The term Urban Heat Island gets its name from industrial and urban development that causes the air temperature within the area to be much warmer than its surrounding rural areas. Unlike surrounding rural areas, an urban area lacks vegetation and moist soil. During the summer, the heat radiating off of buildings can be felt more intensely due to a more direct angle from the sun.
In a large city or urban area that is largely congested with buildings and man made materials, you may find yourself really feeling the heat when it starts to warm up for the summer. Warm asphalt roads, tall steel skyscrapers, and cement buildings absorb and hold onto the incoming energy (shortwave radiation) from the sun. They then heat up to a point where they radiate their own heat (longwave radiation) throughout the day and overnight. With tall buildings blocking most of the air passing through the city, this heat gets trapped, unable to filter out as efficiently as it would with less buildings. This congestion of buildings is not the only thing adding to the heat. Running car engines and exhaust pipes give off heat as well as air conditioners, refrigerators and factory machines. Combined, these continue to contribute to the heat surrounding the urban area. Eventually, as the day persists, the pocket of air within the urban area grows significantly warmer than the air outside of that urban development.
A very large difference between an urban heat island and a rural area is the lack of vegetation and moist soil that covers the surface. For an urban heat island, it is mostly asphalt and roads as opposed to soil, grass and vegetation. The soil and vegetation play a vital role in keeping a rural area cool. One of the main contributions to this is the extra moisture the plants and soil release or have stored. Incoming solar radiation is used as energy to evaporate the water or moisture that is in the soil or was absorbed by the vegetation. During evaporation, latent heat, which is the energy used to transform water from a liquid to a vapor, is absorbed. The absorption of latent heat decreases the air temperature surrounding the water vapor. Therefore, leading to a cooler atmosphere.
As we move into summer, it's important to think about how living within a congested city can add to the temperature you are feeling outside. It could help a bit to escape to a rural area and cool off without the urban heat adding to the daily temperature.
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© 2019 Meteorologist Alex Maynard