Weather Explainers

Urban Heat Islands: Why Cities are Warmer than Rural Areas

If you live in a rural area and drive into the city for work, you may have noticed a temperature rise as you get closer. There's a science to this temperature contrast, and it's known as the urban heat island effect.

An urban heat island is a metropolitan area that is significantly warmer than its surroundings. According to the EPA, the annual mean air temperature of a city with one million people or more can be 1.8 to 5.4 degrees Fahrenheit warmer than surrounding areas. In the evening, the difference can be as much as 22 degrees, as rural areas quickly cool down after sunset, while cities maintain their heat.

This temperature difference is generally greater at night than during the day, larger in winter than in summer and is most apparent when winds are light, according to UCAR.

(MORE: 100-Degree Days Are More Common Than You Think)

Low temperatures in and around New York City on the morning of July 19, 2016.

The image above is a clear representation of the urban heat island effect in New York City. Low temperatures on Tuesday were in the mid-70s at the major airports in and just outside the city, but it was about 10 degrees cooler just 30 miles to the west in northern New Jersey. Farther to the north and west, temperatures in the most rural areas of northern New Jersey and the Hudson Valley of New York were up to 15 degrees cooler than La Guardia Airport.

How Do Urban Heat Islands Form?

As vegetation in rural areas is gradually replaced by asphalt and concrete for roads, buildings and other structures closer to cities, urban heat islands develop. Asphalt and concrete absorb the sun's heat rather than reflect it, causing surface temperatures to rise.

Additionally, tall buildings and narrow streets trap the heat from the afternoon and reduce air flow, preventing the heat from escaping and keeping temperatures warmer overnight.

The urban heat island effect can also be enhanced by heat emitted by vehicles, factories and air conditioners.

(MORE: What Is the Heat Index and Why Is It Used?)

The temperature contrast from rural to urban areas is typically greatest during clear evenings with light winds. Rural areas cool off much faster at night than cities, as urban areas retain a lot of the heat stored in roads, buildings and other structures. In rural areas, the warmth escapes quickly into the atmosphere since those locations don't have the type of infrastructure found within cities to trap the heat.

As a result, the largest temperature contrast between rural and urban areas normally occurs between three and five hours after sunset, UCAR says.

MORE ON WEATHER.COM: Deadly India Heat Wave 2016