Structure of Convective Storms 
Trersuya Fusira 
University of Chicago, Chicago, Illinois 
Abstract—The features of the Fargo tornadoes and the meteorological situations 
producing them are studied in three scales: macroscale, dealing with the precipitation 
and tornado distribution over three-fourths of the United States; mesoscale analysis of 
North and South Dakota and of the rotating cloud which produced four tornadoes 
as it moved over the Fargo area; and, finally, microscale features of the tornado itself. 
These analyses are presented in an animated 16 mm motion picture. Included in this 
report are the results of the computed cyclostrophic wind speed and rotational speed 
of the funnel of the Fargo tornado. A proposed mechanism of an irreversible process 
taking place inside the inflowing air toward the tornado funnel is also presented. 
Life cycle of the Fargo tornado—By using 
the still photographs and movie films collected 
through WDAY-TV, Fargo, and the U. §. 
Weather Bureau at Hector Airport, Fargo, 
changes in the tornado funnel throughout its life 
span were obtained. Figure 1 reveals that the fun- 
nel dropped from the cloud base to the ground 
within a matter of minutes. As soon as the tip of 
the funnel reached the surface, the lower portion 
of the funnel was sheared off, and then rounded; 
meanwhile, the funnel diameter above the 
rounded bottom kept increasing. Soon after, the 
tornado funnel as a whole started shrinking at 
about 18h 80m CST. 
The diameter of the funnel is also given in 
Figure 1 as a function of time and the height 
above the surface. 
Funnel diameter and centrifugal acceleration— 
An expanded analysis of the funnel diameter is 
made for the period of only 242 minutes during 
which the tornado funnel reached the ground and 
was rounded at its bottom. The rounded funnel 
bottom shows an extremely high rate of increase 
in diameter along the vertical (Fig. 2). 
Assuming that the condensation is taking place 
at the funnel edge and that the condensation pres- 
sure is the same everywhere at the smooth edge 
of the funnel, the hydrostatic assumption and the 
eyclostrophie wind equation enable us to de- 
scribe 
oo os UE 
p AR R 
AP = —pgAZ 
Therefore, we have 
AZ y? 
AR 
61 
where V is the cyclostrophic wind speed; p, the 
density; R, the radius; P, the pressure. This 
equation shows that the tangent of the funnel 
slope is proportional to the centrifugal accelera- 
tion in the unit of gravitational acceleration. It is 
of interest to see the maximum centrifugal ac- 
celeration 10.8 g oceurring at about 18h 28.8m 
at 120m above the ground (Fig. 3). 
Cyclostrophic wind speed—The cyclostrophic 
wind speed computed from the centrifugal ac- 
celeration is shown in Figure 4 as a function of 
time and the height above the ground. The diam- 
eter of the funnel is also indicated by broken 
lines. The computed wind speed is the cyclo- 
strophic wind speed at the funnel edge. No speed 
was computed at the bottom of the rounded fun- 
nel because its gradient is so small that it would 
give less than 10 m/see, while heavy damage was 
taking place directly beneath the funnel. 
In any case, the tangential speed at the edge 
of the funnel above the rounded bottom gives a 
fairly high value, which would be expected from 
the tornado damage. The maximum speed of 103 
m/sec, or 230 mph, occurred slightly before 1Sh 
29m when the funnel diameter, 130 m above the 
ground, was 200 m. 
A film taken by WDAY-TV, Fargo, at the time 
of the Fargo tornado permits us to make an inde- 
pendent computation of the rotation speed of the 
funnel. Figure 5 indicates the tangential speeds 
computed by eyelostrophic assumption (double 
lines) and tornado film (heavy lines) for three 
different times: 18h 28.0m, 18h 28.9m, and 18h 
29.6m CST. The computation is made along the 
dotted line, along the funnels shown at the top of 
the figure. At the present time, so far as the Fargo 
