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What Exactly Is the Heat Index?

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Is it really not the heat, but the humidity? With temperatures spiking around the country, we thought it might be a good time to answer some questions about the heat index.

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 humidity is the amount of water the air contains. 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’s 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 weatherman just needs to know the air temperature and the relative humidity, and the chart will tell him the rest.

Is the 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 the 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, the day is classified as “extremely hot,” 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 “very hot” 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.

What’s the highest heat index that’s been recorded?

In their book Extreme Weather Christopher C. Burt and Mark Stroud tackle this topic. The authors explain that the worst spots in the world for a combination of heat and humidity are along Ethiopia’s Red Sea coast, along Somalia’s Gulf of Aden coast, and around the Persian Gulf. July 8, 2003, was a particularly punishing day in Dharan, Saudi Arabia. That day the temperature hit 108 Fahrenheit with a dew point of 95 degrees. This combination led to a heat index in the neighborhood of 155 to 160.

This article originally appeared in 2011.

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Big Questions
Why Do Cats Freak Out After Pooping?
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Cats often exhibit some very peculiar behavior, from getting into deadly combat situations with their own tail to pouncing on unsuspecting humans. Among their most curious habits: running from their litter box like a greyhound after moving their bowels. Are they running from their own fecal matter? Has waste elimination prompted a sense of euphoria?

Experts—if anyone is said to qualify as an expert in post-poop moods—aren’t exactly sure, but they’ve presented a number of entertaining theories. From a biological standpoint, some animal behaviorists suspect that a cat bolting after a deposit might stem from fears that a predator could track them based on the smell of their waste. But researchers are quick to note that they haven’t observed cats run from their BMs in the wild.

Biology also has a little bit to do with another theory, which postulates that cats used to getting their rear ends licked by their mother after defecating as kittens are showing off their independence by sprinting away, their butts having taken on self-cleaning properties in adulthood.

Not convinced? You might find another idea more plausible: Both humans and cats have a vagus nerve running from their brain stem. In both species, the nerve can be stimulated by defecation, leading to a pleasurable sensation and what some have labeled “poo-phoria,” or post-poop elation. In running, the cat may simply be working off excess energy brought on by stimulation of the nerve.

Less interesting is the notion that notoriously hygienic cats may simply want to shake off excess litter or fecal matter by running a 100-meter dash, or that a digestive problem has led to some discomfort they’re attempting to flee from. The fact is, so little research has been done in the field of pooping cat mania that there’s no universally accepted answer. Like so much of what makes cats tick, a definitive motivation will have to remain a mystery.

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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Big Questions
Why Do Baseball Managers Wear Uniforms?
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Basketball and hockey coaches wear business suits on the sidelines. Football coaches wear team-branded shirts and jackets and often ill-fitting pleated khakis. Why are baseball managers the only guys who wear the same outfit as their players?

According to John Thorn, the official historian of Major League Baseball since 2011, it goes back to the earliest days of the game. Back then, the person known as the manager was the business manager: the guy who kept the books in order and the road trips on schedule. Meanwhile, the guy we call the manager today, the one who arranges the roster and decides when to pull a pitcher, was known as the captain. In addition to managing the team on the field, he was usually also on the team as a player. For many years, the “manager” wore a player’s uniform simply because he was a player. There were also a few captains who didn’t play for the team and stuck to making decisions in the dugout, and they usually wore suits.

With the passing of time, it became less common for the captain to play, and on most teams they took on strictly managerial roles. Instead of suits proliferating throughout America’s dugouts, though, non-playing captains largely hung on to the tradition of wearing a player's uniform. By the early to mid 20th century, wearing the uniform was the norm for managers, with a few notable exceptions. The Philadelphia Athletics’s Connie Mack and the Brooklyn Dodgers’s Burt Shotton continued to wear suits and ties to games long after it fell out of favor (though Shotton sometimes liked to layer a team jacket on top of his street clothes). Once those two retired, it’s been uniforms as far as the eye can see.

The adherence to the uniform among managers in the second half of the 20th century leads some people to think that MLB mandates it, but a look through the official major league rules [PDF] doesn’t turn up much on a manager’s dress. Rule 1.11(a) (1) says that “All players on a team shall wear uniforms identical in color, trim and style, and all players’ uniforms shall include minimal six-inch numbers on their backs" and rule 2.00 states that a coach is a "team member in uniform appointed by the manager to perform such duties as the manager may designate, such as but not limited to acting as base coach."

While Rule 2.00 gives a rundown of the manager’s role and some rules that apply to them, it doesn’t specify that they’re uniformed. Further down, Rule 3.15 says that "No person shall be allowed on the playing field during a game except players and coaches in uniform, managers, news photographers authorized by the home team, umpires, officers of the law in uniform and watchmen or other employees of the home club." Again, nothing about the managers being uniformed.

All that said, Rule 2.00 defines the bench or dugout as “the seating facilities reserved for players, substitutes and other team members in uniform when they are not actively engaged on the playing field," and makes no exceptions for managers or anyone else. While the managers’ duds are never addressed anywhere else, this definition does seem to necessitate, in a roundabout way, that managers wear a uniform—at least if they want to have access to the dugout. And, really, where else would they sit?

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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