Unusually high humidity will keep nighttime temperatures very warm, which can be more dangerous than the daytime highs, because our bodies don't have time to recover. The temperature needs to drop to at least 80 degrees for recovery to begin.
Some areas of the United States won't get below 80 degrees some nights this week, resulting in a dangerous combination. Without air-conditioning, these hot temperatures can put a strain on the body that can result in heat exhaustion or even death.
This proved true during a heat wave that struck Chicago in 1995. Hundreds of people died and thousands ended up in the emergency room. Many of the deaths that year occurred at night.
Studies conducted after the heat wave showed that many people didn't feel safe enough to open their windows at night, and the buildings acted like a brick oven. The very young and elderly are especially sensitive to this. Also, a person can lose up to 2 liters of fluids overnight through sweating if the temperature is above 85 degrees.
Overnight lows will stay in the upper 70s to near 80 in many places across the Midwest and the South over the next few days, with very high humidity. Low temperatures are usually reached in early morning, so many of these areas will still have temperatures that feel like the 90s or higher until after midnight.
Humidity plays a key role
This particular heat wave has seen exceptionally high humidity levels. Like the winter equivalent of the wind chill, the heat index -- the temperature combined with the humidity -- gives us a true measure of how hot it feels.
Meteorologists use dew point as a measure of humidity. When the dew point and the temperature are the same, the air is saturated. Dew points in the 50s are considered comfortable.
As dew points climb, so does your lack of comfort. Your body sweats to cool off, and if that sweat doesn't evaporate, your body can overheat.
Dew points this week in the Midwest have been running in the 70s to near 80, which is downright oppressive. The combination of temperature near 100 and those high dew points are pushing the heat index above 120 degrees, or into the very dangerous category, this week and into the weekend.
The urban heat island
The concrete, buildings, asphalt, etc. in cities absorb more heat than rural areas, resulting in something called the "urban heat island effect." This can make cities several degrees warmer during the day, but the real impact can be at night.
The concrete and asphalt release heat at a much slower rate, so cities can run 20 or more degrees above areas outside of the city. This can also add to the dangerous nature of the heat and limit the potential relief that is supposed to come at night.