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During the sticky Iowa summer months, it's common to hear TV meteorologists and weather alerts mention the heat index.

Some days, they'll tell you 90 degrees feels like 90 degrees. Other times it feels like 105.

The difference? The humidity.

According to the National Weather Service, higher humidity means your perspiration doesn't evaporate as quickly, making it feel hotter because your body isn't cooling itself as well. Thus the heat index, or "apparent temperature," measures what it feels like outside.

The NWS notes that heat index values are calculated using thermometers shaded from the sun. That means in direct sunlight, conditions can feel up to 15 degrees hotter.

Fun fact: If the humidity is low, the heat index can actually be below the actual temperature.

Want to calculate heat index yourself? Use this handy tool from the National Weather Service. All you need is the temperature and the dew point or relative humidity.

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