Heat index, decoded. The number your skin already knows.

The thermometer says 90. Your body says otherwise. That gap is the heat index — temperature plus humidity, combined into the number the air actually feels like. It is the forecast number that matters most on a sticky summer afternoon.

WHAT IT ACTUALLY IS.

Temperature, plus the humidity tax.

The heat index combines air temperature with relative humidity to estimate how hot it feels to the human body. Humidity slows how fast sweat evaporates, and evaporation is how your body cools itself — so the same temperature feels much hotter when the air is already full of moisture. A 90-degree day at high humidity can feel like well over 100.

READING THE NUMBER.

It only counts when it is actually hot.

The heat index is defined for warm conditions — roughly 80 degrees and above — and it is calculated in the shade, so direct sun makes it feel even hotter than the number says. As a rough guide: the 90s mean use caution, especially for long outdoor exertion; above 100 means take it seriously, with shade, water, and breaks; above 115 is the range where heat illness can come on fast. Wind helps a little; shade helps a lot. And the index says nothing about the morning — it is a daytime, sun-up number.

WHY IT MATTERS TO YOU.

Plan the outing, not just the outfit.

Two days can share the same temperature and feel completely different — the humid one drains you faster. Checking the feels-like number before a run, a ride, or a long afternoon outside is the difference between a good day and a wiped-out one. Brzzy’s forecast, live radar, and activity ratings turn the same kind of ground truth into a plain-language answer: go outside, or wait it out.

What is the heat index?

The heat index is the “feels like” temperature: air temperature combined with relative humidity to estimate how hot it actually feels to the human body. High humidity slows sweat evaporation, which is how the body cools itself, so humid air feels hotter than dry air at the same temperature.

How is the heat index calculated?

From temperature and relative humidity, using a formula based on how the body loses heat through sweat. It is calculated for the shade — direct sunlight makes it feel hotter than the published number — and it only applies in warm conditions, roughly 80 degrees Fahrenheit and above.

What is the difference between the temperature and the heat index?

The temperature is what the thermometer measures. The heat index is what your body experiences once humidity is factored in. On a dry day the two numbers are close; on a humid day the heat index can run 10 degrees or more above the temperature.

What heat index is dangerous?

As a rough guide: the 90s call for caution during long outdoor exertion, above 100 calls for shade, water, and breaks, and above 115 is the range where heat illness can come on fast. Children, older adults, and anyone working outdoors are affected sooner — when in doubt, cut the outing short.

Does Brzzy show the heat index?

Brzzy shows your local forecast with hourly updates, live radar, severe weather alerts, GIF forecasts, and activity ratings that weigh the sky for your plans. Brzzy is free to download on iPhone.

Download Brzzy Weather today