This lecture discusses tornado formation. In order for a violently rotating column of air to be classified a tornado, the vortex must come from a cloud and be in contact with the ground. If the rotating column doesn't touch the ground it would be a funnel cloud. Doppler radar can see rotation in the atmosphere but can't distinguish when a funnel touches the ground. So all tornadoes need to be visually verified by train spotters or damage surveyed after the fact.
Active tornadoes may or may not be visible in the sky depending upon the appearance of condensation. Tornadoes are generated from severe thunderstorms and hurricanes, although any condition that creates rotation in the atmosphere may generate a vortex. These unusual types of rotating columns are called by other names, like dust devils, fire tornadoes, gustnadoes, and water spouts.
Most tornadoes aren't very large, less than half a mile wide. The average track is 2 to 4 miles long. and the average speed along the track is around 35 miles per hour. The Fujita scale was implemented in 1971 as a means to understand the wind speed in a tornado. Instrumentation can't survive a direct hit, so Ted Fujita developed a way to infer wind speed based on damage.
The enhanced Fujita scale takes into account damage indicators that weren't in the original scale, such as building structures. The historical records are still the same, so an F5 from years ago is still an F5, but its winds may not have been as fast. We've never been able to directly measure wind speed inside a tornado, but researchers were able to remotely measure the Oklahoma City F5 tornado in 1999. Using a portable Doppler radar, they measured wind speed at 318 miles per hour.
It's important to note that the EF scale can't be assigned just by looking at a tornado. There's no relationship between the shape of a tornado and its EF scale. The only way to verify the EF of a tornado is to perform a damage survey after the tornado dissipates. Tornadoes occur all over the world and have been observed in all 50 states. Even New Zealand reports about 20 tornadoes annually.
However, Canada, Argentina, and Bangladesh have the highest numbers of tornadoes outside of the U.S. With around 1,200 annual tornadoes, the United States has 10 times more tornadoes than anywhere in the world. Most of these occur in the Great Plains. However, more long-track and violent tornadoes tend to occur closer to the southeast in Dixie Alley.
As with most extreme events, the weak EF1 tornadoes occur more often. But the EF4 and EF5 tornadoes are the deadliest. Tornadoes are mostly warm season events, although they've been observed every month of the year. The majority of U.S. tornadoes tend to peak in May, and most tornadoes occur in the early evening after surface heating generates more storm energy.
As many tornadoes are built from convective thunderstorms, their occurrence tends to follow the migration of the solar declination. In the United States, tornadoes first appear near the Gulf of Mexico, and then progress toward Canada as the sun's energy moves to the northern tier later in the summer. Most agree that a tornado outbreak happens when 10 or more tornadoes are formed within the same storm system, called families. Some have named the April 27, 2011 tornado outbreak as the Super Dixie outbreak and is ranked as the number two outbreak in history according to severity. While forecasting is much better and most warning time is around 30 minutes for a tornado, researchers are still trying to understand the kinds of atmosphere conditions that generate outbreaks.
The deadliest tornadoes occur from supercells, although only about 10% of all supercells produce a tornado. There's much about the formation process that's still not understood. However, some situations tend to provide a strong setup for tornado genesis.
The standard supercell formation is enhanced by speed shear and directional shear. Shear is just a change in wind speed or direction. Winds increase in speed as they move away from the frictional boundary, which induces a tumbling in the atmosphere. If this tumbling cylinder of winds is uplifted by a strong updraft, vertical rotation is initiated.
Veering winds from the south can act as a lifting mechanism. Assisting tornadic production. The architecture of a supercell is much larger than the tornado that it might spawn.
These systems are usually 2 to 6 miles in diameter, and rotation makes the system a mesocyclone. Inflow bands are low cumulus clouds, usually extending to the south or southeast. These show that the system is bringing in low-level air from miles away. A wall cloud is usually in the rear, behind the precipitation.
and may or may not be rotating. If one exists, it usually sets up 10 to 20 minutes or so before a tornado. The geographic orientation of the storm is important.
If you're driving into a mesocyclone from the northeast, you may drive into the rain core and then directly into a tornado. The rear flank downdraft is on the back side of the storm, and this is where the tornado will appear, if at all. The rear flank downdraft is an essential part of bringing the tornado down. For the mesocyclone to ventilate and grow, the storm must be tilted, usually in a northeast direction.
Otherwise, the updrafts would be choked out by the downdrafts and the atmosphere would stabilize. Radar has been used in meteorology since World War II, but early uses were only for the presence of storms. The Union City tornado in 1973 actually helped confirm that radar can observe rotation in the atmosphere above a tornado. Not all tornadoes behave this way, but radar has certainly become an excellent tool for forecasting tornadoes.
The Union City tornado also allowed the first continuous observation of a tornado through its entire life cycle. Again, not every tornado behaves exactly as scripted, But this life cycle is helpful in understanding the duration of a tornado. The mature stage is usually where most damage occurs, although smaller tornadoes may skip this stage altogether.
The stage of a particular tornado is not an indication of its EF scale or danger. Occasionally, small tornadoes do major damage, and some very large tornadoes over a quarter mile wide have produced only light damage. Tornadoes will typically travel the same path as the storm system in which they're embedded.
In the United States, this means most tornadoes will track along a path that moves from the southwest to the northeast. Tornadoes don't tend to jump around or change direction easily, but that isn't always the case. Faster winds, which are more damaging winds, are found on the right front quadrant of the tornado, where the circulation in the tornado is added to the movement of the tornado along its track.
Tornadoes with multiple section vortices can cause multiple damage areas. What kills most people is not the fast wind, but the flying debris or destabilized structures falling on them. It's important to protect yourself from the debris in a structurally sound location. The safest place is usually below ground.
Safety rooms seem to be effective too. It's important to remember that your car is not the safest location because it's situated above the surface and can be lifted off the ground. While most tornadoes move along their tracks at less than 60 miles per hour, it can be difficult to outrun a tornado if you don't know where the roads will lead you.