Twister Sisters

Weather reports in western Canada may sometimes include references to wall clouds, funnel clouds, cold air funnels, waterspouts or tornadoes. Below are descriptions for these and other related types of weather phenomena, and the conditions under which they typically develop. Treat all such events seriously and take appropriate safety precautions if they are occurring or expected in your area.

Wall Clouds

A wall cloud is a portion of a thunderstorm cloud that appears to hang underneath the main cloud base. It is associated with the main updraft of the storm. If the wall cloud persists for more than a few minutes and appears to rotate, it is a sign of a possible tornado, although only about 10-15% of rotating wall clouds generate tornadoes.

Tornadoes And Funnel Clouds

A tornado is a tightly spinning column of air in contact with the ground beneath a thunderstorm cloud. The rotating column is physically connected to the cloud base or wall cloud and is often visible as a cloud-filled "condensation funnel". If the air is dry enough, the tornado may only appear as a swirl of dirt on the ground without a visible connection to the cloud above.

In contrast, a funnel cloud spins in mid-air without touching the ground. To tell the difference between a funnel cloud and a tornado, look for swirling dust or debris near the ground under the funnel. If present, then a tornado is occurring. Most tornadoes and funnel clouds are preceded by and form approximately in the middle of an accompanying rotating wall cloud. The Fujita scale (F0 to F5) is used to rate the severity of tornadoes after they occur by the extent of the damage they cause. The parent storms often are observable on Doppler radar and can be forecast.

Cold Air Funnel Clouds

These funnel clouds are spawned by large cumulus clouds or weak thunderstorms. Typically, the days when they occur are a bit cooler than normal with large puffy cumulus clouds, showers or weak thunderstorms developing in the late morning or early afternoon. Since the storms are not very strong or well organized, cold air funnels are usually short-lived and normally do not have the energy to reach the ground. However, a small percentage may touch down briefly and can become destructive over a very small area. In general, they are less violent than most other types of tornadoes. These funnel clouds will normally appear with little or no warning. Meteorologists can predict the days when cold air funnels are likely to occur over a general area. When they are predicted or sighted, a special tornado watch will be issued that outlines the need for caution but recognizes their weak and short-lived nature.

Landspouts

Landspouts are a form of tornado that occurs in most parts of Canada. Landspouts usually have a narrow, rope-like condensation funnel extending from the cloud base to the ground, and are seen under small storms or large, growing cumulus clouds. They are similar to a cold-air funnel that has touched down, and while usually weak and short-lived, they can be potentially dangerous. Like their cold air cousin, landspouts can form from rather nondescript developing cumulus clouds or thunderstorms, often before precipitation is visible on radar. Specific tornado warnings are issued when landspouts are predicted or reported.

Waterspouts

The waterspout looks like a slender tornado but occurs only over water, or the nearby shore, in the same conditions that bring cold-air funnels or landspouts. Cool, unstable air masses passing over warmer waters allow vigorous updrafts to form, which can tighten into a spinning column when captured by a passing thundershower. Waterspouts are just as dangerous on water and shoreline areas as landspouts are on land, but often collapse after moving a few hundred metres inland, away from the warm water. A true waterspout forms over the water and is not accompanied by a strong storm. If conditions are cool and cloudy, with showers but no organized storms, then the appearance of a tornado-like funnel over water can be identified as a waterspout. If a severe storm with a tornado happens to pass over a stretch of water, the tornado is sometimes called a tornadic waterspout and would be just as dangerous as a tornado is over land.

Gustnadoes

"Gustnadoes" typically appear as a swirl of dust or debris along the "gust front" of a thunderstorm. They are not directly linked with rotation in the thunderstorm itself and can form a considerable distance away from the parent storm. There is no condensation funnel or other visible connection to the cloud base. Gustnadoes account for a large number of the weakest tornado reports each year. Their localized impact and damaging effects have allowed them to be counted as tornadoes but most are probably not "true" tornadoes. The strong, straight line winds that can follow behind the gustnadoes are likely to cause more damage than the gustnadoes themselves. Gustnadoes are not visible on Doppler radar. Meteorologists do not forecast gustnadoes, but can issue Severe Thunderstorm Warnings for the gust fronts of stronger storms that are detectable on Doppler radar.

Straight Line Winds

A "plough" wind is a strong, straight-line wind associated with downdrafts that spread out ahead of isolated thunderstorms or small clusters of thunderstorms. These winds push across the ground like a blade in front of a snowplow or bulldozer, damaging susceptible objects and scattering the debris. The damage caused is often mistakenly attributed to a tornado. The wind may roar as it passes by. Damage can be heavy and confined to narrow zones like that caused by tornadoes. Plough winds (and the damage they cause) are all in one general direction, rather than rotating as in a tornado. A plough wind is termed "severe" if it exceeds 90 km/h. "Derechos" are areas of greater wind damage associated with clusters of intense thunderstorms or lines of closely-spaced thunderstorms. They are longer-lived and more intense than plough winds.

Dust Devils

On sunny, dry days, heated air near the ground can rise in small, spinning columns. If these columns occur over dusty ground, a dust devil may be observable. The stronger ones become visible when loose grass, hay or dust gathers into the whirl and rises up the column. These dust devils can look like a weak tornado at the bottom but rarely extend higher than 100 metres. They are only seen in fair weather - sometimes without a cloud in the sky. Larger dust devils can extend hundreds of metres high, toss lawn furniture and lift objects weighing a hundred kilograms, but are generally not a threat otherwise. Dust devils near a highway deserve caution as vehicles passing through them can be difficult to control. Like other weak circulations, meteorologists can tell which days and general areas are most likely to have dust devils, but they cannot be forecast and are not observable on Doppler radar. Severe Weather Warnings are not usually issued for dust devils.