Urban Heat Island Research Project

Background

In this project, we investigate the urban heat island (UHI) of the Dallas-Forth Worth metropolitan area. The urban heat island is the characteristic warmth of urban areas compared to their (non-urbanized) surroundings (Voogt, 2003). This phenomenon was first observed by Luke Howard (Howard, 1833), who took detailed temperature measurements in London from 1801 to 1841. While heat islands vary diurnally, seasonally, and in magnitude in different regions of the world, they generally follow a basic structural pattern. The urban heat island peaks over the most densely urbanized areas, with a lower magnitude over suburban and rural areas and a slight shift downwind.

Heat islands are caused by urbanization, when buildings, roads and paved surfaces store the sensible heat during the day and then release it slowly during the evening, keeping urban lands hotter than surrounding areas. (Jusuf et. al, 2007). This results in a diurnal cycle in which the cities remain hotter after sunset, but take longer than the rural areas to heat during the day, sometimes resulting in a "cool island" around mid-day.

Factors affecting the urban heat island intensity

• Anthropogenic heat from air conditioning, cars, etc. increases the city temperature.
  
• Closely spaced buildings, building materials and other urban surfaces (asphault, concrete, etc) store more heat during the day.
  
• High wind speeds reduce the heat island, while low wind speeds or stagnant conditions allow for a greater heat island.
  
• High albedo from cloud cover, snow cover, surface materials, etc. reduces the heat island by reflecting incoming radiation.
  
• Drought conditions and low soil moisture increase the magnitude of the heat island.
  

While there are many factors that affect the formation and intensity of the urban heat island that can vary from city to city, the urban surfaces have the most dramatic influence on its magnitude. This is due to the greater daytime heat storage of urban areas due to the thermal conductivity of building and construction materials (Winguth and Clark, 2013). Anthropogenic heat release from building sides, traffic, air-conditioning during summer, and other electrical consumption (Sailor and Lu, 2004) also increases the urban heat island.

The Dallas-Fort Worth urban heat island

Hourly data collected from air Continual Ambient Monitoring Stations (CAMS) implemented by the Texas Commission for Environmental Quality (TCEQ) were used in the Winguth and Clark (submitted) study. The magnitude of the heat island is calculated from the temperature difference between the urban station (downtown Dallas, CAMS 60) and the rural station (Kaufman, CAMS 71), which is located about 58 km downwind of the Dallas station (Winguth and Clark, submitted). The heat island is known to exhibit its greatest magnitude at night especially during light winds and clear skies (Figuerola and Mazzeo, 1998). Using statistical analysis, the difference between the urban and rural stations was found to be greatest at 9pm, so this was the hour used to further analyze the spatial properties of the Dallas-Fort Worth urban heat island.

In July 2011, there was a ~3.4 °C total change between Kaufman (CAMS 71) and downtown Dallas (CAMS 60), with light south to southeast winds of about 1.9 m/s which advect the warm temperatures northward. This was compared to satellite imagery at the end of July 2011 from the from MODIS/Terra satellite. Land cover data using vegetation types from the International Biosphere Geosphere Program (top figure below) coupled with nighttime surface temperature data (bottom figure below) support the northward transport of the heat island (Winguth and Clark, 2013).

References Cited

1. Winguth, A.M.E., and B. Kelp, 2013. The Urban Heat Island of the North-Central Texas Region and Its Relation to the 2011 Severe Texas Drought. J. of Applied Meterology and Climatology, 52, 2418-2433.
   doi: 10.1175/JAMC-D-12-0195.1