Severe Weather Watcher Handbook

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Once formed, a tornado will reach maximum intensity fairly quickly. Weaker and stronger phases may alternate and the funnel cloud may lift briefly or change in diameter. At peak intensity, you will usually see a dark mass of debris at the base of the tornado, with the debris revolving upward in a tightly controlled fashion. Low, wide tornadoes can be spread out,with several apparent contact points or separate vortices within a broad circular region which show both debris and condensing cloud matter mixing as they ascend.

In the waning stage,the tornado lifts and becomes thinner. Debris decreases suddenly and become diffuse or spreads out horizontally. The condensation funnel shrinks upward and becomes twisted, curved, or less vertical in orientation. It continues to shrivel in this rope stage until it vanishes or retracts into the cloud base. The wall cloud also loses structure and size but may rebuild again in a while.

What it's like near a tornado

If a tornado is nearby and moving closer, forget about the sightseeing and take immediate measures to protect yourself! (See Safety Tips, inside back cover.)

Tornadoes move with the storm cloud, at the same speed and direction, typically 30-50km/hr from the SW or W. Their motion is, therefore, reasonably slow and predictable. Two factors can exist to alter this regular movement: the wall cloud can be rebuilding (often eastward), causing the tornado, especially if it is wide, to also shift and reform forward in steps; and, strong, persistent tornadoes can move along a looping path where straight motion is replaced for short periods by a tightening-up and northward twist in the motion. Tornadoes are highly organized and quite well-behaved structures, and there are rarely any sudden changes or surprises in the direction of movement.

Contrary to popular belief, it is not windy near most tornadoes. Within a few kilometres, the air will be unusually calm since the storm's giant updraft is overhead. Although weaker tornadoes tend to begin and end abruptly, some strong tornadoes induce a steadily increasing wind outside of the destructive vortex itself. Even without strong winds, it is dangerous outside near a tornado because pieces of debris can fall from the sky without warning (most likely NW-NE of the tornado).

In any one place, the severe tornadic wind will last from several seconds to, at most, two minutes and be accompanied by a variety of sounds caused by the damage to buildings, trees, etc. Some people also hear a roar, but many tornadoes have no sound with them at all. If the sound is irregular it is likely the result of damage occurring nearby; but if it is a steady, softer sound that originates in the sky or toward the main storm cloud, it is more likely caused by large hailstones hitting the ground or colliding in mid-air.

Most tornadoes occur near the back side of a worst-last storm, near a brightening sky to the west, and without any precipitation. Heavy rain and hail may precede its arrival but once the updraft core is overhead, only a few scattered hailstones will interrupt the eerie calm. After its passage, some light rain or hail can occur briefly,along with cooler W-NW winds,before clearing takes place.

Other rotating structures

There are several other rotating structures to watch for and report which do not form under a typical wall cloud or storm updraft base.For all of these, your description of their location in the storm, associated features, and general situation will help the weather office determine the true risk and the need for further action.

Cold-air funnels

Cold-air funnels are true funnel clouds by appearance but will not lead to a damaging tornado. They are usually brief and thin, and often occur under the rain-free base of weaker storms or large cumulus during relatively cool, moist weather from spring to fall. A common situation,seen more often on the Prairies, is a spell of cool, windy, unsettled weather in summer. Large cumulus fill the sky and showers are scattered about. Some of the larger clouds will have ragged bases and bits of scud below them. If an updraft is particularly strong, the rising air can tighten into a small vortex which you see as a funnel. A brief, weak touchdown is rare, but possible. The "cold-air" refers to the situation,in which the temperatures aloft are relatively cold for the time of year.A special weather bulletin is issued by MSC for these funnels because they can look so much like a tornadic funnel.

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