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Hot Nights and High Humidity Set This Heat Wave Apart

My colleague Heidi Cullen has an excellent op-ed in the New York Times today on the current heat wave, and the shifting notion of what constitutes a "normal climate" as average global temperatures continue to warm. She didn't have room for details on the heat wave in that story, but it's worth doing so here given its important lessons for climate change adaptation efforts, particularly concerning public health and infrastructure.

The heat wave that is currently roasting much of the United States stands out from typical summertime heat events that we expect to occur during July and August. First of all, the hot weather, which is associated with a sprawling area of high pressure, covers a huge expanse. Today, for example, at least 141 million people under heat advisories or warnings, according to a tweet from NOAA spokesman Justin Kenney (see video below).

A NOAA visualization showing the spread of the heat wave across the US this week.

Second, the heat wave has featured an extraordinary combination of high temperatures and humidity. In combination, these are known as the heat index, and during this heat wave the heat indices have shot up to levels more commonly seen in the brutally hot and humid region near the Red Sea, rather than in Minnesota and South Dakota. As meteorologist Paul Douglas reported on his Minneapolies Star Tribune blog, the heat index in Moorhead, Minn. hit a whopping 134°F yesterday, likely setting a new record for the highest heat index ever reported in Minnesota. The Twin Cities also tied its all-time heat index record, at 119°F.

These extreme heat indices constitute a human health risk, since these conditions make it extremely difficult for the body to cool itself through sweat and evaporation. The extra humidity makes evaporation more difficult, which effectively shuts down our main cooling mechanism. The dew point (another measure of how much water is in the air) has reached the 80s across many parts of the Midwest, which is a level more commonly seen near the Gulf of Mexico during the summertime. For example, Douglas reported that Minneapolis experienced three days in a row with 80°F or higher dew points. According to him, that is unprecedented since the beginning of instrument records there in the late 19th century. In Moorhead, Minn., where the record heat index was observed, the dew point reached 88°F, a new state record.

These dew points are partly related to the heavy rains and flooding that the Midwest experienced this spring. As temperatures soared this week, more and more water evaporated from the moist soils and corn fields, elevating the humidity levels higher than they would normally in a typical summer heat wave. It's no accident that there is a lot of overlap between the highest heat indices seen in the first map below, and the Midwest portion of the spring flood outlook from the National Oceanic and Atmospheric Administration (NOAA).

Dew point temperatures on Sunday July 16, 2011. Credit: UCAR.

Spring flood outlook issued in March. Credit: NOAA.

The current heat wave is consistent with climate science research, which shows that as the climate warms, heat waves in general are becoming more likely. Such heat waves can be especially deadly when they feature high humidity and unusually warm overnight low temperatures that prevent people from cooling off after a hot day. On July 19 alone, 317 record warm minimum temperatures were either set or tied in the U.S., according to the National Climatic Data Center. Of those records, 84 either set or tied all time warm minimum records

Research has shown that climate change has made specific heat waves, such as the deadly 2003 European heat wave, more likely to take place, and projections generated from computer model simulations show summers may become much hotter on average during the next few decades. These projections have implications for public health officials, who need to be prepared for future heat waves. Weather Underground blogger and University of Michigan professor Ricky Rood explored the public health issues in detail on Tuesday, writing:

If I am a public health official here is my algorithm — heat waves are already important to my life, and they are likely to get more dangerous, more frequent, and of longer duration. But by how much? Do I need to know by how much before I decide on a plan for action?

Here in 2011, I see drought and flood and hot weather and warm oceans that interact together to make a period of sustained, dangerous heat. It does not have to “set a record” to convey the reality of the warming earth. It tells me the type of event that is likely to come more often, of longer duration, and of, perhaps, of greater intensity. If I am a public health planner, then I can know this with some certainty. The question becomes, how do I use that information in my planning?

The current heat wave, with its high humidity, hot days and nights, and poor air quality, is not just making conditions stressful for people. Complex systems that we rely on to beat the heat, such as the electrical grid that powers our air conditioners, are also being tested. According to the Star Tribune, XCel energy — Minnesota's largest utility — set a record for high electricity demand in Minnesota and three neighboring states on July 17, and 8,000 customers lost power at one point. Here's a snippet from the paper's story:

The state's largest utility also reported thousands of power outages, usually lasting a few hours or less, as distribution lines failed and fuses on its system blew out, said spokesman Tom Hoen.

"It is not that there is not enough supply," Hoen said. "It's from the demand and stress being put on the system."...

Hoen said Monday's demand was 9,500 megawatts in Xcel's service region in Minnesota, North Dakota, South Dakota and Wisconsin. The previous record was set last Aug. 9 at 9,100 megawatts (MW). By comparison, Xcel's largest power generation station -- the three coal-fired Sherco units near Becker, Minn. -- puts out a total of 2,400 MW.

The utility is using innovative measures to manage the power supply, the newspaper reported:

Xcel on Monday activated remote-controlled power-saving switches on air conditioning units of residential customers who signed up for the conservation program. Hoen said the conservation step, which cycles off customers' air conditioning condensers, was activated for about an hour. 

Iowa's electric utilities also reported record high demand this week, in part because of the warm nighttime temperatures that kept energy demand high. According to the Cedar Rapids Gazette, utilities there were also working with some customers to curtail their power during demand spikes: 

“We’re not seeing that cool-down at night, allowing a little relief for the system,” MidAmerican Energy spokeswoman Tina Pothoff said. “We’re starting with high usage in the morning and seeing it build throughout the day.”

Power outages were kept to a minimum, IP&L President Tom Aller said in prepared remarks. As long as the heat remains, however, he said the electrical grid and Alliant crews will continue to be tested. Alliant’s demand models forecast that electric demand today could match Monday’s record.

To supply enough power, Alliant interrupted service to some industrial customers with special contracts that provide them with lower rates in exchange for the ability to curtail part or all of their power when demand spikes. Alliant also activated a program that cycles air conditioners off and on at 15 minute intervals in households that have signed up voluntarily for the program in exchange for receiving electric bill credits.

Utilities in South Dakota also set new high demand records. Jonathan Fahey of the Associated Press has more details about the energy demand implications of extreme heat. 

Data Journalist David Kroodsma contributed reporting to this blog post.

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