Tornadoes - destructive forces of nature

Recently there were warnings of tornado conditions in the Matara town. Even though we in Sri Lanka normally do not experience tornadoes, weather officials warned of such conditions developing even on a mild scale due to climatic changes.

* What is a tornado?

A tornado is defined as a violently rotating column of air extending from a thunderstorm to the ground.

Although tornadoes occur in many parts of the world, these destructive forces of nature are found most frequently in the United States. The most violent tornadoes are capable of tremendous destruction, with wind speeds of 250 mph or more.

* Requirements for a tornado to occur

Thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large amounts of hail (pellets of frozen rain falling in a shower), strong winds and tornadoes.

Tornadoes in the winter and early spring are often associated with strong, frontal systems. Occasionally, large outbreaks of tornadoes occur with this type of weather pattern.

* Variations of tornadoes

Some tornadoes may form during the early stages of rapidly developing thunderstorms. This type of tornado is the most common. Tornadoes may appear nearly transparent until they pick up dust and debris. Occasionally, two or more tornadoes may occur at the same time.

Waterspouts

Waterspouts are weak tornadoes that form over warm water. These are most common along the Gulf Coast. Waterspouts occasionally move inland, becoming tornadoes, causing damage and injuries.

* How do tornadoes form?

Before thunderstorms develop, a change in wind direction and an increase in wind speed with increasing height create an invisible, horizontal spinning effect in the lower atmosphere.

Rising air within the thunderstorm updraft tilts the rotating air from horizontal to vertical. An area of rotation, two to six miles wide, now extends through much of the storm. Most strong and violent tornadoes form within this area of strong rotation.

There is an area of rotation known as a rotating wall cloud. This area is often nearly rain-free. Moments later, a strong tornado develops in this area. Soft ball-size hail and damaging 'straight-line' winds also occur with this storm.

* Types of tornadoes

1. Weak tornadoes -

69 per cent of all tornadoes are weak tornadoes. Their lifetime is around one to 10 minutes; wind speed is less than 110 mph.

2. Strong tornadoes

29 per cent of all tornadoes are included in this category.

Wind speed is around 110-205 mph; may last for 20 minutes or longer.

3. Violent tornadoes -

Only two per cent of all tornadoes are violent tornadoes. Lifetime of this type of tornadoes can exceed one hour.

* Myths regarding tornadoes

MYTH: Areas near rivers, lakes and mountains are safe from tornadoes.

FACT: No place is safe from tornadoes.

MYTH: The low pressure associated with a tornado causes buildings to explode, resulting in structural damage.

FACT: Strong winds and debris knocking against buildings is the cause of most structural damage.

* What clues does the environment give us?

You must look out for a dark, often greenish sky, wall clouds, large hail, and a loud roar, similar to a freight train. Some tornadoes appear as a visible funnel, extending only partially to the ground. You must look for signs of debris below the visible funnel.

Janani Amarasekara