hard on the body, and can actually give the daily intake of calcium and magnesium a nice little boost.

Do Fish Drink? (McGill OSS)

2 minute read
Originally published here:

Our bodies and fishes’ (yes, fishes is a grammatically correct plural form of fish) bodies as well need water. Without it, the chemical reactions that take place constantly in our bodies would have no solvent and we would die.

Nonetheless, it seems silly that an underwater creature should have to drink. Can’t they just, I don’t know, absorb it or something?

Kind of.

Fish do absorb water through their skin and gills in a process called osmosis. Osmosis is the flow of water across membranes from areas of low concentration of dissolved things (solutes) to areas of high concentration. It serves to equalize the concentrations in the two areas.

In the case of freshwater fish, their blood and bodily fluids are much saltier than the water they swim in, so water will flow in through their gills. The opposite is true for saltwater fish.

As well as getting water through osmosis, saltwater fish need to purposefully drink water in order to get enough into their systems. Where their freshwater counterparts direct all of the water that comes into their mouths out through their gills, saltwater fish direct some into their digestive tract.

But fishes’ bodies, just like ours, need a certain concentration of salt to function best. They can’t just allow the water to diffuse freely through their gills; the saltwater fish would shrivel up and the freshwater fish would explode!

To stop the exploding fish phenomenon, their gills have special cells that selectively pump salt in, or out of their blood. In freshwater fish, the cells constantly pump salt in, and in saltwater fish, they constantly pump salt out. Saltwater fishes’ kidneys also help to filter out some of their salt.

Want to see osmosis for yourself? Submerge some potato slices in salt or fresh water overnight. The saltwater-soaked ones will still be crunchy, but the freshwater ones, having absorbed water, will be softer.

In short: some, but not all, fish drink. Kind of like how some, but not all, fish… fart.

So, keep in mind that next time you’re preparing your fishes’ tank you’re not only creating his environment but his beverages too.

What is Sea Foam and Why is it all Over the Beaches at Dunkirk?

Originally posted here:

If you watched the summer blockbuster Dunkirk, you may have been left with the same question as one of our readers: what is sea foam and why was there so much of it at Dunkirk?

Well, the stuff that makes our beaches look like lattes turns out to be mostly gunk. It’s a collection of organic material, like algae, fish scales or bits of coral, that when agitated by the ocean’s waves and currents act as foaming agents and surfactants.

Surfactants are substances that lower the surface tension of water, basically reducing the attraction between water molecules allowing the surface to stretch around air bubbles. A foamis just a dispersion of a gas, in this case, air. in a liquid. When it comes to sea foam, more organic material means more surfactants, more foaming agents, and more foam, so when algae blooms or large fish schools die, you are likely to see more foam forming.

In 2007 a few giant storms off the coast of Sydney caused a massive influx of sea foam, causing the locals to refer to the beach as the Cappuccino Coast! While we likely won’t ever know exactly what caused the influx of foam during Dunkirk, my guess would be wastesfrom armies being dumped into waterways.

Surfactants don’t only cause bubbly beaches, though; they’re actually responsible for keeping us alive! Pulmonary surfactants in our lungs work to stabilize alveoli: small air-filled membrane bubbles that allow us to diffuse oxygen in, and waste gasses out of our lungs, just as they stabilize air bubbles in the sea.

Those Silica Gel Packs You Toss Out are Filled With Deliquescent Chemicals

Originally published here:

When you buy a new pair of shoes or stereo you probably encounter, and subsequently toss out, lots of little packets bearing the warning ‘Do Not Eat’ filled with silica gel. Did you know though, that those packs are filled with deliquescent chemicals? Deliquescent materials are those that absorb moisture from the air, to the point that, if allowed, they will absorb water until they are dissolved in solution. This moisture affinity is very helpful for keeping water away from sensitive electronics in computers, or stopping the accumulation of water inside double paned windows. Various chemical’s deliquescence is utilized in chemical production, livestock birthing, and flood cleanups. And, deliquescent is an amazing Scrabble word.

Is Carbonated Water Bad for Your Teeth?

Originally published here:

It’s a well-known fact that soda is bad for your health. We’ve all read the reports and articles detailing how the sugar in pop will rot our teeth, cause obesity, or give us diabetes. So perhaps, like many other people, you’ve turned to carbonated water to fill that cold, bubbly niche in your life. Companies like La Croix or Perrier have grown immensely in recent years, signalling a risingpopularity in soda waters- sometimes flavoured, often calorie-free, ‘healthy’ alternative to Coke or Pepsi. But new reports with headlines like ‘Sparkling Water Is ‘Extremely Acidic’ And Can Ruin Teeth, Dentist Warns’‘Tragic News for People Who Love Seltzer Water’, and my personal favourite ‘Flavored waters — yes, including La Croix — are eroding your teeth’are casting doubt on bubbly water’s healthystatus. With claimslike‘seltzers can be bad for your teeth, especially if they are flavoured’ tothe‘citric acid or phosphoric acid that’s in your classic soft drinks [and flavoured bubbly waters can drop below the critical pH of 4, where you then can have dental erosion as an outcome’ it’s becoming less clearwhat’s good for our teeth.

I decided to put some carbonated water to the test, specifically the potential of hydrogen test. The pH of a substance tells us its acidity or basicity- water is roughly neutral with a pH of 7. Here’s a handy reference sheet to give you some context. 

I spent a day in the lab and took a total of 36 pH measurements on 9 different brands of carbonated water that I looked very strange buying at IGA. I opted to test each bottle both cold (4℃, the usual temperature for a fridge) and warm (21℃), and both carbonated and decarbonated. You can see my full results in the chart below.

pH tests

Water BrandWarm, carbonated pHCold, carbonated pHWarm, decarbonated pHCold, decarbonated pH
Sensations (Lemon)5.524.66.335.67
Nestle (Pure Life)5.134.455.85.5
Club Soda4.43.694.994.77
San Pellegrino5.344.286.855.6
Eska (Lemon)4.954.055.435.63

The really important numbers to look at are the ones for cold, carbonated waters, as this is how most people will drink their bubbly water. The average for all cold, carbonated water’s pH is 4.5. There seems to be an idea circulating that unflavoured bubbly waters are actually ok for your teeth, but flavoured ones do damage. That idea isn’t exactly reflected in my data, though I did only test 2 flavoured waters(and both lemon since that’s my favourite). The Eska brand lemon water came out as more acidic, but still rather close to the average, and the Sensations brand actually tested less acidic than some of the unflavoured waters. I’d have to do more tests to be sure, but it seems that there isn’t much truth to the idea that it’s the flavours causing tooth damage.

According to severalsources, teeth begin to demineralize at a pH of 5.5, but if you look closerat this issue it’s not so clear cut. The Journal of the Canadian Dental Association (JCDA) published this issuein 2003, which looks at the idea of a ‘critical pH’, or the pH below which your teeth are in danger of erosion. As they put it, ‘the critical pH does not have a fixed value but rather is inversely proportional to the calcium and phosphate concentrations in the solution’.

Essentially, dental enamel is made mostly of hydroxyapatite, which dissolves in water to form calcium, phosphate and hydroxyl (OH-) ions. So when we drink liquids without calcium or phosphate ions in them, or with decreased hydroxyl ion amounts (such as acidic solutions) some amount of the minerals from our teeth dissolve into the liquid (the universe likes to balance things as much as it can). This occurs every single time you drink water, but only in tiny amounts (about 30 mg in 1 L of water), since the pH of water is neutral. When we drink acidic drinks, like sodas, fruit juices, or (mildly) acidic bubbly waters, the minerals in our teeth dissolve in a process called demineralization.

So how acidic is too acidic then? When is this equilibrium between hydroxyapatite and dissolved minerals tipped too far? Well, that depends on the pH of your saliva. If you have a pool, you may just have litmus strips sitting around, and it could be fun to find the pH of your spit. But if you don’t, take my word for it that saliva pH is ~5.5-6.5, depending on the phosphate and calcium ions in your body. The more ions in your spit, the lower the critical pH, or in other words, the more acid your teeth can withstand before demineralizing.

So, long story short, are carbonated waters good for your teeth? No. But they’re not as bad as most of the other things we drink. The pH of most bubbly waters might be below the critical pH for most mouths, but as a chemistry studentI’d say the claims that these waters are ‘extremely acidic’ are completely false. The pH of Gatorade is about 3, of orange juiceis about 3-4 and of Coke is 2.4 (Read about the pH’s of other common drinks here). If the pH to stay above is ~6, then drinking your daily Perrier warm, or even better warm and flat would be best. But if like me, you think that defeats the purpose, then do what I do and rinse your mouth after with tap water.

It might not be that seltzer water is quite the healthy alternative to soda we all hoped, but a better alternative it certainly is. Skip the Pepsi and quench the craving with some S. Pellegrino, your teeth will thank you, and your blood sugar will too. If this foray in the science of seltzer has taught me anything though, it’s that I need to drink more plainold, flat water.