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tornado to define the polygon could be problematic since tornadoes vary
widely in wind speed, width, and track length. Adding further complexity
to the accurate mapping of tornadoes is their frequent tendency to touch
down, lift up, and touch down again, sometimes multiple times in the same
storm.
In the next activity, you have the opportunity to make maps of tornadoes
yourself, using the tools that produced the maps that you have just examined.
8.5 Activity: Examining the distribution of tornado data
Now it is your turn to investigate tornadoes using a web-based GIS, ArcGIS
Online. Access ArcGIS Online (
http://www.arcgis.com/home
) and start a new
map. Use the Add dropdown menu to add the map layer “USA Historical
Tornadoes” from ArcGIS Online. Select “Done Adding Layers” and click on the
button that allows you to view the map legend. What spatial patterns do you
notice? You should notice that the higher-intensity tornadoes shown at the
scale of the entire country predominantly occur in the central and southern
states. Zoom and pan the map and notice the pattern of the tornadoes spa-
tially and by intensity. What influence does topography, or altitude, especially
in Colorado and Wyoming, seem to have on tornado frequency and intensity?
Why? Mountains and hills seem to have a negative influence on tornadoes,
because they inhibit a smooth track for the tornadoes to travel over and, more
important, they inhibit the merging of the cool and warm air masses and the
vertical updrafts and downdrafts in the storms themselves. What influence
does latitude have on tornado frequency and intensity? Why? Latitude has a
negative influence on tornado frequency because it is more difficult for the
necessary warm, moist air masses to penetrate higher latitudes.
Click on individual tornadoes near the Gulf of Mexico and observe the month
in which they occurred. Repeat this process for tornadoes in Minnesota and
North Dakota. What hypothesis could you make about how the tornadoes
vary spatially by month of the year? Because of latitude, weather patterns, and
the proximity to oceans, January and February tornadoes tend to occur near
the warm waters of the Gulf of Mexico and the Atlantic Ocean off of the coasts
of Alabama, Mississippi, and Florida. They increase in frequency with the
advent of spring, and also their total spatial distribution expands. They reach
a numeric maximum in April when the difference between the cold air masses
from Canada and the warm air masses from the Gulf of Mexico are at their
greatest difference. However, they do not reach a spatial maximum until July,
when most of the continent has warmed up sufficiently to support the merg-
ing of the air masses and the updrafts and downdrafts necessary for the wide-
spread occurrence of tornadoes. Zoom to specific large cities. Can you find
instances where tornadoes moved through the downtown sections of large
cities? Examples include Nashville and Oklahoma City. What safety hazards
are acute in such situations? These hazards include damage to infrastructure
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