Why Big Cities Like Tehran Tend to Be Warmer Than Rural Areas?

According to SAEDNEWS, Here’s a compelling reason why urban residents escape to the countryside during the sweltering summer months: rural areas typically experience cooler temperatures.

Due to the "urban heat island" effect, cities with populations of one million or more can be 1–3°C (1.8–5.4°F) warmer on average than their surrounding areas—and even up to 12°C (22°F) warmer at night, as reported by the U.S. Environmental Protection Agency.

Several factors contribute to the heightened temperatures in cities. The concentration of people, buildings, and machinery generates heat. For example, a recent study from Arizona State University found that air conditioning can raise surrounding temperatures by over 1°C. Urban structures tend to absorb more heat during the day than vegetation, later releasing it at night, which contributes to elevated nighttime temperatures. Cities also have lower reflectivity, allowing them to absorb more of the Sun’s heat.

While many researchers have pointed to the loss of vegetation and the resulting reduction in evaporation as the primary cause of the urban heat island effect, a new study suggests that local climate differences may be more significant. A team led by Lei Zhao from Nanjing University of Information Science and Technology in China published this finding in Nature.

The researchers began by exploring whether cities in different climates experienced the same temperature increases due to urban heat islands. Using NASA satellite data from 65 U.S. cities, they identified trends indicating that larger cities tend to have a greater temperature disparity with surrounding rural areas at midnight, while during midday, wetter cities display a more pronounced heat island effect.

To further investigate, the researchers developed a computer model to assess various factors impacting the urban heat island effect, including human-generated heat, heat retention in cities, land characteristics, and convection—the movement of heat from urban or rural land into the atmosphere.

Their findings revealed that convection and local climate are crucial. In humid regions, such as the U.S. East Coast, "convection is less efficient at dissipating heat from urban land compared to rural land," leading to cities being about 3°C warmer than nearby countryside. In contrast, in arid areas like Las Vegas, the opposite is true; urban regions are approximately 20% more effective at dissipating heat through convection than rural zones, leading some of these cities to avoid the urban heat island effect.

Regardless of climate, at night, the heat absorbed during the day contributes to the heat island effect, as this warmth accumulates in the shallow atmospheric layer near the surface and must disperse horizontally. Larger cities experience this phenomenon more intensely, resulting in significantly warmer conditions at their centers compared to smaller municipalities.

The urban heat island effect poses more than just an inconvenience; it can worsen health issues related to heat stress, particularly during hot, dry years in typically humid areas. This new research sheds light on potential measures to mitigate the problem.

While cutting down on heat generated by air conditioning and machinery may seem like an obvious solution, the model indicates that anthropogenic heat is not the primary contributor, making this approach less effective. Instead, addressing convection efficiency and heat retention would require extensive changes to urban design, which is not practical.

An effective strategy would be increasing the reflectiveness of urban surfaces. This enhancement would reduce heat absorption during the day and lessen the heat available for release at night. Simple measures, such as painting flat city roofs white or using other reflective materials, can make a significant impact.

Additionally, reflective roofs can help lower energy consumption by reducing the demand for air conditioning, which in turn decreases carbon emissions—a benefit that extends beyond just reducing the urban heat island effect.