Summer Danger: Extreme Nighttime Heat
Published: June 26, 2013

Extreme heat is on the way for Arizona, California and Nevada, as a large, hot dome of high pressure builds over the region, according to weather.com meteorologist Chris Dolce. In Death Valley, Calif., searing temperatures could even reach 130 degrees Saturday and Sunday.

The most densely populated area affected, Phoenix, Ariz., won't even see relief at night. With daytime temps expected to hit the mid- to upper 110s this weekend, overnight and early morning lows are only forecast to drop into the low 90s through early next week, Dolce said.

The lack of nighttime cooling pokes a serious hole in most heat-safety advice. Typically, health experts recommend limiting outdoor activity to the early morning and evening when temperatures are lower. But when temperatures don't cool off at night, not only do heat-related health risks remain around the clock, but also a new set of health threats pop up because of urban heat islands, according to Dolce.

"Asphalt and concrete also store heat longer and gradually release heat at night, leading to higher evening temperatures," he said. "This can prevent or reduce nighttime relief from extreme heat in a large city."

In 1995, a severe heat wave killed 465 people in Chicago and an additional 85 people in Milwaukee. In this extreme weather episode, part of the danger came from humid nights that offered no relief from the heat, according to a report from the National Weather Service.

High dew points, a direct measure of the moisture content in the air, were extraordinarily high three nights in a row, according to the NWS report. That, coupled with a high relative humidity (well above 90 percent) "added substantially to the already considerable human stress implied by the high average observed temperatures," officials wrote in the report.

To stay safe in the heat - no matter the time of day - the U.S. Centers for Disease Control and Prevention offers these tips:

Closely monitor people over age 65, infants and children and people with chronic medical conditions. These groups are more prone to heat-related illness.
Seek air conditioning wherever you can. During conditions of extreme heat, spend time in locations with air conditioning such as shopping malls, public libraries or public health sponsored heat-relief shelters in your area.
Get informed. Listen to local news and weather channels, or contact your local public health department during extreme heat conditions for health and safety updates.
Drink cool, nonalcoholic beverages and increase your fluid intake, regardless of your activity level.

Here is the forecast for my area:

I'll never forget the mass deaths in the Paris heatwave; 2002 or so.
  Once we had a heatwave(US) and were in the 10th floor of an apartment when the power failed.  It must have been the synthetic toxic building materials gone airborne in addition to the heat and dehydration.  Everyone was just staggering down the steps when the alarm sounded--it was like being drugged for two days.  Later when we came back I turned the shower on cold, and there was steam for half an hour.  Extreme heat is really dangerous.