TORNADOES AND FUNNEL CLOUDS

A tornado is a tightly spinning column of air in contact with the ground beneath a thunderstorm cloud. It may be visible as a condensation funnel or as a swirl of dirt (debris cloud) on the ground. By comparison, a funnel cloud spins in mid-air and is not touching the ground.

Funnel clouds and tornadoes are relatively rare but there's no reason to assume one cannot happen with a severe storm. Of the 50-70 average annual tornado sightings in Canada, most are first seen and reported by spotters or the public, and they may occur without being previously forecast. The search for this complex,unusual event offers you a most exciting and rewarding challenge. For the sake of reporting and because of the potential threat to people, tornadoes and funnel clouds are grouped together.

How and why they form

It takes an exact set of conditions for a storm to become tornadic. Key storm characteristics are an intense, sustained updraft, a rightward turning of winds with height (e.g. SE at the surface and SW aloft), and strong winds at cloud-top height. The storm's own circulation interacts with the environmental flow to create a structure that can lead to a tightly rotating vortex within the cloud.

91 Spotting tornadoes at a distance is harder than it looks. Even without haze, showers, or low cloud bases, any distant object in the sky quickly becomes tiny beyond about 10 km. You will have to depend on knowing exactly where to look for it in the storm structure. Here, a tornado (light column, bottom centre) is barely visible on the left side of a low wall cloud. The flanking line (darker axis, centre) trails away from a large mesocyclone (note striations, right half) and the RFD has cleared a slot of bright sky behind the storm (left). This F2 tornado followed soon after photo 86.

There are two main tornado types - weak and strong. Weak ones are brief events causing only limited damage. They are formed primarily by the tightening-up of a rotating updraft as the storm intensifies to a maximum,and will be found right under the updraft core,sometimes without a significant lowering. They are more common on the Prairies or with mid-summer storms. This form of "simple "rotation accounts for several tornado variations (see below)and most funnel sightings, including all cold-air funnels.

There is a much stronger variety that occurs with supercells.The updraft becomes a mesocyclone, or rotating column,within the cloud which you see at the bottom as a rotating wall cloud. The mesocyclone derives its rotation from a complex process in which two forces combine. First, positive vorticity is transferred from inflow air to the updraft column. Vorticity refers to the ability of a fluid (air) to rotate about an axis. The initially horizontal inflow is sheared forward and rightward (higher parts move faster and more to the right), causing a "rolling-pin" motion in the flow.When the air turns vertical in the updraft, the rollers angle upward too. This causes the column to rotate slowly as a mesocyclone with a diameter of about 2-5 km. The potential for rotation is highest when the air entering the storm turns sharply to the right with height (undergoes directional wind shear). That's why storms moving from the W-SW with a SE-E inflow are more likely to rotate than those with inflow that is aligned with the flow aloft.

PREV TOP NEXT

Meteorological Service of Canada - Environment Canada - Government of Canada

Graphics : [Turn off] | Formats : [Print] [PDA]