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Thermal Islands: Cycle B

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1). Cities are warmer than surrounding countryside. Ait temperature in a city of one million people can be up to 3 decrees C warmer during the day and as much as 12 degrees C warmer at night.

2). Human influence has made cities warmer by adding buildings, impermeable surfaces, and concrete that both produce additional heat and trap heat in the urban environment.

3). Thermal islands have downstream effects. This frequently takes the form of increased convective weather activity like thunderstorms and rain showers.

4). Urban heat islands prolong the growing season in cities. This affect diminishes away from the urban center but can affect areas as far out as 2.4 times the size of the city itself.

5). The surface urban heat island can be very pronounced during the day when temperatures of exposed surfaces like roofs and pavements can be up to 50 degrees C (90 degrees F) warmer than the surrounding air.

6). The atmospheric urban heat island tends to be weak from late morning throughout the day. It intensifies at night as heat is released from buildings and surfaces.

7). Excessive heat events (heat waves) are enhanced in urban areas resulting in increased mortality and morbidity. In the U.S. over 1,800 deaths per year may be due to urban-enhanced excessive heat events

8). Evapotranspiration is the process by which water is moved from soil through plants then is evaporated into the air. The evaporation process removes heat from the air.

9). In urban areas, surfaces that were once permeable and moist are now impermeable and dry. This adds to the build up of heat in urban areas where usually 75% or more of the surface is impermeable,allowing for much reduced evapotranspiration.

10). Whenever water undergoes a change in phase, heat is either released or absorbed.
The latent heat of condensation is the amount of heat energy released when
water goes from the vapor to the liquid phase. Conversely, the latent heat of vaporization is the amount of heat absorbed from the environment when water evaporates.

11). The albedo of a surface is the percent of incoming solar radiation that is reflected from the surface. The albedo of the Earth-atmosphere system as a whole is about 30%.

12). Specific heat capacity is the amount of energy required to raise the temperature of 1 gram of a substance 1 degree Celsius. For water, it takes one calorie (4.18 joules). For air only about 0.25 caloties(~ 1 joule) is needed


Scenario: A heat wave in Chicago, increased thunderstorm activity in Quincy Illinois, and fogless London days and nights – is it possible these are all related to thermal islands? What is the role of cities in our climate – and more specifically, how does the urban heat island affect climate – not only in cities but in the surrounding countryside?

Numerous studies have shown how the concrete pavements and buildings retain heat in cities, making cities several degrees warmer than the surrounding countryside. The research of Tim Oke from the University of British Columbia has shown that cities of a million people can be 1 to 3 degrees Celsius warmer than the surrounding countryside during the day and as much as 12 degrees Celsius warmer at night.

Increased morbidity and mortality rates in cities during heat waves (sometimes referred to as Excessive Heat Events or EHEs) are exacerbated by the urban heat island effect. For this and other reasons, many believe mitigation of urban heat islands should be pursued. Some strategies being recommended include increasing trees and vegetation, and developing roofs that are green and/or cool.

As cities have grown, they have warmed. One result has been a decrease in fog. London, for example, used to be known for its "pea soup" fogs, but today, dense fog is rare in the city. New York, Tokyo and Los Angeles show similar trends. According to a November, 2005 article in Nature, changes in land cover in both cities and the countryside is responsible for part of the warming the United States has experienced in the past century.

In a 2003 paper in the Journal of Applied Meteorology , Rozoff, Cotton and Adegoke demonstrated how the urban heat island of St. Louis enhanced convective activity (thunderstorms) downstream of the city.

Not all the consequences of an urban heat island are negative. For example, savings in winter heating costs, less ice and snow, and longer growing seasons in urban areas are all positive results.


Author: Michael Witiw, Seattle Pacific University



Date: 1/22/2010


Scenario Images

Temperature profile in the vicinity of an urban heat island
This image shows how both nocturnal and daylight temperatures vary in the vicinity of an urban heat island, and the fact that they have a different magnitude, especially in the daytime. More... Image: Courtesy: EPA (modified after Voogt, J.A., 2002: Urban heat island, in Vol. 3, Encyclopedia of Global Environmental Change, Ed. Ted Munn, John Wiley & Sons, Ltd, Chichester, 660-666)

Contributors to thermal islands
Buildings, asphalt, concrete, and industry all contribute to the Urban Heat Island by their uptake and subsequent release of heat, and, in the case of industry, by adding heat to the atmosphere. More... Image: Courtesy NASA Earth Observatory



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Team Assignment
<Event Team Study

This cycle you and your Event Team will build on the sphere to sphere interactions you identified last cycle to synthesize them into causal chains or causal matrices. You will use the resources listed under Readings, discuss your ideas in the event team discussion space, and then submit your team's work to ESSEA for a grade.



  • Read about the scenario.
  • Review the Team Event Study Rubric.
  • Serve as a "Sphere Expert" to your Event Team.
  • Read your teammates' summaries about the other individual sphere effects.
  • Identify intersecting and overlapping effects in your event team discussion space. Refer to An Example of an ESS Analysis reading if you would like to review causal relationships.
Upload to ESSEA your team's most accurate analysis of the Earth System interactions with reasoning and support and complete the team rubric.
Deadline: Friday, March 17 2017 11:59 PM (Eastern Time)
Upload Assignments

What's hot at the mall? (Cycle B)
Shows how shopping malls change the natural environment trough deforeststion and contribution to the urban heat island
Difficulty: beginner

Why is the city hot? (Cycle B)
Examines the formation of urban heat islands with Atlanta as an example using NASA data.
Difficulty: beginner

Air Pollution Prevention Through Urban Heat Island Mitigation: (Cycle B)
Describes the relation between air polution and heat island in the urban environment. Also looks at mitigation and remote sensing of the urban environment.

Heat Island Group (Cycle B)
Describes the urban heat island including effects on air quality and energy use. Includes links to mitigation strategies including vegetation , cool roofs, and cool pavements.

LAUNCH (Cycle B)
LAUNCH is a joint NASA, USAID and Nike program looking toward sustainable agriculture. Vertical hydroponic farming is among the projects described.

Mitigating New York City's heat island with urban forestry, living roofs, and light surfaces (Cycle B)
This conference paper outlines a strategy for New York City and estimates the various mitigation effects.

NASA Satellite Confirms Urban Heat Islands Increase Rainfall Around Cities (Cycle B)
Describes how urban heat islands enhace precipitation (especially summer precipitation) downwind of a city.

Reducing the Urban Heat Island: Compendium of strategies (Cycle B)
Contains an overall review of urban heat islands and some suggested mitigation strategies.

Urban climatology and air quality (Cycle B)
Although this site has not been updated in a while, it contains good basic information and links.

Urban heat islands make cities greener. (Cycle B)
Shows how urban heat islands prolong the growing season and make cities greener.

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