What makes heatwaves more dangerous for urbanites

Built-up areas trap the heat and humidity, intensifying the threat to vulnerable city-dwellers like senior citizens
Turner Field vendors use bags of ice to stay cool during the heatwave in Atlanta
Turner Field vendors Demetrius Smith (left) and James Jernigan use bags of ice to stay cool while working at a baseball game in Atlanta, Georgia, 30 June 2012. Photograph: Tami Chappell/Reuters

Few are finding this heatwave over central and eastern US pleasant. Most, however, can take the record temperatures in their stride. Unfortunately, the elderly and other vulnerable members of our society suffer tremendously during heatwaves.

The heat has already claimed lives from Illinois to Maryland. The forecast for the next few days suggests that deaths attributable to the weather will mount up more quickly than in past days. Six key factors are involved.

National Weather Service heatmap July 2012
National Weather Service heatmap, 6 July 2012. Source: weather.gov

1. The heatwave is migrating into densely-populated areas

This map of the current heat advisories and warnings tells the story. While the heat in recent weeks has been unbearable in cities like Washington, DC, many of the highest temperatures have been recorded in the south and lower midwest. These areas are less populated than the major cities of the upper midwest and northern mid-Atlantic. Now, that heat has shifted to the upper midwest and northern mid-Atlantic cities of Chicago, New York, and Philadelphia.

2. Heat waves in well-populated areas are exacerbated by the "urban heat island" effect

Urban areas are almost always warmer than their surrounding suburbs because of the greater amount of asphalt, tall buildings, and lack of green areas.

One study of an intense 2008 heat wave in Oklahoma found that Oklahoma City's temperatures were, on average, 1F degree (Fahrenheit) warmer during the day and 4F warmer at night than surrounding areas. That may not seem like a lot, but heat stroke occurs when the body temperature rises to 105F, so the difference between 102F and 106F can be critical.

The built environment also causes moisture to be trapped within cities, which causes humidity levels to stay elevated. When high temperatures are combined with high dew points, you get high humidity and it feels warmer. The Oklahoma City study found that when you examine the heat index (a combination of temperature and dew point), cities feel 2 degrees warmer during the day and an astounding 9F warmer at night than rural areas. And the fact that it doesn't cool down at night means the heat is prevented from dissipating.

3. Heat waves in urban areas are worse in tall buildings

Warm air rises. People who live outside urban areas tend to dwell in houses; their size and profile helps keep them cooler. Most people in urban areas are not as fortunate. During the 1995 Chicago heatwave, it was estimated that the upper floors of high-rises without ventilation felt about 10-15F warmer than it felt out in the street.

4. Many senior citizens live in urban areas

Americans over 65 account for 40% of all heatwave-related deaths, three times their percentage in the overall population. One reason why older people are so susceptible to heatwaves is because of their concentration in urban areas. The senior citizen population of New York City is about 1 million, over 200,000 in Philadelphia, and 400,000 in Chicago. As the heatwave intensifies in these cities, it will disproportionately affect older people.

5. The duration of the heatwave

As evidenced by the excessively warm nights, a long heatwave builds up a reservoir of hot air. Apartment buildings will trap warm air for longer and longer periods, in an almost exponential fashion, the longer the heat wave goes on. The high temperature in Washington, DC, for instance, has not dropped below 95F since 28 June. Even if the heat relents at the beginning of next week, it will be days before many apartment buildings actually cool down.

6. Heatwave-related power outages

Because of "derecho" storms, power remains out to hundreds of thousands from Ohio to Washington, DC. Brownouts (that is, cuts in electicity due to a combination of high demand and a labor dispute) have occurred in both the Bronx and Brooklyn, New York. When power is limited or cut, fewer people will be able to run their air conditioners.


This heatwave is likely to get worse before it gets better for the major cities in the upper midwest and northern mid-Atlantic. People contending with the hot conditions should try to stay in air-conditioned facilities, stay hydrated, and avoid strenuous outdoor activities. For those affected by power outages, or who do not have air conditioning, news outlets and local government will be providing details of heat relief shelters.