Table 4: The Fujita tornado damage scale.

The F scale is based on tornado damage (primarily to buildings), so there is some ambiguity in the scale. For example, a tornado which moves over open country will tend to receive a lower rating than a tornado which strikes a populated area. Since buildings have a wide variation in age, quality of design, and quality of building materials, more uncertainties are thrown into the mix. Tornadoes over open country will probably encounter varying types of vegetation, leading to uncertainties in these cases. Still, the Fujita scale provides a good baseline for classifying tornadoes according to their intensities.

Tornado/Funnel Cloud Look-Alikes

Experienced spotters are probably aware that a number of features (both natural and man-made) can bear a resemblance to a tornado or funnel cloud. Some of these natural features include rain shafts and scud clouds. Some of the man-made features include smoke from oil flares and factories. If a suspicious-looking cloud formation is observed, watch it for a minute or two. Look for organized rotation about a vertical or near-vertical axis. Figure 34 depicts a number of tornado look-alikes.

Figure 34(a): Some tornado/wall cloud look-alikes. Scud clouds.

Figure 34(b): Rain shaft. Photos - NWS, NSSL.

Another phenomenon which must be discussed is the gustnado. Gustnados are small vortices which sometimes form along a gust front (figure 35). Gustnados are generally not associated with the updraft area of the storm and do not originate in mesocyclones, so in some ways they are not "legitimate" tornadoes. They can cause damage to lightweight structures and are hazardous to people in the open, though, so they do pose a threat and should be reported to the controller/dispatcher.

Figure 35: Gustnados are small vortices which sometimes form along strong gust fronts. View is to the southwest. Photo - Charles Doswell III.

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