Past studies of urban heat islands — metropolitan areas with significantly higher temperatures than surrounding lands — have relied on data collected from satellites. Starting a few years ago, a new approach where citizen scientists are using inexpensive thermal monitors attached to their vehicles and following zigzagging city routes — once in the morning, once mid-afternoon, and again in the early evening — is collecting data that reveals a more complicated and varied picture of the heat island effect.
The 24 cities studied using this on-the-ground approach are showing 15-20 degree Fahrenheit differences within a city. Understanding what is causing the neighborhoods with the highest temperatures is important because heat is the number-one weather-related killer. The 115°F days in southern France last summer killed 1,500 people. In 2010, 11,000 people died in Moscow from extreme heat. Death can come directly from heat stroke, or indirectly because of escalating risk of heart attack or stroke.
These more detailed studies have uncovered six factors that impact temperatures at or close to the ground. Three are living: the size of tree canopy, the height of the tree overstory, and ground-level vegetation. Three are human-made: volume of buildings, variety in building heights, and color of buildings. Because tall buildings cast shadows, afternoon temperatures are frequently lower than they would otherwise be. On the other hand, a collection of low-rise box stores generates hotter afternoon temperatures. Increasing differences in the height of buildings in a grouped area creates more air currents, and thus, a cooling effect. A surprise finding was that a large expanse of water or grass that is infrequently watered can be almost as hot as concrete or asphalt.
Collecting data on street-level temperatures reveals a complicated patchwork of hot spots and cool spots, largely determined by urban design. Analyzing the data has led to creating heat maps and vulnerability maps. In most of the cities studied, the hottest neighborhoods are where lower-income people often live and work. The explanation is that these areas are largely lacking trees but have a predominance of gray and black paving.
Creating cooler cities doesn’t necessarily mean building less densely. What matters is varying building heights, increasing the tree canopy, limiting sunbaked parking lots, encouraging vegetated roofs on commercial buildings, narrowing streets or whitewashing black asphalt surfaces, and requiring landscaped areas along streets and around buildings. Understandably, our cities have developed without attention to the increasing incidence of heat waves linked to climate change, but now, with more sophisticated data and understanding, we can design our urban spaces more thoughtfully and equitably.