what is the heat index?

Heat is more than the temperature showing on the thermometer. It is a combination of temperature and humidity. See this National Weather Service explanation of the heat index

Way back in the 1970s the US Army called it a “wet bulb” temperature to factor in the impact of humidity on the human body.

This is what New York City Emergency Management recently wrote on X about their ongoing heatwave.

Remember, it is only June and there will be many more heat waves coming our way in the months to come.