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HUNTSVILLE, Ala. (WHNT) — As Tuesday night temperatures dip close to record lows across the Tennessee Valley, downtown Huntsville will remain a little hotter, thanks to a phenomenon known as the Urban Heat Island effect.

WHNT News 19 hit the road in Storm Hunter 19 Tuesday evening to measure the temperature difference from downtown Huntsville to areas in rural Madison County.

Here are some fun facts about the effect, provided by the Environmental Protection Agency.

What Is an Urban Heat Island?

As urban areas develop, changes occur in their landscape. Buildings, roads, and other infrastructure replace open land and vegetation. Surfaces that were once permeable and moist become impermeable and dry.  These changes cause urban regions to become warmer than their rural surroundings, forming an “island” of higher temperatures in the landscape, according to the US Environmental Protection Agency website. 

Heat islands occur on the surface and in the atmosphere. On a hot, sunny summer day, the sun can heat dry, exposed urban surfaces, such as roofs and pavement, to temperatures 50–90°F  hotter than the air,  while shaded or moist surfaces—often in more rural surroundings—remain close to air temperatures. Surface urban heat islands are typically present day and night, but tend to be strongest during the day when the sun is shining.

In contrast, atmospheric urban heat islands are often weak during the late morning and throughout the day and become more pronounced after sunset due to the slow release of heat from urban infrastructure. The annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F  warmer than its surroundings On a clear, calm night, however, the temperature difference can be as much as 22°F

Surface and atmospheric temperatures vary over different land use areas. Surface temperatures vary more than air temperatures during the day, but they both are fairly similar at night. The dip and spike in surface temperatures over the pond show how water maintains a fairly constant temperature day and night, due to its high heat capacity.

Why Do We Care About Heat Islands?

Elevated temperature from urban heat islands, particularly during the summer, can affect a community’s environment and quality of life. While some heat island impacts seem positive, such as lengthening the plant-growing season, most impacts are negative and include:

  • Increased energy consumption: Higher temperatures in summer increase energy demand for cooling and add pressure to the electricity grid during peak periods of demand. One study estimates that the heat island effect is responsible for 5–10% of peak electricity demand for cooling buildings in cities.3
  • Elevated emissions of air pollutants and greenhouse gases: Increasing energy demand generally results in greater emissions of air pollutants and greenhouse gas emissions from power plants. Higher air temperatures also promote the formation of ground-level ozone.
  • Compromised human health and comfort: Warmer days and nights, along with higher air pollution levels, can contribute to general discomfort, respiratory difficulties, heat cramps and exhaustion, non-fatal heat stroke, and heat-related mortality.
  • Impaired water quality: Hot pavement and rooftop surfaces transfer their excess heat to stormwater, which then drains into storm sewers and raises water temperatures as it is released into streams, rivers, ponds, and lakes. Rapid temperature changes can be stressful to aquatic ecosystems.

What Can Be Done?

Communities can take a number of steps to reduce the heat island effect, using four main strategies:

Typically heat island mitigation is part of a community’s energy, air quality, water, or sustainability effort. Activities to reduce heat islands range from voluntary initiatives, such as cool pavement demonstration projects, to policy actions, such as requiring cool roofs via building codes. Most mitigation activities have multiple benefits, including cleaner air, improved human health and comfort, reduced energy costs, and lower greenhouse gas emissions.

For more information on heat island mitigation strategies and activities: