Why Does Humidity Make Us Feel Hotter?

Tomwang112/iStock via Getty Images
Tomwang112/iStock via Getty Images

With temperatures spiking around the country, we thought it might be a good time to answer some questions about the heat index—and why humidity makes us feel hotter.

Why does humidity make us feel hotter?

To answer that question, we need to talk about getting sweaty.

As you probably remember from your high school biology class, one of the ways our bodies cool themselves is by sweating. The sweat then evaporates from our skin, and it carries heat away from the body as it leaves.

Humidity throws a wrench in that system of evaporative cooling, though. As relative humidity increases, the evaporation of sweat from our skin slows down. Instead, the sweat just drips off of us, which leaves us with all of the stinkiness and none of the cooling effect. Thus, when the humidity spikes, our bodies effectively lose a key tool that could normally be used to cool us down.

What's relative about relative humidity?

We all know that humidity refers to the amount of water contained in the air. However, as the air’s temperature changes, so does the amount of water the air can hold. (Air can hold more water vapor as the temperature heats up.) Relative humidity compares the actual humidity to the maximum amount of water vapor the air can hold at any given temperature.

Whose idea was the heat index?

While the notion of humidity making days feel warmer is painfully apparent to anyone who has ever been outside on a soupy day, our current system owes a big debt to Robert G. Steadman, an academic textile researcher. In a 1979 research paper called, “An Assessment of Sultriness, Parts I and II,” Steadman laid out the basic factors that would affect how hot a person felt under a given set of conditions, and meteorologists soon used his work to derive a simplified formula for calculating heat index.

The formula is long and cumbersome, but luckily it can be transformed into easy-to-read charts. Today your local meteorologist just needs to know the air temperature and the relative humidity, and the chart will tell him or her the rest.

Is the heat index calculation the same for everyone?

Not quite, but it’s close. Steadman’s original research was founded on the idea of a “typical” person who was outdoors under a very precise set of conditions. Specifically, Steadman’s everyman was 5’7” tall, weighed 147 pounds, wore long pants and a short-sleeved shirt, and was walking at just over three miles per hour into a slight breeze in the shade. Any deviations from these conditions will affect how the heat/humidity combo feels to a certain person.

What difference does being in the shade make?

Quite a big one. All of the National Weather Service’s charts for calculating the heat index make the reasonable assumption that folks will look for shade when it’s oppressively hot and muggy out. Direct sunlight can add up to 15 degrees to the calculated heat index.

How does wind affect how dangerous the heat is?

Normally, when we think of wind on a hot day, we think of a nice, cooling breeze. That’s the normal state of affairs, but when the weather is really, really hot—think high-90s hot—a dry wind actually heats us up. When it’s that hot out, wind actually draws sweat away from our bodies before it can evaporate to help cool us down. Thanks to this effect, what might have been a cool breeze acts more like a convection oven.

When should I start worrying about high heat index readings?

The National Weather Service has a handy four-tiered system to tell you how dire the heat situation is. At the most severe level, when the heat index is over 130, that's classified as "Extreme Danger" and the risk of heat stroke is highly likely with continued exposure. Things get less scary as you move down the ladder, but even on "Danger" days, when the heat index ranges from 105 to 130, you probably don’t want to be outside. According to the service, that’s when prolonged exposure and/or physical activity make sunstroke, heat cramps, and heat exhaustion likely, while heat stroke is possible.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

This article has been updated for 2019.

Why Are Poinsettias Associated with Christmas?

iStock
iStock

Certain Christmas traditions never seem to go out of style. Along with wreaths, gingerbread cookies, and reruns of A Christmas Story sits the poinsettia, a red-tinged leafy arrangement that’s become synonymous with the holiday. Upwards of 100 million of them are sold in the six weeks before December 25.

Why do people associate the potted plant with seasonal cheer? Chalk it up to some brilliant marketing.

In 1900, a German immigrant named Albert Ecke was planning to move his family to Fiji. Along the way, they became enamored of the beautiful sights found in Los Angeles—specifically, the wild-growing poinsettia, which was named after Joel Roberts Poinsett, the U.S.-Mexican ambassador who first brought it to the States in 1828. Ecke saw the appeal of the plant’s bright red leaves that blossomed in winter (it’s not actually a flower, despite the common assumption) and began marketing it from roadside stands to local growers as "the Christmas plant."

The response was so strong that poinsettias became the Ecke family business, with their crop making up more than 90 percent of all poinsettias sold throughout most of the 20th century: Ecke, his son Paul, and Paul’s son, Paul Jr., offered a unique single-stem arrangement that stood up to shipping, which their competitors couldn’t duplicate. When Paul III took over the business in the 1960s, he began sending arrangements to television networks for use during their holiday specials. In a priceless bit of advertising, stars like Ronald Reagan, Dinah Shore, and Bob Hope were sharing screen time with the plant, leading millions of Americans to associate it with the holiday.

While the Ecke single-stem secret was eventually cracked by other florists—it involved grafting two stems to make one—and their market share dwindled, their innovative marketing ensured that the poinsettia would forever be linked to Christmas.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

Why Does Santa Claus Live at the North Pole?

allanswart/iStock via Getty Images
allanswart/iStock via Getty Images

As children settle in for a restless night’s sleep this Christmas Eve, they’ll no doubt be picturing Santa Claus on his way from the snowy ’scapes of the North Pole to deliver them Star Wars LEGO sets, Frozen 2 dolls, and everything else on their wish list. They picture Santa at the North Pole, of course, because they’ve seen him living there in numerous Christmas movies, books, and television specials, from perennial Rankin/Bass programs to more modern classics like 2003’s Elf.

While it might seem a little more magical if we told you that nobody really knows why Santa lives there, there is a relatively traceable paper trail: The first known reference to Santa’s North Pole residence is in an 1866 cartoon from Harper’s Weekly.

According to Smithsonian.com, famed political cartoonist Thomas Nast—who was also responsible for establishing the donkey and elephant as the symbols for the Democratic and Republican parties, respectively—first started creating Harper’s Weekly Christmas cartoons as Union propaganda for the Civil War in January 1863. Borrowing imagery from Clement Clarke Moore’s (alleged) 1823 poem “A Visit From St. Nicholas” (which you’d probably recognize as “’Twas the Night Before Christmas”), Nast developed the white-bearded, rosy-cheeked, all-around jolly guy that we know today, and showed him passing out gifts to Union soldiers, climbing into a chimney as a soldier’s wife prays, and more.

harper's weekly santa claus at camp by thomas nast
Thomas Nast, Harris Brisbane Dick Fund, Wikimedia Commons // Public Domain

The cartoons became so popular that Nast branched out from his source material and began inventing his own details to add to Saint Nick—like where he’s from, for example. A December 29, 1866 issue of Harper’s Weekly debuted a multi-image cartoon titled “Santa Claus and His Works,” which includes a small inscription along the circular border that reads Santa Claussville, N.P. According to The New York Times, we don’t know exactly why Nast chose the North Pole (or if it was even his own idea), but there are a few reasons it made sense for the time period.

For one, Santa Claus was already widely associated with snow because most of the publishing companies producing Christmas cards and other content were located in New England, where it actually snows around Christmas. Furthermore, the 1840s and 1850s were partly characterized by high-profile—and ill-fated, in the Franklin expedition's case—attempts to explore the Arctic, and the public was generally interested in the mysterious, poorly-charted region. Because the Pole was unoccupied, Santa and his elves could toil the year away without interference from prying eyes; and, because it was unclaimed, Santa could remain a bastion of benevolence for every nation.

merry old santa claus by thomas nast
"Merry Old Santa Claus," perhaps Nast's most famous illustration of Santa, from the January 1, 1881 edition of Harper's Weekly.
Thomas Nast, Wikimedia Commons // Public Domain

Though we’ll likely never know Nast’s personal rationale behind placing Santa Claus in the North Pole, one thing’s for sure: At this point, it’s hard to imagine him living anywhere else. It’s also hard to imagine him riding a broom, wielding a gun, or smoking cigarettes (find out the stories behind those early Santas here).

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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