It’s not your imagination — some hot days really do feel worse than others. Some feel like stepping into an oven, while others feel more like wading through a steam room.

As an intense heat wave grips much of the United States, from the Midwest to the East Coast, it’s the steam room that’s dominating for many places this week.

We talk a lot about temperature, but the thermometer tells only part of the story. A dry 99 degrees Fahrenheit in Phoenix might feel harsh but bearable. The same temperature in New York or Philadelphia this week can feel far more oppressive, partly because of higher humidity — that’s increased water vapor in the air.

Here’s how that works.

When humidity surges, the body struggles to cool itself. Normally, when your body gets too hot, it cools itself by perspiring. The evaporation of sweat carries heat away, acting like a natural air-conditioner. But when the air is already saturated with moisture, that evaporation process slows or stalls. As the National Weather Service puts it, “the human body feels warmer in humid conditions.”

The opposite is true when humidity decreases, so the body feels cooler in dry conditions, even if the temperature is climbing.