The "Expanding Bull's-Eye Effect"

A conceptual model of the “expanding bull’s-eye effect” for a hypothetical metropolitan region that is characterized by increasing development spreading from an urban core over time. A sample tornado scenario is overlaid to show how expanding development creates larger areas of potential impacts from hazards. A series of figures representing this conceptual model are available for download here.


What is the expanding bull's eye effect? “Targets”—i.e., humans and their possessions—of geophysical hazards are enlarging as populations grow and spread. It is not solely the population magnitude that is important in creating disaster potential, it is how the population and built environment are distributed across the landscape that defines how the fundamental components of risk and vulnerability are realized in a disaster. 

Why is it important? While climate change may amplify the risk of certain hazards, the root cause of escalating disasters is not necessarily event frequency, or risk, related.  Rather, Ashley et al. (2014) confirm that the upward trend in disasters is predicated on increasing exposure and vulnerability of populations. 



Ashley, W. S., S. Strader, T. Rosencrants, and A. J. Krmenec, 2014: Spatiotemporal changes in tornado hazard exposure: The case of the expanding bull's eye effect in Chicago, IL. Weather, Climate, and Society, 6, 175-193. Also, see the summary in the April 2014 Bulletin of the American Meteorological Society's "Paper of Note".

Strader, S. M., and W. S. Ashley, 2015: The expanding bull's-eye effect. Weatherwise, 68, 23-29.

Ashley, W. S., and S. M. Strader, 2016: Recipe for disaster: How the dynamic ingredients of risk and exposure are changing the tornado disaster landscape. Bulletin of the American Meteorological Society, 97, 767-786.

Other research exploring this topic:

Rosencrants, T., and W. S. Ashley, 2015: Spatiotemporal analysis of tornado exposure in five U.S. metropolitan areas. Natural Hazards, 78, 121-140.

Strader, S. M., W. S. Ashley, and J. Walker, 2015: Changes in volcanic hazard exposure in the Northwest USA from 1940 to 2100. Natural Hazards, 77, 1365-1392. 

Strader, S., W. S. Ashley, A. Irizarry, and S. Hall, 2015: A climatology of tornado intensity assessments. Meteorological Applications, 22, 513-524. 

Paulikas, M. J., and W. S. Ashley, 2011: Thunderstorm hazard vulnerability for the Atlanta, Georgia metropolitan region. Natural Hazards, 58, 1077-1092.

Hall, S. G., and W. S. Ashley, 2008: The effects of urban sprawl on the vulnerability to a significant tornado impact in northeastern Illinois. Natural Hazards Review, 9, 209-219.