At first glance, it may seem difficult to tell a severe thunderstorm from a "garden variety" thunderstorm. There are, however, a number of visual clues which can be used to gain an idea of a thunderstorm's potential strength and organization, and the environment in which the storm is developing. Many of these visual clues are interrelated, but for discussion's sake, we will classify these clues as upper-level, mid-level, and low-level features of the storm which is being observed.

Upper-Level Features

     Most of the upper-level clues are associated with the thunderstorm's anvil. Recall that the anvil is a flat cloud formation at the top of the storm. Air (and cloud material) rising in the updraft reaches a point where it begins to slow down. This level is called the equilibrium level. The air (and cloud material) rapidly slows its upward motion after passing the equilibrium level. As the air (and cloud material) spreads out, the anvil is formed.

     If the storm you are watching has a vigorous updraft, a small portion of the updraft air will rise higher than the surrounding anvil. This will form a "bubble" of cloud sticking up above the rest of the anvil. The bubble is called an overshooting top. Most thunderstorms will have small, short-lived overshooting tops. However, if you observe a storm with a large, dome-like overshooting top that lasts for a fairly long time (more than 10 minutes), chances are good that the storm's updraft is strong enough and persistent enough to produce severe weather.

     The anvil itself will also provide clues to the storm's strength and persistence. If the anvil is thick, smooth-edged, and cumuliform (puffy, like the lower part of the storm), then the storm probably has a strong updraft and is a good candidate to produce severe weather. If the anvil is thin, fuzzy, and glaciated (wispy, similar to cirrus clouds), then the updraft is probably not as strong, and the storm is less likely to produce severe weather. If the anvil is large and seems to be streaming away from the storm in one particular direction, then there are probably strong upper-level winds in the storm's environment. The storm will be well ventilated, meaning precipitation will probably be blown downstream away from the updraft rather than fall through the updraft.

Mid-Level Features

     Most of the mid-level cloud features are associated with the storm's main updraft tower. If the clouds in the main updraft area are sharply outlined with a distinct cauliflower appearance, then the clouds are probably associated with a strong updraft which may produce severe weather. If they have a fuzzy, "mushy" appearance to them, then the updraft probably is not as strong. If the updraft tower itself is vertical (almost perfectly upright), then the storm probably has an updraft strong enough to resist the upper-level winds blowing against it . On the other hand, if the updraft leans downwind (usually northeast), then the updraft is weaker.

     Thunderstorms with good storm-scale organization typically have a series of smaller cloud towers to the south or southwest of the main storm tower. These smaller towers are called a flanking line and usually have a stair-step appearance as they build toward the main storm tower. Some supercells, as their mesocyclones develop, will show signs of rotation in the updraft tower. You may see striations on the sides of the storm tower. Striations are streaks of cloud material which give the storm tower a "corkscrew" or "barber pole" appearance and strongly suggest rotation. A mid-level cloud band may also be apparent. The mid-level cloud band is a ring of cloud material about halfway up the updraft tower encircling the tower like a ring around a planet. This is another sign of possible rotation within the storm.

     As a storm increases in size and intensity, it will begin to dominate its local environment (within about 20 miles). If cumulus clouds and other storms 5-15 miles away from the storm of interest dissipate, it may be a sign that the storm of interest is taking control in the local area. Sinking motion on the edges of the storm may be suppressing any nearby storms. All of the instability and energy available locally may be focused into the storm of interest which could result in its continued development.

Low-Level Features

     Some of the most critical cloud features for assessing thunderstorm severity and tornado potential are found at or below the level of the cloud base. While there is a lot of information to be discerned in these low-level cloud features, most of the confusion (and frustration) associated with storm spotting stems from attempting to interpret these similar appearing but meteorologically distinct cloud formations.

     Perhaps the easiest low-level feature to identify is the rain-free base. As its name suggests, this is an area of smooth, flat cloud base beneath the main storm tower from which little or no precipitation is falling. The rain free base is usually just to the rear (generally south or southwest) of the precipitation area. The rain-free base marks the main area of inflow where warm, moist air at low levels enters the storm. Some have called the rain-free base the "intake area" of the storm.

     We earlier discussed the domination by a storm of its local environment. Besides suppressing any nearby storms or clouds, this local domination can also show itself through the presence of inflow bands, ragged bands of low cumulus clouds which extend from the main storm tower to the southeast or south. The presence of inflow bands suggests that the storm is gathering low-level air from several miles away. The inaow bands may also have a spiralling nature to them, suggesting the presence of a mesocyclone.

     The beaver s tail is another significant type of cloud band. The beaver's tail is a smooth, flat cloud band which extends from the eastern edge of the rain free base to the east or northeast. It usually skirts around the southern edge of the precipitation area. The beaver's tail is usually seen with high-precipitation supercells and suggests that rotation exists within the storm.

     Lowerings of the rain-free base and "accessory clouds," such as shelf clouds and roll clouds, mark important areas of the storm. The next chapter will discuss wall clouds and other lowerings in more detail.

Wall Clouds and Other Lowerings