Heat index vs feels like temperature

Question

I hear it both ways some will say a feels like temperature or a heat index. I want to know if there is a difference between the two and if so what is it.

Thanks in advance

Answer 1

One-liner: Wikipedia thinks its the same. Although Wikipedia is not always perfect about information it posts, the justification there make sense to me.

Looked up at Heat Index wikipedia, it says,

The heat index (HI) or humiture is an index that combines air temperature and relative humidity, in shaded areas, to posit a human-perceived equivalent temperature, as how hot it would feel if the humidity were some other value in the shade. The result is also known as the "felt air temperature", "apparent temperature", "real feel" or "feels like"

Quoting other source such as weather.gov:

The heat index, also known as the apparent temperature, is what the temperature feels like to the human body when relative humidity is combined with the air temperature. This has important considerations for the human body's comfort.

Additional information for those who don't know that this term means, quoting from the same source again,

When the body gets too hot, it begins to perspire or sweat to cool itself off. If the perspiration is not able to evaporate, the body cannot regulate its temperature. Evaporation is a cooling process. When perspiration is evaporated off the body, it effectively reduces the body's temperature. When the atmospheric moisture content (i.e. relative humidity) is high, the rate of perspiration from the body decreases. In other words, the human body feels warmer in humid conditions. The opposite is true when the relative humidity decreases because the rate of perspiration increases. The body actually feels cooler in arid conditions. There is direct relationship between the air temperature and relative humidity and the heat index, meaning as the air temperature and relative humidity increase (decrease), the heat index increases (decreases).

Thanks Ken Graham for the source: For those who prefer values of temperature in degree Celsius, please refer the table here

Answer 2

As someone that lives in a high humidity and high-temperature area, I can tell you for certain that "feels like" and "heat index" is very much the same thing.

It's an insanely important number because, at 90 degrees Fahrenheit in 40% humidity, you can feel very comfortable, but at 90 degrees in 95% humidity you can wish you were dead. (90 degrees Fahrenheit is about 32 degrees Celsius.)

Of course, this also affects how you stay hydrated, and how you try to cool your body. In lower humidity, you can just sweat and drink a lot, but in higher humidity, the sweat doesn't evaporate as well so is less effective.

Mostly it's attempt to quantify a comfort level. But they do mean the same thing.

Answer 3

In simple terms:

"Heat Index" is what it feels like based on temperature + relative humidity.

"Feels Like" is temperature + relative humidity + wind speed. With most any breeze, "Feels Like" is usually a lower number than heat index due to evaporative cooling of your skin by the wind.

Answer 4

Heat Index is a combination of temperature and humidity.

Feels Like also takes the wind speed into account.

I live in a hot and humid climate and I know from my time on the golf course that both temp and humidity are more tolerable when the wind blows.

Answer 5

Too long, didn't read: Watch the Feels Like measurement (Australian version / Steadman calculation)

While the US Heat Index and Australian "Feels Like" temperatures are NOT the same, they are very similar.
Of options, the better one to watch is the Australian style Feels Like temperature because it:

• Measures the wind speed (which may blow away the layer of insulation that our clothes may produce to protect us)
• Assumes people are wearing clothes
• Is less error prone
• Works at temperatures below 80°F (26.67°C)
• Accounts for air saturation above 100% humidity (supersaturation)

Many heat index calculations have an error rate of ±1.3°F or more, depending on when their calculation was developed because initial formulas had changes to them over time based on further observations.

Wet bulb calculations have an error rate between −1° to +0.65°C and are only that accurate between relative humidities of 5% and 99% and air temperatures between −20° and 50°C.

The reason it is important to pay attention to Feels Like or at least the Heat Index instead of just the temperature is because at a certain combination of sunlight, heat, wind, and humidity the air around us can no longer absorb more water, which means our sweat stops evaporating. When that happens, we still sweat, but instead of cooling off, we cook.

Limit to mammalian temperature control

A resting human body generates ∼100 W of metabolic heat that (in addition to any absorbed solar heating) must be carried away via a combination of heat conduction, evaporative cooling, and net infrared radiative cooling. Net conductive and evaporative cooling can occur only if an object is warmer than the environmental wet-bulb temperature.

Humans maintain a core body temperature near 37°C that varies slightly among individuals but does not adapt to local climate. Human skin temperature is strongly regulated at 35°C or below under normal conditions, because the skin must be cooler than body core in order for metabolic heat to be conducted to the skin. Sustained skin temperatures above 35°C imply elevated core body temperatures (hyperthermia), which reach lethal values (42–43°C) for skin temperatures of 37–38°C even for acclimated and fit individuals.

So this affects everyone, regardless of physical fitness level. The very healthy may survive a little longer if their core temperature gets too high, but they are still cooking.

Also, when we think it is comfortable we may go out without a jacket, but wind chill can have a drastic effect on how the temperature and humidity affect our bodies. So in medium to cool temperatures, we can actually get hypothermia (freeze) if we don't pay attention to humidity and wind chill as well as temperature. Feels Like temperatures account for this, but heat index often does not.

In summary, the Australian / Steadman 'Feels Like' temperature is preferred, but the Heat Index is better than only looking at temperature. Wet bulb can tell you when the humidity + heat are getting too high, but says little and can even be misleading about colder temperatures.

Look at the links above for details on the studies that went into making and evaluating the different temperature measurement methods.