INTRODUCTION

The tools of remote sensing can be used to monitor land cover and the climatic environment of the Earth's surface.  It is also possible to use these tools to investigate the interaction between land cover and local climate.  This lecture will focus upon the "urban heat island" phenomenon as observed through the use of digital Landsat thematic mapper image data, and will address questions concerned with the link between surface land cover and local climate.  Exercise number one, of Volume 4, in the Remote Sensing Core Curriculum titled: "Identification of Urban Heat Islands Using Remotely Sensed Data: A Multi Sensor Approach" provides an extensive discussion of the urban heat island phenomenon.  In exercise number one, Dr. Kevin Gallo and Tim Owen have provided illustrations of observed urban temperatures as compared to surrounding rural landscapes.  In their examples, remotely sensed images from the one-kilometer resolution AVHRR imaging system are used.

For many years it has been reported that city centers are generally warmer than the surrounding countryside.  This idea was first observed using a few insitu thermograph observations placed at points within the metropolitan regions.  More recent use of coarse resolution thermal remotely sensed imagery (usually from satellite platforms) has shown that many cities are warmer near their center.  (See Exercise #1 of this applications volume, and figures 1 and 2.)  This pattern is generally true for cities in humid climatic regions, but not necessarily the case for cities in arid environments.

Moreover, this general "heat island" concept does not explain thermal variations within the city, and does not explain why the city is warmer.  It is incorrectly thought, and stated in many textbooks, that urban materials of concrete, steel and asphalt absorb and hold more of the sun's radiant energy.  In fact, city surfaces often reflect more energy away from the earth's surface, and they also emit greater amounts of terrestrial radiation.  Thus, the city actually has a much lower net radiant energy level than the humid rural landscape, and is holding much less energy.

The answer to why the city is warmer lies in how radiant energy is used.  Where moisture is available the majority of radiant energy will be used for evapotranspiration. Since most city centers have less biomass for evapotranspiration, and less open water, most of the radiant energy is used for heating the air and the ground in the city.  Air takes on the characteristics of the surface below.  Thus, city surface temperature controls the near-surface air temperature.  Environmental planning to provide greater moisture availability within the city can control neighborhood temperature to a significant level.  Works by Schmid and Oke (1992) and Hubble (1993) have shown that microclimatic spatial interaction at a sale of less that 200 meters is critical to explain urban thermal variations.  Fine resolution remotely sensed data are necessary to monitor and analyze these phenomena.  The coarse resolution "urban heat island" pattern is a valid generalization, but misses the important intricacies necessary to study and understand the environmental processes taking place within the city environment.

In this lecture we will look at cites in both humid and arid environments.  First, Rochester, New York will be viewed as an example of a city in a humid climate.  As a comparison, research conducted in the area of Phoenix, Arizona will be presented to illustrate the climate of a city in an arid climate.  Figures 1, 2, and 3 are Landsat 5 images of Rochester, New York.  Rochester is located just south of Lake Ontario, in the humid continental climate of the northeastern United States.  These images were obtained in the month of June.  Figure 1 is a color composite image where green vegetation is shown in shades of green, bare soil appears in shades of pink, water appears almost black, and urban pavement and rooftops appear in shades of dark blue.  Figure 2 is a thermal image of Rochester, New York.  Lighter tones in figure 2 represent warmer surface temperatures.  It can be seen that these images portray a city in a humid climate displaying the classic thermal patterns of the urban heat island, with hotter surface temperatures found towards the center of the city.  Figure number 3 is an image of the normalized difference vegetation index for Rochester, New York.  In general, brighter areas of figure 3 depict areas of greater biomass.  By comparing figures 1, 2, and 3, it can be seen that, for land areas, cooler temperatures are associated with regions having greater amounts of vegetation.

 
[FIG 1]                      [FIG 2]                       [FIG 3]

Urban centers in arid climates do not necessarily exhibit the classic pattern of an urban heat island, as discussed above.  Figures 4 and 5 show landsat images of the Phoenix, Arizona metropolitan region.  Figure 4 is a color composite image where green colors correspond to vegetated surfaces, while figure 5 shows the surface temperatures of this area.  It is interesting to note that the hottest regions are in the desert outside of the Phoenix metropolitan area, and the coolest surface temperatures are associated with areas of irrigated agriculture.  In Figure 4, beige tones correspond to barren landscape and desert; bluish tones represent pavement and urban impervious surfaces, while green depicts vegetation.

 
[FIG 4]                      [FIG 5]

In this lecture we'll look at the surficial climate of Phoenix, Arizona at verious spatial scales.  Figures 4 and 5 show the whole metropolitan area and its surroundings, while figures 6 and 7 show a smaller portion of the city, and environmental variation within this small area.  The standard "urban heat island" concept also does not work well when viewing specific portions of a city closely.  This lecture will utilize digital remotely sensed Landsat imagery focusing upon the metropolitan region of Phoenix, Arizona, and the suburb of Scottsdale, Arizona.  Some neighborhoods are much hotter than others.  This differential in temperature can be from a few degrees to as much as twenty degrees (deg. F).  Hotter neighborhood temperatures are not only more uncomfortable in summer months, but demand much greater amounts of electrical energy for air conditioning.  Figures 6 and 7 show Landsat images of Scottsdale, Arizona region, within the Phoenix metropolitan area.  Figure 6 shows a color composite image similar to that of Figure 4, while figure 7 shows patterns of thermal emittanace (i.e., surface temperature) similar to figure 5.   Past work has shown that much of the thermal variation of the city surface can be explained by the presence of moisture for evapotranspiration (Lougeay et. al., 1996).  The more evapotranspiration that takes place, the cooler the neighborhood.  Thus, irrigated parkland, open pools of water, and irrigated residential lawns and shrubbery reduce the energy demand.  Environmental managers can literally trade water for electricity by controlling the land cover characteristics of a neighborhood.  Currently Scottsdale, Arizona and the whole country of Singapore (Nichol, 1994) are using this information in their city planning .

 
[FIG 6]                      [FIG 7]

Images of Phoenix, AZ (figures 4, 5, 8, and 9) show a city which is actually cooler than the surrounding desert rural landscape.  The coolest regions in the Phoenix metropolitan area are those regions of irrigated agriculture or well irrigated residential land use.  Figure 8 displays surface temperatures for Phoenix.  Figure 9 is an image showing variations in biomass across the Phoenix metropolitan area, with brighter area representing greater biomass and rates of evapotranspiration.

 
[FIG 8]                      [FIG 9]

This lecture focuses upon the utilization of remotely sensed data provided by the Landsat thematic mapper thermal band, having a pixel resolution of 120 meters.  This is an environmental scale experienced by the people who live in these cities.  As stated above, there is interest, and a few active applications, in using these procedures of image analysis to control neighborhood thermal environments by planning for strategically placed green belts of irrigated landscape.

For further information concerning current research on the topic of urban climatology the reader may wish to consult the Urban Climate Network at the following Internet address: http://www.urbanclimate.net/