Students

What is An Urban Heat Island? 

This effect occurs when cities replace natural vegetation and trees with impermeable surfaces, such as pavement, buildings, asphalt and other structures necessary to accommodate growing populations. The loss of trees and plants reduces the natural cooling effects provided by shade and evapotranspiration (the process of water evaporating from leaf surfaces). Asphalt and dark, impervious, manmade materials absorb, rather than reflect, the sun’s energy – and can be 50º to 70º hotter than other surfaces. The ambient temperature difference between urban centers and the exurban border can be as much as 10 F.

What are the Consequences? 

Thermal conductivity

• Remember that the Urban Heat Island Effect is a night phenomena. 
• The noctural heat normally dissipated during the night from conductive surfaces, like asphalt, is not sufficiently dispersed.

Hydraulic conductivity

• Impermeable surfaces mean less water is absorbed into the ground. 
• Water is instead gathered in runoff. 
• The volume and velocity of runoff contributes to erosion.

Albedo

• Albedo is the ratio of light reflected by a body to the light received by it. 
• Dark urban surfaces absorb and retain heat, rather than reflect and disperse it.

Emissions

• Ozone is produced through a thermochemical reaction between heat and greenhouse gases, such as carbon dioxide. 
• Highly urbanized population centers displace trees and plants for surfaces that increase temperatures. 
• As a result, urban centers use more energy to moderate living and work space temperatures. This increased energy consumption further increases temperatures through release of fossil fuel and greenhouse gases. 
  • The combination creates a smelting pot of ozone production in the ambient space (up to 10’ above the ground surface).

What are the Effects?

• Poor Air Quality
• Risks to Public Health
  • Risks can include: heat exhaustion, heat stroke, respiratory illnesses including asthma, targeting children, the elderly, the weak and the poor.
• Higher Energy Consumption
• Local Climate Changes 
  • Convection cloud layers expand and can cause (microclimates) of increased rainfall, with intensified storm events, including lightning. 
  • This is more pronounced in warmer, more variable climates such as we find in Florida.
• Biodiversity Shifts
  • Plant and animal colonies may have to migrate to seek their natural temperature ranges.

What Can You Do?

The principal formula for countering the negative effects of urbanization promotes:

• Green Roofs – roofs are the hottest urban surfaces, reaching as high as 160ºF.
• Switch to cool paving and building materials. Materials that are light-colored and reflective.
• Plant trees!
  • The United States carbon sink (forests, grasslands, and agricultural lands) absorb over 25% of the total U.S. global warming emissions. 
  • Trees and plants reduce temperatures, save energy consumption and costs, improve air quality, advance a healthier environment, avert excess ozone production and foster the biodiversity chain.