Words of Wisdom from a Tornado

Article by, Adrian Vatchinsky

Tornadoes have ravaged the continental United States year after year as they await the right conditions to allow for their formation. What these conditions exactly are, is still up for debate as scientists still face difficulties in coming up with ways to confidently predict and expect tornado formations. However, an interesting observation from tornado formations - a low infrasound frequency - may allow us to develop more advanced last minute warning systems (1).

The average human can hear sound in the frequency range of 20 Hz to 20kHz. Infrasound is sound waves below the 20Hz mark. Near-infrasound is sound in the range of 1Hz to 20Hz and infrasound is in the range of 0.05Hz to 1 Hz. This is akin to how light outside of the visible spectrum is called near-infrared and infrared.

It has been observed that various events, such as explosions, avalanches, storms, produce infrasound waves. These waves, due to their low frequency have very long wavelengths and thus are capable of traveling long distances. Infrasound monitoring has been successfully used as a last-minute warning system for avalanches (2) and also as a monitoring system for nuclear testing (3). Infrasound monitoring systems are also of interest as a warning and tracking system for tornadoes. It is also possible to infer the diameter of a tornado from the fundamental frequency it produces giving us a better idea of the scale this tornado falls under (4).

A violent rotating column of air which has made contact with the ground is a simple way of describing what a tornado is. The formation of this column of air results from the right combination of wind directions, storm conditions, and relative humidity and temperature in the surroundings. These conditions are most favorable to occur during a thunderstorm.

If warm moist air is to be found near the ground and a block of cold dry air moves overhead, the lighter warmer air will move up due to an updraft caused by this new arrangement of air blocks. As the warm air moves along with the updraft it may encounter wind shear. An example of wind shear would be wind near the ground traveling at about 5mph southeast, but then wind about 5000 feet up is blowing at 25mph southwest (5). The wind shear will cause the warm updraft to begin rotating on its way up. This rotation creates a mesocyclone – an area of organized rotation a few kilometers up in the atmosphere, typically 2-10 km across. Mesocyclones are detectable by DOPPLER radar. Once a mesocyclone forms, the thunderstorm is classified as a supercell. As rainfall increases, a downdraft in the storm may occur which is called the rear flank downdraft (RFD). The RFD may bring the mesocyclone down forming a visible condensation funnel at the base of the supercell narrowing the mesocyclone. Once this mesocyclone makes contact with the ground the storm is classified as a tornado.

For tornado enthusiasts, check out the Tornado History Project which is an excellent online database which has a wide range on information on basically all reported tornadoes in the United States.

Image credit goes to Wikipedia.