What Is the Heat Index? Feels-Like Temperature Explained
When summer rolls in, checking the weather forecast is no longer as simple as looking at the thermometer. You might see a temperature of 32°C, but with a warning that it actually feels like 39°C. That apparent figure is the heat index.
Why Air Temperature Doesn't Tell the Whole Story
Your body cools itself primarily through perspiration. As sweat evaporates from your skin, it absorbs and removes heat from your body. However, relative humidity levels dictate how quickly this evaporation can happen.
When relative humidity is high, the air is already filled with moisture, making it much harder for your sweat to evaporate. Consequently, your body retains heat, your heart rate rises, and you feel significantly hotter than the actual temperature of the surrounding air.
Topical Authority Check
On our live map, cities like Athens and Rome frequently experience high relative humidity along the coast, raising their feels-like index to critical levels even when nominal air temperatures are lower. Check how they are doing on the live map leaderboard.
The Heat Index Chart: When Does It Become Dangerous?
Meteorologists categorize heat index values into threat levels to help the public stay safe:
- 27°C - 32°C (Caution): Fatigue is possible with prolonged exposure and activity.
- 32°C - 41°C (Extreme Caution): Heat cramps and heat exhaustion are possible.
- 41°C - 54°C (Danger): Heat exhaustion is likely; heat stroke is possible.
- 54°C or higher (Extreme Danger): Heat stroke is highly imminent.
Frequently Asked Questions
How is the heat index calculated?
The index is calculated using a multi-parameter equation developed by Rothfusz. It applies dry-bulb temperature and relative humidity to estimate subjective sensory discomfort.
Does wind affect the feels-like temperature?
In hot conditions, wind can speed up evaporation slightly, providing a minor cooling sensation. However, if the air is extremely hot (above skin temperature), wind can behave like a convection oven, worsening heat stress.
Are You Cooked?
Before heading out today, calculate your personal comfort index. Take our quick Am I Cooked? Quiz to check your home and lifestyle vulnerability and discover which temperature tier you are sitting in.