Anatomy of a Tornado

TORONTO – They are nature’s most violent and catastrophic storms.

Often referred to as the “˜finger of God,’ these powerful twisters form swiftly, often leaving a trail of devastation in their wake.

Across Tornado Alley, an area spanning the U.S. Midwest between the Rockies and Appalachian Mountains, residents were reminded of the destructive power of the tornado on Monday.

A massive EF-5 tornado tore a 10 km path across southwestern Missouri, wiping an entire community off the map in what’s now being called the deadliest single tornado in more than six decades.

Meteorologists Bill Coulter and Anthony Farnell talk to us about the driving force behind these fast-forming storms.

Q: How is a tornado formed?

Anthony Farnell: A typical tornado forms in what is called a supercell storm. A supercell is a strong thunderstorm that is rotating. Wind shear is necessary for tornadoes to form which means the wind changes direction with height as you go up in the atmosphere.

Q: What factors must be present to turn an ordinary storm into a tornado?

Bill Coulter: Two factors are necessary, lift and shear. Lift refers to rising air. That air can either be forced as along a front or by buoyancy when the air cools rapidly with height.

The difference in temperature is directly related to a difference in pressure and density.

Less air wants to rise over the cooler more dense air above it in much the same way as a basketball held down at the bottom of a pool wants to rise to the surface.

Shear simply refers to winds changing direction with height. If the wind is coming in out of the south at the surface and out of the northeast at some level far off the ground, this twist or spin will assist in the development of the tornado vortex.

Q: Are twisters always born from severe thunderstorms?

Bill Coulter: Any severe thunderstorm is capable of creating a tornado. But, even a weak thunderstorm can sometimes produce a tornado if the conditions are right. Thunderstorms are considered severe if the meet certain conditions dictated by a national weather agency. But, just because a storm isn’t classified as severe, doesn’t mean it isn’t strong and capable of damage or generating tornadoes.

Q: How does a tornado function?

Bill Coulter: Most of the time, tornadoes live and die with the life cycle of the thunderstorm cell of which is was born, which is usually only a couple of minutes up to around 20minutes. However, the outflow of a dying storm can provide the fuel for another storm in almost exactly the same place creating a 10-20minute life cycle of a new storm. If there is still shear and uplift present, it can appear as if the tornado continues well longer than 20 minutes, even if it may in fact be a second tornado following the same path.

Q: What forms the funnel shape?

Bill Coulter: Variable winds at different levels of the atmosphere feed the updraft and cause the formation of the tornado’s characteristic funnel shape. More air flowing into the column as it rises up to the cloud base fattens the top of the funnel. The tighter the funnel is, the faster the winds are likely rotating.

Q: Why are April and May the most common months for killer tornadoes?

Anthony Farnell: April and May have the most storms in the U.S. because this is when the air masses clash the most. Warm moist summer air sweeps north as cold left over winter air masses sweep in from the Rockies.