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ated with less organized storm systems, in which precipita- tion bands shroud the region of formation of the tornado. Tornados generated by this mechanism tend to be weaker, but they are very dangerous due to their lack of predictabil- ity and the poor visibility associated with the phenomenon. Roughly one half of the storms in the Southeastern United States are of this variety, with almost 100% of the tornados spawned in Florida being non-supercell in origin (Kelly et al., 1978).
Tornado Alley
Tornados in the US Plains States represent an all too famil- iar threat to life, health, and property. What is not typically well appreciated is that the area that has the largest threat for loss of life is the United States Mid-South. Contributors to the higher mortality rate include rain-wrapped thunder- storms, which obscure tornados within the storm cell as well more convoluted terrain and the presence of tall forests that obscure the horizon. Without advanced warning from the emergency broadcast system that provides alerts to oncom- ing tornados, people who live in rural areas in these states are subject to the whims of nature.
In the Great Plains, low-precipitation supercells are a rela- tively common occurrence (Bluestein, 1999). Because of the lack of significant precipitation, very clear views of tornadic activity are usually possible, making them ideal for the study of tornados. The relatively smooth terrain and lack of tall forests also allow views to the horizon in many cases.
In Figure 3 we show histograms of the total number of tor- nados for each day over the period 1950–2009 summed over the South-Central and over the Mid-South regions (as de- fined in Figure 1). These figures show that the tornado sea- son is roughly concentrated in the band from days 80–180 of the year (mid-March through the end of June of each year). For the Mid-South, there is strong activity from Janu- ary through March but also late season activity in November and December.
The relatively narrow window for tornadic activity and a safer operating environment, due to good visibility and road systems that are on a regular grid, make the South-Central region the preferred region for tornado research and tor- nado chasing. The downside of this circumstance is that most research on tornado genesis arises from data obtained from low-precipitation supercells, and tornado genesis from high-precipitation supercells and front boundaries (more common in the Mid-South) tend to be understudied (which ultimately may contribute to the higher mortality rates re- corded in the Mid-South).
Figure 3. Histogram of tornadic events for (a) the "Mid-South" geo- graphical region and (b) the “South-Central” geographical region. The states used for each region are shown by their state abbreviations. The green shaded area is for all tornadoes, and the magenta area is for just violent (EF3-EF5) tornados.
In spite of the South-Central region’s deserved reputation for having a high rate of tornados, we see that the Mid-South has nearly identical rates for tornados as the South-Central, and both the East-North-Central (Figure 1) and Mid-South regions suffer higher fatality rates than does the South-Cen- tral region. Lower visibility due to terrain, presence of trees, and a higher frequency of rain-wrapped tornados are likely contributors to these higher fatality rates. Possibly the fact that Plains States tornados and the meteorological factors associated with their genesis are well-studied has reduced the risk of fatality associated with South-Central tornados.
Infrasound from Tornados
Since the early 1970s, convective systems have been known to be sources of infrasound (e.g., Bedard and Georges, 2000). The typical frequency band for these sounds is about 0.017 –1 Hz (Georges, 1973). However, there is a great deal of un- certainty about the origins and significance of the sounds. A variety of potential source mechanisms have been proposed for infrasound generated by convective storms. Georges (1976) considered many of these and concluded that vortex motion was the most plausible candidate. While the sug- gestion has been made that these signals are a precursor to tornadic activity, Bedard (2005) concluded that “it seems unlikely that the much lower frequencies detected by these geoacoustic observatories had any direct connection with tornado formation.”
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