the Urban Heat Island Group at UT Austin
Civil, Architectural, and Environmental Engineering Department
So what is an urban heat island ? In a general sense, it's a phenomenon in which developed areas experience higher temperatures than less developed areas nearby.
For example, Austin probably experiences higher average temperatures than Lago Vista, a nearby town that's much smaller and less urbanized.
So what causes an urban heat island? As urban features like buildings, highways, and paving reduce or replace natural features, like bodies of water and vegetation, an area's temperatures rise compared to undeveloped areas.
Let's explore the causes of heat islands in more detail...Causes
Heat islands are caused by many variables. A few significant ones are
For example, with an increase in impervious cover , there's an associated decrease in vegetation. But impervious cover doesn't absorb water like vegetation does, leading to an increase in run-off water. Plus, while vegetation can cool air by evapotranspiration, impervious cover heats air!
Certain building materials that have a low albedo (or solar reflectivity) also contribute to heat island effects. Rather than reflecting solar radiation, a lot of building materials absorb it and then emit heat to their surroundings.
But wait - how do we know heat islands exist? Let's take a look at the data!Data
Heat islands can be studied using satellite imagery, which allows researchers to visualize and analyze the energy reflected and emitted from Earth's surface. Instruments on satellites can measure the thermal energy emitted from surfaces as wavelengths, creating thermal images, on which colors are associated with temperatures. By comparing thermal images of developed and undeveloped areas, the urban heat island effect can be quantified - simply take the difference between the temperatures of different areas and map the results.
One source of satellite images is NASA’s Moderate Resolution Imaging Spectroradiometer (MODIS). On board the Terra and Aqua satellites, MODIS is able to capture images of and data from the Earth’s entire surface every 1 to 2 days. In addition, MODIS can help with obtaining historical data, helping us to understand how urban heat islands have evolved.
After collecting relevant satellite images, information can be plotted on a coordinate system, like ArcGIS, to create a visual representation of where the peak temperature differential occurs. A heat map of the city can be created and a fundamental heat island can be visualized.
Now that we know a little about heat islands, why should we care about them?Impacts
Increased energy and peak energy demands, leading to brownouts.
More energy demand = more toxic pollution and carbon emissions.
Hotter cities can be uncomfortable and dangerous.
Hot runoff water harms aquatic life.
So heat islands have a lot of negative impacts on people and our environment.
Let's investigate how heat islands impact our health.Health
Urban heat islands cause both an increase in temperature as well as in air and water pollution. This results in major health issues in addition to general discomfort for urban residents.
Heat islands can contribute to respiratory issues, heat stroke, water-borne illnesses, and even mortality. The elderly and the ill are especially vulnerable sectors of the population. Low-income urban residents are also in a vulnerable position if they lack access to air conditioning, healthy buildings, and healthcare.
If you'd like to read more in-depth about heat islands' impact on mortality, we encourage you to look at this recent study on climate-mortality relationships in the US, or at this study specific to Washington, D.C.
In many ways, heat islands are a human issue.
So our next question has to be - what can we do about heat islands?Mitigation
Increased vegetation - looks good, reduces runoff, and can lower temperatures.
Use reflective building and paving materials.
Add shading to buildings and outdoor areas.
Reduce your energy use.
So there are strategies we can implement to mitigate the effects of heat islands. Of course, some strategies may be more effective for certain places - what's best for our city, Austin?Our Project
Our goal is to quantify and model the urban heat island effect in Austin.
Currently, we're conducting a comprehensive literature review and evaluating accessible sources of data.
As we progress, check back for updates.
If you'd like to get in touch with us, find our contact information here.Contact
If you'd like to know even more about UHIs, please look at the resources below.Resources