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Obviously we can't put as much material as we would like into the web site. We can however highlight some of the basic principles of what we expect spotters to look for during severe weather days.
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Many summer severe weather days begin with cumulus cloud that may develop as early as 10-11 am. A general "rule of thumb" suggests that cumulus cloud forming during the early morning hours may produce thunderstorms during the afternoon.
Cumulus clouds are characterized by small to medium vertical extent caused by convection with consistent flat bases sometimes darkened or shadowed by the clouds above them. These are quite often called "popcorn cumulus" for obvious reasons. In this picture you can see both cumulus clouds and higher cirrus clouds. |
| As daytime heating occurs, convection increases causing cumulus to grow into taller towers. Not all cells grow at the same rate as you can clearly see in this image. The type of terrain beneath the cloud will often impact on it's growth. Rapid growth can be a sign that the atmosphere is ripe for thunderstorm develop. |  |
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The cumulonimbus cloud (CB). This is the cloud that spotters normally watch for knowing it may or may not produce severe weather. This CB which formed in Alberta was an isolated event which produced golf-ball sized hail. Note the cauliflower appearance near the top. This type of crunchy detail is common with rapid growth CBs. Also note how the anvil has reached the level of the tropopause and has spread outward forming a large flange. Some thunderstorms do not possess enough vertical strength to punch through the tropopause and subsequently spread the energy sideways. Ones that do can often go on to produce severe weather events. |
| In this picture the main cell at the rear of the picture has developed quickly to it's full potential. It may continue this stage for quite some time, or it may collapse. Note the towering cumulus in the foreground. Sometimes a collapse of one storm can help propagate another. The towering cumulus in the foreground may grow on it's own and possibly join forces with the one in the background, or it may develop rapidly if the rear storm collapses sending large downdrafts in the direction of the new storm. |  |
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This picture taken was taken in Alberta and demonstrates some of the features we look for. Note the rain free base at the lower left of the shot in opposition to the heavy rain area to the right of it. The heavy rain area is normally coincident with the tallest towers. At the centre just below the tallest towers is a lowering called a wall cloud. It has taken on a characteristic pedestal appearance. Wall clouds are distinct lowerings, often dark in appearance and frequently shows signs of rotation. The wall cloud is a feature which frequently precedes a tornadic event and subsequently should be reported. |
| Because of Ontario's geographical features we aren't always as fortunate to see storms with this type of scope and clarity. This shot from north-central Texas (looking WNW) again demonstrates large storm cell features. Rain-free base to the left....heavy rain and hail area to the right. The intersection of the rain and rain-free base is quite often where wall clouds will form directly below the tallest towers. To the left of the image can be seen flanking clusters of cumulus cloud and bright sunshine off to the west. |  |
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Note the remarkable similarity in this image to the Doswell visual schematic (Doswell, 1985) of a tornadic thunderstorm shown on the right. Study the features of these images and try and relate them to storms you may encounter. |  |
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In this photograph, a wall cloud is trying to emerge from the large multi-celled storm system. The wall cloud is ragged, dark and did show signs of weak rotation. In this case, the wall cloud eventually weakened, possibly due to competition from another storm complex. Where wall clouds are concerned, watch for storm cell position and signs of rotation for a more comprehensive report. |
| Another ragged wall cloud. There was strong inflow with this cell and there were signs of rotation. The scud cloud (ragged, dark bits and pieces of cloud that appear and disappear) at the centre-right just to the right of the highway indicates rotation. This is far too close for spotters to observe such features safely. Watch your distance. Environment Canada does not endorse "chasing" - but rather observing and reporting. |  |
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The Shelf Cloud. Shelf clouds are an indication of outflow, normally where advancing cold air meets warm air, ploughing forward forcing it to rise. The shelf cloud takes on the appearance of a wedge or circular flange normally at the leading edge of the approaching storm. The flange can be smooth or ragged depending on the stability of the air. As storms gain strength as they move forward, they are often called "front-loaders". Squall lines are one such storm and are often accompanied by a shelf cloud. |
| The Roll Cloud. Where cold air moves into the region of warm air on an outflow boundary, it may roll (sometimes along its own axis) forming the cloud you see here. Also a shelf cloud can become detached from the leading edge of an approaching system forming a roll cloud. Roll clouds are normally seen at the leading edge of an approaching storm system. |  |
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Another dramatic example of a roll cloud. This shot taken in Colorado shows how the roll cloud is detached from the main cloud. This and the shelf cloud should not be confused with a wall cloud. |
| Mamma. These cloud formations usually occur on the rear side of the storm in the overhanging anvil although they can be seen at the leading edge as well. These elements can appear threatening to a fledgling observer, but they don't guarantee severe weather. The breast-like appendages in this image are highlighted by the sun. The explanation for mamma is not completely clear. |  |
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What tornado training page would be complete without a funnel. But since we are training, take note that not all funnels show dark connections to the parent cloud. This photograph taken on the north-east side of Denver CO shows debris at the surface and an incomplete funnel. Tornados can form before the actual funnel becomes visible - the debris can be the first sign. |
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