The heat wave that affected at least 200 million people in the United States during the past week and a half has finally subsided, but not before shattering or tying thousands of records. The geographic extent of the heat wave was highly unusual, with temperatures of 100 degrees Fahrenheit or greater and heat indices much higher than that stretching from Texas to North Dakota, and eastward all the way to Maine.

Every heat wave has a particular calling card, be it furnace-like hot and dry heat, sopping wet humidity, or other characteristics. This one will long be remembered for its widespread scope, high humidity levels, and dangerously warm nighttime low temperatures that remained above 80 F in many locations.

A chart of surface temperature anomalies (or departures from average) on July 22, as the heat was stretching into the Northeast. (NOAA ESRL)

Nationally, 1,966 daily high maximum temperature records have been broken or tied so far this month (through July 23). Sixty-six of those records were all-time maximum temperature records.More impressive, however, are the figures for highest minimum temperature records. Because of the extremely high humidity levels during this heat wave, a whopping 4,376 record highest minimum temperature records were broken or tied through July 23. Of those minimum temperature records, 158 were all-time records

South Central and Plains States

For some states, such as Texas, Oklahoma, and Kansas, the heat wave was more of a continuation of the intense heat and drought conditions they have already experienced so far this summer. The heat wave pushed some places across the finish line and into record territory, but intense heat is still ongoing in much of this region.

• Fort Smith, Arkansas has set a new record for the greatest number of consecutive 100 F days, with 20 through July 24.

• As of Sunday, the 23 days in a row with 100 F heat or greater in Dallas/Ft. Worth stood at number five on that city’s all-time list of 100-degree streaks.

• San Angelo, Texas has already had 58 100-degree days, which is ranked number 3 of all-time.

• Abilene has had 44 days this year with highs of 100 F or greater, and as of yesterday only needed two more such days to tie its all-time record, set in 1934.

• Amarillo, Texas, has set a new record for the greatest number of 100-degree days in a calendar year, with 28 such days through July 23rd.

• In Wichita, KS the average high temperature so far this summer is 101.2 F, which ranks 3rd on the list of all-time warmest summers.

Midwest and Upper Midwest

In the Upper Midwest, the heat wave broke records for temperature, dew point, and heat indices.

A chart of maximum heat indices observed in Wisconsin on July 18. (National Weather Service)

• On July 18, a weather balloon recorded a precipitable water value - a measurement used to describe the moisture in a vertical column of air, from the surface to the top of the troposphere - of 2.44 inches. According to the NWS, that may have been a new state record for Minnesota. Precipitable water values of greater than 1.75 inches are more typical of tropical air masses, and are rarely seen in the Upper Midwest.

• The heat index of 134 F reported at Moorhead, Minnesota on July 19 was not representative of the true atmospheric conditions at the time, the Weather Service found. “While it is possible the Moorhead dew-point did reach 88 F, it did so because the weather station is located in an agricultural field surrounded by water, or very wet soils, and crops that release a great deal of water vapor into the atmosphere,” the NWS stated on its website.

• Rochester, Minn. set a record for a consecutive 4-day warm low temperature of 77 F, recorded during July 17-20, breaking the previous record of 76 F set during July 1-4 1911. Rochester also set a new high dew point record of 83 F on July 18 and 19th, and a heat index record of 118 F on July 18.

• According to the NWS in Aberdeen, SD, high dew point temperatures recorded there “were likely some of the highest values ever recorded in South Dakota.”

• This heat wave produced 2 of the 5 warmest low temperatures ever recorded in Huron, SD. The low temperature there of 81 F on July 19 was the 2nd warmest low temperature in that station’s history (tied with July 20, 1934 & July 15, 1931).

• Chicago (Midway) hit 100 on back-to-back days for the first time since 1995.

The East

The heat peaked in the East on July 22 and 23, when numerous daily and all-time temperature records fell.

• Newark, NJ set a new all-time record high of 108 F on July 22, breaking the old record of 105 degrees, set on August 9, 2001.

• New York’s Central Park reached 104 F on July 22, which was the city’s second-hottest temperature on record.

• Atlantic City, NJ set a new daily record high of 105 degrees on July 22, making it the city’s second-hottest day on record. (This reading was tied the next day.)

• Washington Dulles International Airport broke it’s all-time record July 22, reaching 105 degrees.

• Baltimore Washington International Airport set it’s second highest temperature all-time July 22, reaching 106 degrees.

• Washington Reagan National Airport tied its all-time record high minimum temperature of 84 on July 23 and 24.

• Hartford, CT set a new all-time record high of 103 F on July 22.

• Bridgeport, CT tied its all-time record high of 103 F on July 22.

• The high of 103 F in Boston on July 22 was the city’s hottest day in exactly 85 years

• Raleigh-Durham North Carolina hit at least 100 for five consecutive days (July 20-24), breaking the old record of four.

A “ heat dome”

A map showing the unusual atmospheric circulation during the heat wave period and the strength of the blocking high. The bright colors indicate unusually high heights of the 500 mb pressure surface, which has been pushed upwards by the bubble of very warm air underneath. (NOAA ESRL)

The proximate cause of this heat wave was a huge ridge of high pressure at the upper levels of the atmosphere, which led to sinking air that inhibits rainfall and boosts air temperatures. The clockwise circulation around the High helped pump humid air northward from the Gulf of Mexico. The abnormally strong high pressure cell peaked in the central states on July 17 and 18, and then stretched eastward, moving the bubble of heat with it as the week wore on.

Are heat waves like this one part of the new normal?

In many ways, this heat wave exemplified the type of extreme heat events that climate science studies show are becoming more common in many parts of the world, likely due at least in part to manmade emissions of greenhouse gases, such as carbon dioxide. Studies of specific extreme heat events, such as the deadly 2003 European heat wave and last summer’s heat wave and wildfires in Russia, have demonstrated that they were also associated with unusually large, intense high-pressure systems.

One study, published in the Journal of Geophysical Research in 2008, stated:

The risk of hot summers is currently rapidly increasing, raising the likelihood of record breaking heat waves around the World, as seen in Europe in 2003 and 2006 and in North America in 1995 and 2006.

Climate projections for the rest of this century show that heat waves of similar magnitude to what we just experienced may become much more common over the course of this century, possibly occurring once or twice per year if emissions continue to increase at present rates. In fact, one study published earlier this year projected a dramatic shift towards permanently hotter summers, with more frequent and intense heat waves, during the next few decades (starting first in tropical regions), which is far sooner than was previously thought.

Another study raised the disturbing possibility that the limited human tolerance to heat stress constitutes an “upper limit” to our species’ ability to adapt to global climate change, and estimated that this limit might be reached if warming reached about 7 degrees Celsius above preindustrial levels, which is within the range of projections of many studies looking at how the climate will respond to greater amounts of greenhouse gases over the longer term.

The bottom line: Heat waves such as the one we’ve just experienced may soon become part of the typical American summer, and we’d be wise to start taking actions to ensure that we are prepared for that very real possibility.