Tornadoes

Once a wall cloud is identified, the next step is to determine its tornadic potential. Wall CLouds can form at the base of any thunderstorm - supercell or otherwise - with a sufficiently strong updraft and adequate moisture available in the layer below the cloud.

Tornadic Wall Cloud Characteristics

• Persistence - Wall Cloud will exist for 10 to 20 minutes before a tornado appears.
• Persistent Rotation (more than 10 minutes)
• Strong Surface-Based Inflow - winds will blow in toward the Wall Cloud from the East or Southeast. Typical speeds of 25-35 mph are observed near tornadic wall clouds.
• Rapid Vertical Motion - Small cloud elements near the wall cloud will quickly rise up into the rain-free base.

Tornado Life Cycle

A tornado typically evolves through a three stage cycle:

• Developing Stage - a rotating wall cloud is evident, with tighter rotation in the base of the cloud. As the tornadic circulation continues to develop, the condensation funnel appears.
• Mature Stage - a good inflow of warm, moist air continues into the storm. The circualtion is at a maximum diameter and the intensity is a maximum as well.
• Dissipating Stage - sometimes called the "rope stage." The condensation funnel becomes tilted and shrinks into a contorted rope-like configuration. Remember: the tornado is still dangerous, even at this late stage.

Tornado Intensity Classification

Tornado intensities are classified by the Fujita Damage Scale, developed by the late Professor T. Theodore Fujita of the University of Chicago.

Fujita Tornado Intensity Scale

• F0 (weak) - wind speed 40-72 mph - Damage light - Frequency 29%
• F1 (weak) - 73-112 mph - Moderate - 40%
• F2 (strong) - 113-157 mph - Considerable - 24%
• F3 (strong) - 158-206 mph - Severe - 6%
• F4 (violent) - 207-260 mph - Devastating - 2%
• F5 (violent) - 261-318 mph - Incredible (rare) - <1%

Something to note is that the difference between a funnel cloud and a tornado, is that a funnel cloud does not extend more than half way to the ground. However, funnel clouds are called tornadoes if there is debris along the ground.

Tornado Look-alikes

Another significant challenge to the spotter is correctly discriminating between a tornado and a tornado "look-alike".

• Rainshafts - Lack of debris cloud near the ground and organized rotatioon about a vertical axis.
• Smoke Column - From a distance, a smoke column presents a difficult challenge. However, watch closely for rotation in the cloud and smoke.
• Scud clouds typically are not attached to the rain-free base as are wall clouds, funnel clouds, and tornadoes. And again, loof for rotation.
• Shelf Clouds - Shelf clouds do not ahve considerable motion. But unlike wall clouds and tornadoes, shelf clouds lack persistent, organized rotation.

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