Imagine your usual day. You wake up, get yourself prepared for the day, then head to your table to grab some breakfast. You grab your phone to browse through what you missed on news and social media while you were asleep, but you spot your phone’s recent notification on the lock screen: “Clear: 30°C, feels like 27°C.” “Why two temperatures?” you ask, scratching your head. The weather updates you saw on TV last night mostly mention just one. Then you notice the words lodged between the update: “feels like.” What’s all that supposed to mean?
That was your phone informing you of the heat index for that specific time of day. Heat index, an index presented to us usually through weather updates and is commonly preceded with words like “feels like,” “real feel,” or “apparent temperature,” is the way the current temperature feels to a person in the shade (Yes! The heat index assumes you’re standing in the shade during the time of measurement.) with respect to the current relative humidity and air temperature. Developed by Robert G. Steadman back in 1979, it helped bring a human element to the weather by making the temperature readings closer to what we people really feel—or at least close to it. Now, understandably, that can be a lot to take in, so let’s break it down.
What are the details?
As any person frequently blessed—or pestered by—rains will tell you, the air can hold water. This water is usually held in the form of water vapor, and is usually the culprit for when you just swear you feel the air is a bit heavier than usual, especially during rainstorms. Now, as it stands, the amount of water air can hold changes depending on how hot or hot cold it currently is. Relative humidity simply describes how much water vapor air actually holds compared to what it can hold at a certain temperature. The hotter the air, the more water vapor it can hold. The higher the relative humidity, the closer the current water vapor content in the air is to its maximum amount (an amount referred to as a saturation point)—therefore the less water vapor it can take in. You’ve probably already heard about air temperature; it’s the usual temperature reading your local weatherman tells you every night to expect for the coming day.
How are we involved in all this?
Humans sweat—a lot. It’s our body’s tried and tested way of cooling down. By releasing sweat from our sweat glands, we expose our glistening skin to the air. The air then brings a little bit of the sweat along with it, evaporating the sweat and cooling off our skin—and indeed us—as a consequence.
Now, imagine this situation: it’s a hot day, and you’re outside cleaning the yard. You’re sweating from all the hard work you’ve been doing, but there’s not much evaporation happening: the air is too humid—rather, the current relative humidity is high. As a result, your sweat isn’t evaporating off your body much, meaning your body isn’t cooling down as much as it needs to. Your body then feels hotter as a result.
It’s this difference between how hot the air feels versus how hot your body feels that makes the heat index so important in everyday life. Misjudging the reported air temperature versus what your body actually experiences can lead to issues like heat stroke and heat exhaustion. Let’s not forget that this goes the other way, too: the body can feel colder compared to how cold the air actually is, as well. These are the problems that the heat index tries to address.
So, how do they figure it out?
Following the earlier work of Steadman from years back, meteorologists use specific calculations and assumptions to determine the heat index for a specific time of a certain day. Like what Steadman has done before, they make things a bit simpler by assuming certain values and measurements, like an average person’s weight, how much clothing they’re wearing, and how fast the breeze is at the time. To make things easier for everyone, meteorologists have devised heat index charts that show how hot, or indeed how cold, an average person feels with respect to the current air temperature and relative humidity, and are often color-coded to show how dangerous it might be to someone who’s willing to go outside at that time. Of course, every person is different from one another: we don’t all have the same weight or amount of clothing compared to what was assumed in the calculations. However, it’s close enough to be considered a good enough estimate to be used in everyday life.
And yes, just as mentioned above, the heat index does assume that the person is standing in the shade: the thinking goes that you will naturally want to stay in the shade instead of staying out in the sun. Your body will, of course, feel differently when you are standing in direct sunlight or when you are actively engaging in activities at the time.
Conclusion
And that’s it! In an odd mix of meteorology with a seemingly more human aspect, the heat index is a convenient way to make everyday weather more relatable to us, condensing weather information into a simpler estimate that’s easy to understand, no calculators required. The concept that Steadman developed decades ago has been refined by scientists throughout the years since, and has proven useful time and time again, giving us an accessible way to judge how safe it is to head out on a hot summer’s day or through a cold winter breeze. Just make sure to check on your phone’s weather app or the local weatherman before heading out next time.
Bibliography
- National Weather Service. (n.d.). What is the heat index? National Weather Service. Retrieved September 1, 2021, from https://www.weather.gov/ama/heatindex
- Steadman, R. G. (1979, July 1). The Assessment of Sultriness. Part I: A Temperature-Humidity Index Based on Human Physiology and Clothing Science. Journal of Applied Meteorology and Climatology, 18(7), 861-873. https://doi.org/10.1175/1520-0450(1979)018<0861:TAOSPI>2.0.CO;2
- Steadman, R. G. (1979, July 1). The Assessment of Sultriness. Part II: Effects of Wind, Extra Radiation and Barometric Pressure on Apparent Temperature. Journal of Applied Meteorology and Climatology, 18(7), 874–885. https://doi.org/10.1175/1520-0450(1979)018<0874:TAOSPI>2.0.CO;2
- Trex, E. (2019, July 20). Why Does Humidity Make Us Feel Hotter? Mental Floss. Retrieved September 1, 2021, from https://www.mentalfloss.com/article/30964/what-exactly-heat-index
