What does "feels like" temperature mean?

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📓 The short answer

“Feels like” temperature reflects how wind and humidity change how cold or hot the air feels to the human body. Wind chill increases heat loss in cold conditions, while the heat index reflects how humidity interferes with sweating in hot weather.

📚 The long answer

There's temperature. And then there's "feels like" temperature. I had no clue they could be so different until I spent a hot, humid summer in Washington, D.C.:

"Feels like" temperature, otherwise known as apparent temperature, is an estimate of how hot or cold the temperature will feel to the human body. On hot days, the "feels like" temperature might be several degrees hotter, and on cold days, it might be way lower.

"Feels like" temperature has to do with the impact of wind and humidity on our perception of cold and hot days.

🥶 What is wind chill?

On cold days, meteorologists use wind chill to determine the "feels like" temperature. The basic principle is this: Wind makes us feel colder because it's removing our body heat's layer of warm air and replacing it with cooler air.

Our bodies naturally radiate heat, creating a layer of insulation known as the boundary layer. On a cold but windless day, this layer keeps us from rapidly losing body heat.

Wind blows. More specifically, it blows away this boundary layer. This leads your body to lose more heat as it tries to warm the new cold air, which wind blows away again. A vicious, chilly cycle that leaves you feeling much colder.

Meteorologists use mathematical calculations factoring in the true temperature and the wind speed to come up with a wind chill index:

🥵 What is the heat index?

I'm sure we've all heard the phrase, "It's not the heat that gets you; it's the humidity." And as clichè as it sounds, it's accurate.

The reason why humidity makes us feel so much warmer has to do with how we sweat. When we're hot, our bodies produce sweat to cool us down. But it's not enough to simply sweat; we need the sweat to evaporate for it to have a cooling effect.

We cool down because evaporation requires energy. More energy allows water molecules to move faster so they can escape into the air. Our body heat provides this energy, which reduces our skin temperature.

But on humid days, sweat evaporates more slowly. If the air is humid, it already has lots of water vapor in it, close to the point of saturation. This is why humidity makes it feel so much hotter — sweating is simply less effective at cooling us.

Meteorologists use mathematical calculations factoring in the true temperature and the relative humidity to come up with a heat index:

Interestingly, the heat index also explains why some hot days feel cooler. If the relative humidity is extremely low, it can speed up evaporation and make us feel cooler than the true temperature. For example, if the true temperature in Phoenix, Arizona is 100°F (38°C) and the relative humidity is 15%, the heat index is 96°F (36°C).

🧠 Bonus brain points

What is the wet-bulb globe temperature?

Along with "feels like" temperature, you may have also heard about the wet-bulb globe temperature (WBGT) on hot days. The heat index is helpful, but it doesn't take into account the impact from the sun (i.e. all figures assume you're in the shade.) If you're exposed to direct sunlight, it can feel up to +15°F (+9°C) hotter than the heat index suggests.

The wet-bulb globe temperature creates a more accurate "feels like" temperature by factoring in air temperature, humidity, wind speed, and solar radiation.

• A black globe thermometer measures the radiant heat from the sun and surrounding environment.
• A wet-bulb thermometer measures the cooling effect of evaporation.
• A dry-bulb thermometer measures the air temperature in the shade.

--- Thanks for reading this week's newsletter! If you have any thoughts, questions, or favorite GIFs, my inbox is always open. Just hit reply to send me a note! :) ​ All my best, Caitlin ​Click here to read five facts about Caitlin​

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​Sources for this week's newsletter​

🌐 Wikipedia article of the week

​Pocari Sweat​

"Pocari Sweat is a Japanese sports drink, manufactured by Otsuka Pharmaceutical....

Pocari Sweat is a mild-tasting, non-carbonated sweet beverage and is advertised as an "ion supply drink", "refreshment water" (1992), "body request" (1999), and "electrolyte beverage" in Thailand. It has a mild grapefruit flavor with little aftertaste....
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The name Pocari Sweat was chosen by the manufacturers for the purpose of marketing the product as a sports drink in Japan, where English words are used differently. It was largely derived from the notion of what it is intended to supply to the drinker: all of the nutrients and electrolytes lost when sweating. The first part of the name, Pocari, has no meaning but was intended to sound refreshing."

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👀 Catch up on other curious questions

• ​What happens to the temperature of the atmosphere the higher you go?​​
• ​What is freeze drying?​​
• ​Do sports teams that train in high altitudes have a home advantage?​​

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