TORNADOES AND RELATED PHENOMENA 
By EDWARD M. BROOKS 
Saint Louis University 
Introduction 
Definition. The word “tornado” comes from the 
Spanish tronada (thunderstorm). In western and north 
central Africa, “tornado” refers to a thunderstorm. 
But elsewhere it means an intense spiral motion around 
a vertical or inclined axis, averaging about 250 yards 
across, with its lower part [17] often characterized by a 
narrow pendant cloud [12] extending from a cumulo- 
nimbus cloud base to or nearly to the ground. 
Frequency. A tabulation of tornadoes which have 
occurred over a long period of years is needed to de- 
termine frequencies. Unfortunately, many severe wind- 
storms are difficult to classify correctly. The number of 
tornadoes is erroneously decreased by unreported tor- 
nadoes and by tornadoes classified as other storms, and 
it is erroneously increased by nontornadic storms re- 
ported as tornadoes [20]. — 
Tornadoes occur on all continents, but are rare except 
in Australia and the United States. The numbers of 
tornadoes reported in these two countries [1] average 
about 140 and 145 per year, respectively, and the fre- 
quencies per unit area are similar. Australian tornadoes, 
however, do not become as severe. In the United States 
the highest annual frequencies per ten thousand square 
miles are found in northeastern Kansas (3.2), central 
Arkansas (3.0), and throughout Iowa (2.5 to 2.3). 
’ However, some of these differences between states may 
be due to their different methods of classifying wind- 
storms. Because the area swept by a tornado averages 
only about two-thirds of a square mile, the probability 
of a tornado’s hitting a given square mile of the Midwest 
is generally less than 1 per cent per century. 
The number of tornadoes in any one year may vary 
considerably from the annual average. The seasons, 
listed in order of tornado frequency for the United 
States, are spring, summer, fall, and winter. May has 
the greatest number; December, the least. For an indi- 
vidual state, the month of maximum tornado frequency 
is usually one month before the maximum hailstorm 
frequency and two months before the maximum thun- 
derstorm frequency. Tornadoes appear most frequently 
in the following months: in the southern states in 
March; in the central states in April or May; and in the 
northern states in Juneor July [12]. Multiple tornadoes 
within a general cyclone are most likely in April or 
May [12]. 
Over 80 per cent of reported tornadoes occur between 
noon and 9 p.m. This preponderance of afternoon storms 
seems to be least pronounced in winter. 
Weather Conditions Associated with Tornadoes 
Synoptic Situations. The usual United States weather 
map, favorable for tornadoes, shows a deep extra- 
tropical cyclone with central sea-level pressure below 
1002 mb located in the central or northern part of the 
country, usually with a curved trough extending gen- 
erally southeastward from it and having a large wind 
shift across it. The most likely place for tornadoes is 
within the portion of the trough generally 100-600 mi 
southeast of the center of the parent low. Tornadoes 
rarely occur within 50 mi of the primary, or parent, 
low center, but when they do they are usually very 
violent. 
The following extratropical frontal conditions are 
associated with many tornadoes. Along the trough there 
is usually a cold front aloft, or a surface cold front 
where a maritime tropical air mass is being displaced 
by a modified continental polar or maritime polar air 
mass in 90 per cent of the cold front cases and by a 
fresh continental polar air mass in 10 per cent. Some- 
times the trough contains an occluded front or a warm 
front instead. 
Other tornadoes occurring with a deep parent cyclone 
are not located on a front. Tornadoes occasionally occur 
in the maritime tropical air warm sector southeast of 
the parent low center. These tornadoes might be located 
on a squall line, or at the points of intersection of two 
or more squall lines [21], or on several closely associated 
squall lines separated by relatively quiet areas. Some- 
times tornadoes occur on the north or west side of the 
parent low center in the polar air coid sector which 
may have maritime tropical air over it. Occasionally 
the parent cyclone is a tropical hurricane, in which 
tornadoes occur in maritime tropical air usually north 
rather than southeast of the center. About four-fifths 
of the tornadoes in the United States are ‘‘cyclonic” 
and the rest “convective.” By definition, ‘‘cyclonic” 
tornadoes occur with a well-developed parent low, 
whereas ‘‘convective” tornadoes are either frontal or 
nonfrontal and occur in a weak parent low, in a weak 
pressure trough, or even in a high-pressure area. Tor- 
nadoes are known to occur along a tropical or inter- 
tropical front [9]. 
Typical aerological soundings [19] of the maritime 
air in which tornadoes develop show that it has high 
relative humidity usually only up to about 1-38 km. 
A thin stable layer, which may be an inversion, sepa- 
rates the maritime air from the dry superior air aloft, 
which is characterized by a steep lapse rate. The thin 
layer is convectively unstable because of the rapid 
decrease of humidity with height. 
Nearby Weather Conditions and Preliminary Signs. 
Tornadoes are not to be expected unless there are some 
preliminary signs of cumulonimbus activity, since the 
pendant cloud, sometimes not visible, extends down- 
ward from a cumulonimbus cloud base. The under side 
of the cumulonimbus frequently exhibits a mammatus 
appearance before the tornado [9]. Along the path of a 
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