You have been there: the thermometer reads 35°C but stepping outside feels like walking into an oven. The forecast says "feels like 42°C." What is the "feels like" temperature, how is it calculated, and why does it matter beyond a number on a screen?

What Is the Heat Index?

The heat index — also called the "apparent temperature" or "feels like" temperature — is a measure of how hot it actually feels to the human body when both temperature and humidity are taken into account. It was developed by meteorologist Robert Steadman in 1979 and is now a standard component of weather forecasting worldwide.

The basic principle is this: your body cools itself through sweating. When sweat evaporates from your skin, it carries heat away from your body. But humid air is already saturated with water vapour — there is less capacity for your sweat to evaporate. When sweat cannot evaporate efficiently, your body's cooling mechanism fails, and you feel much hotter than the air temperature alone would suggest.

How Is It Calculated?

The full Steadman formula is a complex polynomial equation involving temperature and relative humidity. A simplified approximation: at 35°C and 40% humidity, the heat index is approximately 35°C (not much difference). At 35°C and 80% humidity, the heat index is approximately 46°C. At 40°C and 70% humidity — roughly what you might experience in Dubai in August — the heat index can exceed 55°C.

This is why tropical cities like Bangkok, Singapore, and Mumbai, despite having lower maximum temperatures than Dubai, can feel oppressively hot in ways that desert cities do not — the combination of heat and extreme humidity creates a heat index that is often far higher than the actual temperature.

When Does Heat Become Dangerous?

Health authorities use heat index thresholds to classify risk:

  • 27–32°C heat index: Caution — fatigue is possible with prolonged exposure
  • 32–41°C heat index: Extreme Caution — heat cramps and heat exhaustion possible
  • 41–54°C heat index: Danger — heat cramps and exhaustion likely, heat stroke possible
  • Above 54°C heat index: Extreme Danger — heat stroke imminent

Children, elderly people, and those with cardiovascular or respiratory conditions are especially vulnerable at elevated heat index levels.

Wind Chill: The Cold-Weather Equivalent

The opposite effect exists in cold weather. Wind chill is the perceived temperature when wind speed is factored in — wind accelerates heat loss from exposed skin, making cold air feel significantly colder. At -10°C with a 30 km/h wind, the wind chill can be equivalent to -20°C. Wind chill, like heat index, is about heat transfer from your body to the environment, not the actual temperature of the air.

On the SunorSnow dashboard, the "Feels like" reading incorporates both heat index (for warm, humid conditions) and wind chill (for cold, windy conditions), giving you the most practically useful temperature number for planning your day.