Infrasonics Program - Frequently Asked Questions
- How early can tornadoes be predicted?
In many cases the weather service can provide warnings with better than 10 minutes lead time. However, the likelihood of correctly detecting a tornado depends on a number of factors such as distance to the tornado from the radar, the type and orientation of the funnel or tornado, and whether there are any obstacles preventing radar detection. Doppler radar has provided a key tool for forecasters but efforts are being made to further increase warning lead time and detection accuracy.
- What is the latest in tornado researching technology?
ETL is currently working on a system for detecting very low frequency sounds from tornadoes as a system complementary to Doppler radar. It continues to show promise as a technique for filling detection gaps and improving prediction times. This is a "work in progress". NOAA supports or has supported a number of other technologies, including:
- A seismic detection technique to warn of and locate tornado touchdown points.
- An electromagnetic detection technique (with a hope that something like a low-cost tornado "smoke alarm" could evolve)
- A pressure detector prototype suitable for large scale deployment in tornado prone areas to provide needed knowledge about the cores of tornadoes.
- Is it possible to prevent tornadoes, such as introducing "seeding agents"?
Once a large tornado has formed, the energy involved is tremendous and it is difficult to think of practical ways of reducing the hazard at that point. However, it is a good question to continue to ask. As our knowledge of tornadoes, as well as the formation processes, increases, we may find that there are opportunities to intervene and perhaps change the direction of the formation process at a critical moment. We are not yet close to having the knowledge to even assess whether this may someday be possible.
- What exactly is the vortex of a tornado and how is it formed?
There has not been a direct measurement within the core of a tornado vortex to date, but mobile Doppler radars have provided us with unprecedented views of the flows in and near the core. We can also look to measurements of other vortices such as dust devils, aircraft wake vortices, mesocyclones, and hurricanes to improve our knowledge. Most vortices have a "solid body" rotation within the core. This means that the flow is near zero at the center and increases outward to a maximum. After this point the flow tends to decrease inversely as the radius. I think we understand why this is so.
I like to think that nature has a deck of cards having a number of ways to deal out and change vorticity. These methods include tilting horizontal vortices whether they are near the ground or aloft, and then stretching them (increasing the speed of rotation). This stretching could be done, for example, by downdrafts or updrafts. The analogy of a skater increasing rotational speed by bringing in their arms, while conserving angular momentum is a good one. The point is that nature has a lot of ways of creating, manipulating and organizing vortices.
- Why do most tornadoes occur in the spring and summer in the afternoon and early evening?
There is a close relationship between tornadic activity and severe weather, which tends to peak in activity in the late afternoon and evening in the central US where tornadoes are frequent. As usual there are always exceptions to this trend, but the association is quite close.
- Why do tornadoes make a freight train sound?
There are a number of possible sources for the audible sounds accompanying tornadoes. A paper by Arnold et al. (Journal of the Acoustical Soc. America, Vol.60, pages 584-593) reviews measurements and some of the possibilities. Interactions with the ground will produce strong turbulent eddies which will create a continuous roar. Also, the strong winds interacting with obstacles can produce aeolian tones over a range of frequencies. Even funnels above the ground can produce sound, mainly associated with windshear; but probably also from smaller, imbedded vortices. We have also studied sound from aircraft wake vortices, observing a number of distinct sound generation processes. In other words the sound from a tornado can result from an ensemble of different mechanisms.
- Would tornadoes be visible if they were not picking up dust?
Under some conditions cloud tags can provide visual clues of the presence of a tornado. In other situations if there is enough moisture present and the pressure drop in the core is large enough, a condensation funnel will occur and be quite visible. You can experience this effect when looking out the window of an aircraft taking off or landing on a moist, foggy day. You will typically see an array of vortices around the wings made visible by condensation.
- Occasionally, a tornado will spin in an opposite direction to most tornadoes (which usually rotate counter clockwise in the northern hemisphere). Why does this happen?
The fact that most tornadoes rotate counterclockwise indicates that the rotation of the earth is an important factor in the formation process. For smaller dust devils about half rotate clockwise and about half rotate counterclockwise, indicating that the rotation of the earth does not dominate the formation process. The occasional reverse rotation tornado could have been created in a different way than usual. One possible example is that eddies of different directions of rotation were alternately shed in the lee of a thunderstorm acting as an obstacle to the mean flow.
- More Tornado Resources:
- The Online Tornado FAQ from the NOAA Storm Prediction Center
- NOAA National Severe Storms Laboratory Tornado Page
- NOAA NWS Tornado Fact Sheet
- Tornadoes... Nature's Most Violent Storms: A Preparedness Guide from NOAA, FEMA and the American Red Cross
- Maps of Tornado Statistics by State: from the National Climatic Data Center
- University of Illinois World Weather 2010 Project
- Oklahoma Weather Center Symposium on the Southern Great Plains Tornado Outbreak of 3 May 1999
- The Why Files
Anyone interested in this project should contact Dr. Alfred Bedard, (303) 497-6508