Australian Storm Spotters' Guide
4. Assessing severity from storm features
Once a thunderstorm develops, various features can be assessed as a guide to its potential severity. The anvil, for instance, can tell us many things about the age, strength and organisation of the storm system. Unevenness on top indicates erratic growth, and a diffuse edge suggests weak updraughts, hence a weaker system. This photograph (4.1) from Brisbane is a good example of a weak, fibrous anvil from a non-severe storm.
The anvil on a long-lived, regenerating storm may be very extensive, with small notches or dents along the edge which correspond to the separate updraught pulses. Notches can be seen here (4.2) on the right edge (or front) of this thunderstorm in Wellington, New Zealand.
The top of an anvil is normally restricted by the tropopause (the stable layer at the top of the weather-producing part of the atmosphere) and blown forward on strong winds aloft. However, when the main updraught within the storm is very strong, a portion of the anvil may push upward above the general level. This feature is known as an overshooting top and can be seen above on the top left of the photograph as an upward "bulge".
When the overshooting top becomes prominent, as in this case, and persists for longer than several minutes, it can be regarded as an excellent sign of storm severity. Other characteristics of severe thunderstorms illustrated well in this photograph (4.3) near Tamworth, NSW, are:
Advancing outflow air acts like a plough, mixing the cool, moist air at its boundary with warmer inflow air and forcing it to rise. As we have seen, this can result in the spectacular low cloud bank called shelf cloud (or arcus) near the leading edge of the storm. The shelf often has a smooth, laminar or banded surface and black, turbulent base. Here (4.4) we see a smooth shelf cloud near Tewantin, Queensland. The rain curtain beneath the core can be seen in the left background. Very humid conditions will promote a thick, low cloud bank. A sharp, strong gust front will cause the lowest part of the leading edge of the shelf to be ragged and lined with rising scud cloud.
Wind squalls may also be generated by downbursts, concentrated, severe downdraughts usually accompanied by a descending deluge of precipitation. These induce an outward (horizontal) burst of damaging wind at the surface. On a smaller scale, this wind feature is known as a microburst. The rain curtain below this storm (4.5) has a "foot" which extends to the right, close to the ground. Outwardly curved rain shafts such as this, are a good sign of strong downburst or microburst winds and the steeper the angle, the stronger the flow. Note the rising rain or dust well to the right. (Tamworth, NSW)
There are other clues to storm severity that can be found in the rain curtain. If it is
dark and smooth, such as in this photograph (4.6) near Tamworth, very heavy rain is
likely. Rainfall in severe storms will generally become progressively heavier, and
sometimes mixed with hail whilst in weaker storms, rainfall will be patchy or confined to
One small cloud feature which is particularly valuable in assessing a thunderstorm's
severe potential, can sometimes be found beneath the rain-free cloud base toward the rear
of the storm. This localised cloud-base lowering occurs at the site of the main, focused
updraught into the system. It forms when cool, moist air from the rain area, is drawn into
the updraught and condenses below the main cloud base in a process similar to that which
forms shelf cloud at the leading edge of the storm.
Before moving on to discuss tornadoes in more detail, we will summarise the storm features seen so far that may give indications of thunderstorm severity. It must be stressed that these are indicators only and should not be used as the basis of storm spotter reports.
Assessing severity from storm features
This diagram (5.8) of a supercell thunderstorm illustrates many indicators of storm severity that we have discussed. The storm is moving towards the left of the screen and we are viewing it from the northeast looking towards the southwest.
Updraughts along a flanking line of cumulus clouds, lead to a solid, boiling cloud mass which is the main core of the storm. The anvil above is high and crisp, with an overshooting top and back-sheared rear edge. Beneath the storm, precipitation falls in a dark, smooth rain curtain which becomes progressively heavier towards the core. The boundary between storm inflow and outflow is marked by a low ragged shelf cloud, whilst beneath the core, a tornado touches down below the wall cloud.