Module Learning Objectives

To use some data to characterize the magnitude of the UHI phenomena.

To understand the energy balance and energy flows at the urban/rural interface

To develop mitigation strategies

Components of the Urban Heat Island

Clearly, an Urban Heat Island (UHI) occurs as a result of converting green space into asphalt/concrete. The absorption and emission profiles of asphalt/concrete or substantially different than green space and/or soil.

In particular, asphalt/concrete have relatively low specific heats but relatively high thermal conductivity. This physics means that concrete/asphalt can absorb heat and increase their temperature relatively fast and once they have become heated, they can effectively radiate that excess heat to the atmosphere for some time.

Because it takes a few hours for this material to absorb the incident sunshine, the temperature contrast between the urban and rural areas usually maximizes in the late afternoon. The actual value of the temperature contrast varies as a function of concentration of materials on the landscape. This is summarized in the Figure below:

Figure 1: Cross Sectional Profile of a typical summertime UHI

The 7 degree maximum temperature differential shown above is the typical average contrast seen in well studied urban areas such as Atlanta GA, Washington DC or London England.

Measuring the amplitude of the UHI

Characterization of the amplitude of the UHI effect is done by making daily temperature measurements between the urban and rural areas. The data sets below are daily maximum (high) and daily minimum (low) temperatures for December, January and February (representing winter) and June, July and August (representing summer) for two years for downtown Wilkes-Barre, PA, and the nearby, less urban Wilkes-Barre/Scranton International Airport:

• December 1995, January 1996, February 1996
• December 1996, January 1997, February 1997
• June 1996, July 1996, August 1996
• June 1997, July 1997, August 1997

Second Student Exercise: Using the data links above, before the following analysis: Determine the average difference in winter and summer time high temperatures between the Urban (downtown) and Rural (airport locations). You will want to average all of the daily data to do this. Determine the average difference in winter and summer low temperatures between Urban an Rural Areas. Determine if there is a correlation between the temperature difference and the actual rural temperature itself. Based on this analysis, under which scenario (e.g. summertime high, wintertime low, etc) is the UHI effect the strongest? Does the actual UHI effect (e.g. the temperature difference) dependent on ambient rural temperature or does the UHI represent a constant offset independent of actual air temperature? If its the former, suggest a reason. If its the latter, suggest a reason why the UHI effect would be temperature independent.

Scaling the UHI with Population Increases

Now that you have used data to determine the amplitude of the UHI for a specific location, it is of interest to examine scaling behavior. That is, how much does the temperature differential increase with the size or area of the city.

The data below refer to some averages for cities in europe.



Third Student Exercise: Using the data shown above, predict what the temperature differential would be for a city size of 20,000,000 and, god forbid, 100,000,000. Is this relation linear or non-linear? That is, if you double the size of the city, does the temperature difference also double? What factors do you think are limiting the overall amplitude of the temperature differential?

Clearly the relation between city size and maximum temperature difference is complex and depends upon the landscape the city is set against. For instance, a large metropolitan area in a desert, probably has a smaller signature than a similar size metropolis set in a cow pasture.

The data below, show the trends in the UHI effect, in units of degrees C or F per decade, for different urban environments for which sufficient data exists to define a trend. Note that this data is averaged annually and does not refer to the maximum tempearture differential as we have used previously. Thus, for instance, Los Angeles shows a growth rate of .8 degrees F per decade meaning that every 10 years, the average annual temperature in the LA metro area increases by .8 degrees relative to the average annual temperature in the surrounding (rural) environment.



Fourth Student Exercise: Suggest a set of reasons why the growth of the UHI effect is so much larger in Los Angeles and San Diego than in San Francisco or Baltimore.




Pima County Arizona, where Tucson is located, is a rapidly sprawling urban disaster in an arid, naturally hot climate. The figure below shows the very good correlation between average annual temperature and the county population. In this case, its actual average temperature which plotted and not tempearture differential between the rural and urban environments. That part is coming next.



Firth Student Exercise: Using the above graph, determine what the average mean temperature will be for Pima county when it reaches population levels of 1.6 million and 3.2 million. Comment on the reliabilty of your extrapolations. Do you think some other factors might come in to play to reduce the amplitude of the future UHI effect as population in the area increases?

The figure below shows the period of 1969-1998 which compares the average annual temperature between the urban areas of Pima county (e.g. Tucson) and the surrounding rural area (which is mosly inhabited by cacti and prairie dogs).



Sixth Student Exercise: Using the above trends which have been fit to the data, estimate the rural/urban temperature difference in the years 2030 and 2060? Comment on the possible effect this might have on urban weather in this area.

Lastly, the image below as constructed using data from small towns in Austrailia. This study is unique as its the only one that has studied small scale systems. The formula that shown as the fit to the data is a non-linear formula.



Seventh Student Exercise: Using the formula, determine the maximum urban/rural temperature differences for population sizes of 100,000; 500,000; 2,000,000 and 10,000,000. Compare those estimates to the ones you did earlier from different data sets. Based on that comparison, do you think the UHI as defined on small scales, accurately connects to the large scale population centers. If so, why might this be the case. If not, why don't small population centers scale up to large ones in their UHI behavior?

A Changed Energy Balance

In the broadest physical sense, the alteration from green space/soil to a more concrete and asphalt environment changes the heat flow at the macrolevel.

In particular, new pathways for heat flow now exist. To better understand these pathways we need to introduce some flow terms.

• Q_I = Incident (direct and diffuse) solar radiation. Urbanization obviously does not change this.

• Q_R = Reflected solar radiation. The ability for a surface to reflect incoming solar radiation is known as an Albedo. Paving over green space clearly changes the Albedo.

• Q_L = Surface emission of long-wave length radiation. This depends on the heat retention/capacity of the surface material. Most of this radiation is directed upwards but that radiation is absorbed by the atmosphere (greenhouse gases) and some fraction of that is re-radiated downwards.

• Q_F = Anthropogenic energy such as from industry, transportation, heating and AC

• Q_E = Latent heat from evaporation of water from trees, soil, bodies of water, etc.

• Q_H = Sensible heat carried by vertical and horizontal air motion, including wind. The creation of urban wind alleys has an influence here.

• Q_S = Storage heat flux within the system (ground, buildings, etc.). This can best be thought of as material that absorbs heat in the daytime and then releases that heat slowly over night time hours. Concrete has material properties that maximize this process.

We can define two Net energy gain/loss terms as well:

• Q* = Q_i - Q_R = net solar energy (can be positive or zero)

• QL = the difference between upward directed and downward directed long wavelengh radiation. This is generally zero, or could be slightly negative (when its clear and the ground more effectively radiates away heat).


In the Figure below we show these various heat pathways for a typical unaltered rural landscape:


First Student Exercise: Calculate Q* and QL for the case shown above Comment on the reasons that Q* + QL do not equal zero. That is, where is this excess heat temporarily stored and how is it re-radiated. in this area.

Now let's exam the same Q's in the case of incoming solar radiation that is interacting with an urban environment.



Second Student Exercise: Calculate Q* for the urban environment and compare it with what you did previously. Comment on the reasons that it is so different. Calculate QL for the urban environment and compare it with what you did previously. Again, what are the reasons for this difference? Now compare the sum Q* + QL for the urban and rural environment. You should find them to be similar. If so, what change in the energy balance does this reflect? Explain why Qs is significantly larger in the urban environment than in the rural environment and explain why Qe is significantly lower.

Mitigation Strategies

By now you have examined various data streams that portray a pretty consistent picture. That is, the conversion of green space to asphalt/buildings produces a significant micro climate whose impace on the local environment scales (perhaps drastically) with population.

Given this dilemma, mitigation strategies now require serious consideration, particularly because in the summer, the UHI phenomenon will drive up the urban temperature which increases the demand for Air Conditioning. Thus mitigation, which reduces air conditioning needs, ultimately may save dollars. Saving dollars is always good incentive.

There are basically only two forms of sensible mitigation to consider:

Strategies that increase the overall reflectivity of the urban environment

Strategies that incorporate more green space in the urban environment

In what follows, you will be asked to critically think about both of these strategies in order to obtain some optimal balance.

Cool Roofs:






Student Exercise: Research and report on what kinds of materials can be best used to increase roof reflectivity and how practical (or not) this investment would be. In addition to roofs, what other kind of surface in the urban environment could be modified to have higher solar reflectivity? Consult this case study for some ideas as well as this study that shows the human health benefit of mitigation for the case of Philadelphia.



Increasing urban vegetative cover:

As seen below, trees offer a multifaceted defense against some of the impacts of urbanization. Logically, we should simply plant more trees in cities, but there are significant issues of practicality and significance.

There are numerous case studies of the utility of Urban Forestry . Below are references to two well known case studies.
• Urban Forestry

• European Case Study

Student Exercise: Read those two case studies and prepare a report on the effectiveness of urban forestry. Using the relations you worked with earlier between UHI impact and population, argue that there is an optimal size city where increasing vegetative cover would make a significant difference and that there also is some threshold population, above which its impractical to pusure this strategy