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Why Does the Shower Curtain Always Try to Get Me?

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For those of you who take baths or are simply unconcerned with hygiene, let me explain the horror that is the “shower curtain effect.” You get in a nice hot shower first thing in the morning. You’re barely awake, but the womb-like confines offer the perfect space to transition from sleepy cretin to functioning human. Until, of course, the shower curtain – who you thought was your friend, who you thought you could trust – gives in to its strange, powerful attraction to running water and billows in towards you. On a good day, it’s a minor annoyance. On a bad day, it’s an unspeakable horror and the curtain actually, like, touches you. And it’s cold. And maybe even a little slimy.

Why does the curtain do this to us? Great minds have been struggling with the problem for years, but until recently all anyone offered were hypotheses. Nobody ever actually tested these explanations and made the results public until a few years ago. That experiment gave us a pretty solid answer, but the theoretical work that came before it is pretty interesting, too. Let’s explore the small handful of different answers that have been suggested over time.

The Bernoulli Principle Hypothesis
We talked about the Bernoulli’s principle the last time I wrote about bathroom science. It states, basically, that an increase in velocity of a fluid (liquid or gas) results in a decrease in pressure around it. With shower curtains, the principle was thought to come into play like this: The water coming out of the shower head causes the air in the shower to start flowing in the same direction that it's traveling, which is parallel to the curtain. The air moving across the inside of the shower curtain causes the air pressure to drop, and the difference in pressure between the two sides of the curtain causes it to move in toward the lower-pressure area. For most of the time that people have been talking about the shower curtain effect scientifically, this has been the leading explanation.

The Buoyancy Hypothesis
Warm air rises up and out of the shower. The air density in the shower is reduced and, like in the Bernoulli hypothesis, the difference in pressure between the shower and outside of the curtain makes the curtain move inward. The big problem with this hypothesis? The curtain still moves in even if you run an ice cold shower.

The Coand? Effect Hypothesis
Jearl Walker, a professor of physics at Cleveland State University who used to write Scientific American’s “Amateur Scientist” column, suggested that the Coand? effect, the tendency of a fluid in motion to adhere to a surface or vice versa, was at work.
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Now, these hypotheses are all well and good. They’re plausible explanations. One was even suggested by a guy who knows a thing or two about physics (and has put his hand in molten lava and poured liquid nitrogen in his mouth to demonstrate its principles). But they don’t mean much without data to back them up.

In 2001, David Schmidt, from the University of Massachusetts, put his own hypothesis to the test and gave us the first evidence-backed explanation to every showerer’s pet peeve.

The Horizontal Vortex Hypothesis
Using a computer model of a shower, Schmidt found that the shower head’s spray creates a horizontal vortex with a low-pressure area center that sucks in the shower curtain. We’ll let Schmidt, who won the 2001 Ig Nobel Prize in Physics for his work, explain his study and its results a little more. As he explained it to Scientific American:

To do the calculation, I drafted a model of a typical shower and divided the shower area into 50,000 minuscule cells. The tub, the showerhead, the curtain rod and the room outside of the shower were all included. I ran [the modeling software] for two weeks on my home computer in the evening and on weekends (when my wife wasn't using the computer). The simulation [which ran some 1.5 trillion calculations] revealed 30 seconds of actual shower time.

When the simulation was complete, it showed that the spray drove a vortex. The center of this vortex, much like the center of a cyclone, is a low-pressure region. This low-pressure region is what pulls the shower curtain in… It is a bit like a sideways dust devil. But unlike a dust devil, this vortex doesn't die out because it is driven continuously by the shower.

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Health
Growing Up With Headphones May Not Damage Kids’ Hearing
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A study published in the American Medical Association's JAMA Otolaryngology-Head & Neck Surgery finds no increase in child and adolescent hearing loss despite a rise in headphone and earbud use.

"Hearing impairment in children is a major public health burden given its impact on early speech and language development, and subsequently on academic and workforce performance later in life," the authors write. "Even mild levels of hearing loss have been found to negatively affect educational outcomes and social functioning."

As portable music players continue to grow in popularity, parents, doctors, and researchers have begun to worry that all the music pouring directly into kids' ears could be damaging their health. It seems a reasonable enough concern, and some studies on American kids' hearing have identified more hearing loss.

To take a closer look, researchers at the University of California-San Francisco analyzed data from the National Health and Nutrition Examination Survey (NHANES), collected from 1988 to 2010. They reviewed records from 7036 kids and teens between the ages of 12 and 19, checking each participant's hearing tests against their exposure to noise.

As expected, the authors write, they did find a gradual increase in headphone use and other "recreational noise exposure." And they did see an uptick in hearing loss from 1988 to 2008 from 17 percent to 22.5 percent. But after that, the trend seemed to reverse, sinking all the way down to 15.2 percent—lower than 1988 levels. They also found no significant relationship between noise exposure and hearing loss.

The results were not uniform; some groups of kids were worse off than others. Participants who identified as nonwhite, and those of lower socioeconomic status, were more likely to have hearing problems, but the researchers can't say for sure why that is. "Ongoing monitoring of hearing loss in this population is necessary," they write, "to elucidate long-term trends and identify targets for intervention."

Before you go wild blasting music, we should mention that this study has some major limitations. Hearing loss and other data points were not measured the same way through the entire data collection period. Participants had to self-report things like hearing loss and health care use—elements that are routinely under-reported in surveys. As with just about any health research, more studies are still needed to confirm these findings.

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Weather Watch
NASA Figures Out Why When It Rains, It (Sometimes) Drizzles
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What’s the difference between drizzle and rain? It has to do with updrafts, according to new research by NASA scientists into the previously unexplained phenomenon of why drizzle occurs where it does.

The answer, published in the Quarterly Journal of the Royal Meteorological Society, could help improve how weather and climate models treat rainfall, making predictions more accurate.

Previously, climate researchers thought that drizzle could be explained by the presence of aerosols in the atmosphere. The microscopic particles are present in greater quantities over land than over the ocean, and by that logic, there should be more drizzle over land than over the ocean. But that's not the case, as Hanii Takahashi and her colleagues at the Jet Propulsion Laboratory found. Instead, whether or not rain becomes full droplets or stays as a fine drizzle depends on updrafts—a warm current of air that rises from the ground.

Stronger updrafts keep drizzle droplets (which are four times smaller than a raindrop) floating inside a cloud longer, allowing them to grow into full-sized rain drops that fall to the ground in the splatters we all know and love. In weaker updrafts, though, the precipitation falls before the drops form, as that light drizzle. That explains why it drizzles more over the ocean than over land—because updrafts are weaker over the ocean. A low-lying cloud over the ocean is more likely to produce drizzle than a low-lying cloud over land, which will probably produce rain.

This could have an impact on climate modeling as well as short-term weather forecasts. Current models make it difficult to model future surface temperatures of the Earth while still maintaining accurate projections about the amount of precipitation. Right now, most models that project realistic surface temperatures predict an unrealistic amount of drizzle in the future, according to a NASA statement. This finding could bring those predictions back down to a more realistic level.

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