678 
some of the environment by an upward current [3, 22]. 
Tt can also occur if the inflow is reduced, which would 
be the case with the development of a strong centrifugal 
force, as described above. Because of the smaller hori- 
zontal pressure gradient aloft, the centrifugal force of 
the whirl overbalances it, producing the outflow needed 
to maintain the tornado. This is an argument in favor 
of accepting rotation in addition to some inflow as a 
necessary feature of tornadoes. An analogy can be 
found in the case of water running down a drain, high 
horizontal water speeds being obtained only when there 
is a whirl. Redfield and Ferrel [6] in the nineteenth 
century realized that gyratory motion as well as an 
unstable condition was necessary for the formation of a 
tornado. 
Protection of Life and Property 
Forecasting and Tracking. Unlike the ordinary daily 
weather forecast, reliable specific tornado forecasts can- 
not be made at the present time [8]. At best, conditions 
may be found which are favorable for the development 
of tornadoes over a wide area. Ever since Finley [7] 
first attempted to limit this area to a quarter of a state, 
others have been trying to limit the area or time of 
occurrence by taking into account variations in surface 
and upper-air conditions.! For more than fifty years the 
U. S. Weather Bureau has been issuing warnings of 
severe local storms within the next 24 hr without men- 
tioning tornadoes specifically. 
Although the exact place and time a tornado wil 
strike are not known, Lloyd [16] and others realized 
that, once a tornado had formed, its future course 
could be predicted with reasonable accuracy from a 
knowledge of winds aloft in the warm air mass. In 
some midwestern cities, the U. S. Weather Bureau has 
considered plans for the detection and tracking of 
tornadoes by telephone, by short-wave radio sets with 
independent power supplies, and on radarscopes. Identi- 
fication and tracking should be improved by the use of 
sferics, a method now being tested experimentally [15]. 
Injuries, Loss of Life, and Public Safety Measures. 
The average annual death toll from tornadoes in the 
United States is about 245 [1]. Since the population of 
the United States exceeds 100,000,000, the average 
chance in the United States of being killed by a tornado 
in any given year is less than 1 in 400,000. Much of the 
loss of life and many injuries are caused by objects 
striking people’s heads, and by fires starting after the 
tornado. The greatest loss of life in a single tornado was 
689 (March 18, 1925) and the greatest on a single day 
was about 1200 (Feb. 19, 1884). 
People can protect themselves better by learning to 
recognize local signs of a tornado and to watch the sky 
when public forecasts call for severe local storms. If a 
tornado cloud appears, it is advisable Gf time permits) 
to shut off immediately the electric power and gas 
supplies and to extinguish all fires (in fireplace, furnace, 
1. For an account of some recent work of this nature see p. 
792 in “‘A Procedure of Short-Range Weather Forecasting”’ by 
R. C. Bundgaard in this Compendium. 
LOCAL CIRCULATIONS 
etc.) so that a conflagration will not occur and burn to 
death someone trapped by heavy debris. The next step 
is to seek shelter quickly im a tornado cellar, or in the 
southwest corner of the basement of a frame house, or 
beside an inside partition on a lower floor of a reim- 
forced concrete or modern steel building, but not in a 
house with brick walls. In a city, it is generally danger- 
ous to try to get in a car and drive away from an ap- 
proaching tornado because excessively high winds, often 
with flying debris and hail, could wreck the car and even 
lull the occupants. If a person is caught in the open 
without available shelter, to avoid injury or death he 
should lie flat in a ditch or culvert, hold on to a fixed 
object to keep from being blown away, and cover him- 
self, especially his head, to protect himself from mis- 
siles. 
Property Damage, Building Safety, and Insurance. 
The average annual United States property loss from 
tornadoes is over $11,000,000 [1]. The yearly damage is 
extremely variable, depending on the size of population 
centers hit. Also, the total amount of property and the 
value of the dollar undergo considerable changes over 
a long period of years. The greatest damage on a single 
day, with 57 tornadoes, was about $35,000,000 (Feb. 
19, 1884); however, that much damage has been caused 
by only two tornadoes in St. Louis (1896, 1927). In- 
direct property losses are from fires and looting after 
the storm. 
The Western Society of Engineers in 1925 recom- 
mended that a building be designed for wind pressures 
of 65 lb ft with a factor of safety of 4, which would 
probably save it in a minor tornado or on the outer edge 
of a major tornado. A building, especially a public 
meeting place, should be better protected by being 
bonded together, being anchored to a basement, having 
sufficient openings or automatic vents to relieve pres- 
sure, and having a grove of large trees (preferably oak) 
on its southwest side to diminish the wind speed. 
Finley, on the other hand, thought buildings should 
be constructed as if there were no tornadoes, and then 
be covered by insurance. Most tornado insurance is 
lumped together with all other windstorm insurance to 
avoid arguments about whether a windstorm was or 
was not a tornado. The highest risks in the United 
States are along the South Atlantic and Gulf Coasts 
because hurricanes there cause about seven and one- 
half times as much damage as do tornadoes in the 
Middle West. 
Factors usually taken into account in writing wind- 
storm insurance policies are (1) the location of the 
building, (2) the construction of the building, and (3) 
the susceptibility of the contents of the building to 
damage. 
Other Whirlwinds 
Since whirlwinds in general have received less atten- 
tion than tornadoes, they are not well understood. 
The information given below is based on only a few 
good sources. 
Both dust devils and waterspouts are, on the aver- 
age, less violent and have higher central pressures 
