What is Heat Index?

A hot day can feel more uncomfortable when the humidity is high. It’s that sticky, muggy feeling when there is excess moisture in the air. This makes you feel hotter because your body is unable to cool itself by sweating.

Heat index is a measurement of how hot it really feels outside. The heat index formula relies on environmental data including the ambient air temperature and relative humidity, and is only calculated when temperatures exceed 80 degrees Fahrenheit (27 degrees Celsius) for and relative humidity above 40 percent.

The heat alerts initiated by the National Weather Service are mainly based on heat index values. Excessive heat can be a safety and health hazard. For an example of heat index, let's say your weather station records an outdoor temperature of 90 degrees Fahrenheit (32.2 degrees Celsius) and a humidity reading of 60%. The temperature at which these two values intersect is the heat index. In this example, it will feel like it is 100 degrees Fahrenheit (38 degrees Celsius) outside.

Take caution during outdoor activities when the heat index is above 91°F (32 °C) to reduce risk of heat exhaustion and heat stroke – especially for young children and the elderly.

As you can see, at cooler temperatures, humidity isn’t that large of a factor. But when it’s very hot outside, humidity can really make a big difference in how you feel. For example, on a 100-degree Fahrenheit day (38 degrees Celsius), a relatively low 40% humidity already raises the heat index to 109 (42 degrees Celsius). At 55% humidity, you’re feeling a scorching (and dangerous) heat index of 124 degrees Fahrenheit (51 degrees Celsius).

To help you stay safe and comfortable, the AcuRite weather displays and My AcuRite dashboard calculate Heat Index for you based on the information reported from your weather station and outdoor sensors. The calculation used in the AcuRite products for Heat Index were obtained from the publicly available Heat Index Calculator from the National Weather Service.

Be prepared to diagnose and treat heat exhaustion or heat stroke by visiting the Mayo Clinic articles on heat exhaustion and heatstroke.