Why Some Cities Get Way Hotter Than Others During The Summer

Summer in the city can be brutal, but just how bad it gets depends on the city itself. Most cities are hotter than their surroundings, but some are more susceptible to this “heat island” effect than others. A new study, released July 9 in the journal Nature, has pinpointed why — and which cities are the most at risk.

For example, New Orleans is about 10 degrees F hotter than its surroundings, but in Los Angeles this difference is less than 6 degrees F. It turns out, this effect is worsened in wet areas because of the different kinds of plants that live around those cities, according to the study.

Half of the world’s population lives in urban areas. Unfortunately, these cities are causing “one of the most significant human-induced changes to Earth’s surface climate,” the Urban Heat Island (UHI) effect, according to the study.

Below you can see what a heat island looks like for a generic city:

The study looked at the annual mean average temperature of 65 locations from 2003 to 2012 for both the city and the surrounding rural area at 1:30 p.m. and 1:30 a.m. local time.

Their findings showed that how much and what kind of vegetation was present in the rural areas — which is greatly influenced by precipitation rates — played a large role in the heat island effect. “If you look across the North American continent, in places with higher precipitation, you get stronger daytime UHI,” study researcher Xuhui Lee, of Yale University, told Business Insider in an email.

When climates are naturally wet, forests act as natural ‘heat converters’ moving heat from the ground into the atmosphere, Lee said. “In comparison, cities in these climates are not as efficient in convecting heat — our estimate is that they are 58% less efficient than the natural landscape — and hence we have strong urban heat island,” he said.

You can see how your city stacks up below. The “daytime ΔT” refers to how much warmer the urban areas are compared to the surrounding rural areas in Celsius (1 degree C = 1.8 degrees F).

The worst offenders

You can see those cities on the map below. The size of the dot shows the temperature difference between the city and surrounding areas (the numbers in the legend are in Celsius, 1 degree C = 1.8 degrees F). The colour of the dot denotes the amount of annual mean precipitation. Red is less than about 20 inches of precipitation. Green is between 20 and about 43 inches. Blue is over 43 inches.

You can see the blue dots are the biggest dots:

Cities in the southeastern U.S. tended to show the greatest heat island effect with an average 7 degrees F daytime and 4 degrees F nighttime greater temperature than surrounding areas, Lee said. The nighttime urban heat island effect temperatures are due to the release of heat stored in buildings, which absorb some of the heat during the day.

Here are those same cities at night:

“Cities in the semi-arid climates, such as Tucson, Arizona and Reno, Nevada, have the lowest urban heat island,” Lee said. This is because low-lying vegetation, like shrubs and grasses, aren’t good at removing heat from the ground. “In comparison, urban landscape is on average 20% more efficient in heat convection,” Lee said (Still the surrounding landscape remains cooler since other factors are at play such as the increased reflectivity of the landscape and water evaporation by trees).

Large cities also naturally have a greater heat island effect. “In a large city, heat has more time to build up,” Lee said.

The effect can change from year to year with different precipitation rates. “In the humid southeastern U.S., urban heat island tends to be stronger in the drier (and hotter) years than in the wetter years,” Lee said. This could be due to the fact that urban trees help cool the landscape through water evaporation, an effect which the researchers think is not as strong within the city in drier years as it is outside, he said.

So what can cities do?

Lee suggests they use building materials that reflect heat away from the city rather than absorb it. “A good case in point is Chicago,” Lee said.

The windy city experienced a record heat wave in 1995 which killed hundreds of people. In response, the city changed its building codes to encourage the use of reflective roofs yielding a cooler city.

While urban greening can help with cooling, it has to be done right to be effective. The popular grassy green roofs, for example, don’t to much for heat mitigation, Lee said. However, previous studies have shown that green areas, like tree-speckled parks, can help keep city dwellers cool. This is because “Trees tend to offer more cooling power than shrubs and grasses,” Lee said.

“Light coloured roofs also offer cooling benefits,” he said. In a previous study, Lee and a team compared the cooling power of street trees versus reflective roofs, with the latter taking the win.

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