Chapter 15 — METEOROLOGICAL ELEMENTS 



forms as such directly without going through the 

 liquid stage. 



TORNADOES 



A tornado is an exceedingly violent whirling 

 storm with a small diameter, usually a quarter 

 of a mile or less. The length of the track of a 

 tornado on the ground may be from a few 

 hundred feet to 300 miles; the average is less 

 than 25 miles. When not touching the ground 

 It is termed a "Funnel Cloud." Data from 

 recent tornado studies indicate that the velocities 

 of tornadic winds are in the general range of 

 150 to 300 miles per hour. A large reduction of 

 pressure in the center due to the spiraling of the 

 air seems to cause buildings in the path of the 

 storm to explode. The speed of the storm over 

 the earth's surface is comparatively slow — 

 usually 25 to 40 mph. 



Most of the tornadoes in the United States 

 occur in the late spring and early summer, and 

 are associated with thunderstorm activity and 

 heavy rain. Tornadoes have been observed with 

 various synoptic situations and usually are as- 

 sociated with overrunning cold air. Statistics 

 show that the majority of tornadoes appear about 

 75 to 180 miles ahead of a cold front along 

 the prefrontal squall line. Figure 15-1 2 shows 

 the various stages of development of a tornado. 



A situation that is noticeably favorable to 

 tornado activity is cold air advection aloft. 

 When mP air moves across the United States, 

 it becomes heated in the low levels from the 

 Western Plateaus. Having a density equal to 

 or less than the mT air moving northward over 

 the Mississippi Valley, the mP air rides up 

 over the mT air. The mP air still maintains 

 low temperatures at higher altitudes; this causes 

 extreme instability. 



The following conditions may indicate possible 

 tornado activity: 



1. Pronounced horizontal wind shear. (Wind 

 shear is the rate of change of wind velocity 

 with distance.) 



2. Rapidly moving cold front. 



3. Strong convergence. 



4. Marked convective instability. 



5. Dry air mass superimposed on a moist 

 air mass. Abrup. change in moisture content, 

 usually below 10,000 feet. 



209.63 

 Figure 15-12. — Stages of development of a 

 tornado. 



6. Marked convection to the minus 10°C iso- 

 therm. 



WATERSPOUTS 



Waterspouts are tornadoes that form over 

 ocean areas. They may be divided into two 

 classes. One is the true waterspout in which 

 the vortex forms at the cloud and extends to 

 the surface. This type occurs mainly in advance 

 of a squall line. The second type, often called 

 the pseudo-waterspout, originates just above the 

 water surface and builds upward; this type 

 is identical with whirling dust often seen on 

 deserts. 



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