Tornadoes
What is a tornado?  A tornado is a violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud. For a vortex to be classified as a tornado, it must be in contact with both the ground and the cloud base.

The Fujita Scale
The Fujita scale (F-Scale), or Fujita-Pearson scale, is a scale for rating tornado intensity, based primarily on the damage tornadoes inflict on human-built structures and vegetation. The official Fujita scale category is determined by meteorologists (and engineers) after a ground and/or aerial damage survey; and depending on the circumstances, ground-swirl patterns (cycloidal marks), radar tracking, eyewitness testimonies, media reports and damage imagery, as well as photogrammetry/videogrammetry if motion picture recording is available.
Tornado Safety Tips

When a tornado warning has been issued, you may have very little time to prepare.  How you respond now is critical. And how you react depends on where you are.

In a Frame Home
- Make sure you have a portable radio, preferably a NOAA weather radio, for information.
- Seek shelter in the lowest level of your home (basement or storm cellar). If there is no basement, go to an inner hallway, a smaller inner room, or a closet. Keep away from all windows.
- You can cushion yourself with a mattress, but do not use one to cover yourself. Do cover your head and eyes with a blanket or jacket to protect against flying debris and broken glass. Don't waste time moving mattresses around.
- Keep your pet on a leash or in a carrier.
- Multiple tornadoes can emerge from the same storm, so do not go out until the storm has passed.
- Do not leave a building to attempt to "escape" a tornado.

In a Mobile Home
- Leave your mobile home immediately and take shelter elsewhere.

Outside
- Try to get inside and seek a small protected space with no windows.
- Avoid large-span roof areas such as school gymnasiums, arenas, or shopping malls.
- If you cannot get inside, crouch for protection beside a strong structure, or lie flat in a ditch or low-lying area and cover your head and neck with your arms or a piece of clothing.

In a Car
- Ideally, you should avoid driving when tornadoes or other kinds of dangerous weather threaten, because a vehicle is a very unsafe place to be. If, however, this is not possible, stay as calm as possible, and assess the situation.
- Your best option might be to get out of the car and lie flat in a ditch or other low-lying area that is sufficiently deep enough to protect against the wind.
- If you do so, beware of water runoff from heavy rain that could pose a hazard; get as far away from the vehicle as possible and shield your head from flying debris.
- Or, if possible, take shelter immediately in a nearby building.

Tornado Tracking Records
Hook echo
As its name implies, this refers to an appendage that appears as a hook on radar and usually comes out of the southwest (lower left) portion of some supercell thunderstorms. It's an indication the storm has a strong updraft and sometimes rotation. When tornadoes are spawned by supercells, they are located near the tip of the hook echo on radar. But a hook echo doesn't always mean a tornado is in progress.
Hook echo
The mention of a debris ball on radar is an indication that a damaging tornado is in progress. It means the radar is sensing an area of higher reflectivity in the hook echo, which is often associated with debris that has been lofted thousands of feet in the air.
Debris Ball
Bow Echo
This name is given to radar echoes that bow outward from a larger line of thunderstorms (see arrows in the image above). It's an indication of a concentrated area of damaging straight-line winds in a line of thunderstorms.

T​he bow echo shows where the rain-cooled thunderstorm downdraft is pushing down to the Earth's surface and spreading out horizontally. Wind gusts in bow echoes are frequently 60 to 80 mph, sometimes higher.

One or more of these curved lines of thunderstorms can sometimes be long-lived enough to be called a derecho, which is a widespread wind damage event.
Tornadic Debris Signature
T​DS is an acronym you might hear that stands for "tornadic debris signature." Like a debris ball, it means the radar has detected debris lofted into the air from a tornado in progress.

The upgrade of National Weather Service radars to dual polarization technology earlier this century allowed meteorologists to see a TDS and communicate that a tornado is in progress and causing damage, even if it happens at night or no spotters have seen it yet. It's often seen using what's called correlation coefficient (CC) radar data.
Airborne tornado debris consists of items that are vastly different sizes and shapes, falling to the ground much differently than precipitation, which is what the CC parameter detects.