Photo 2. Thunderstorm at Larapinta, Northern Territory.
Photograph courtesy of Steve Strike.
A thunderstorm is associated with a very tall cloud mass, a cumulonimbus cloud, that has a flat, dark base from which heavy rain and hail can fall (Photo 2). When not obscured by haze or other clouds, the top of a cumulonimbus is bright and tall, reaching up to an altitude of 10-16 km (lower in higher latitudes and higher in the tropics). The top may appear to be 'boiling' with cauliflower-shaped lumps but more often has a fibrous, frozen appearance.
Although a thunderstorm is a three-dimensional structure, it should be thought of as a constantly evolving process rather than an object. Each thunderstorm, or cluster of thunderstorms, is a self-contained system with organised regions of updraughts (upward moving air) and downdraughts (downward moving air). Their movement within the cloud and interaction with prevailing winds at various heights in the atmosphere form changing cloud features that you can see and interpret. The whole process is an example of convection, which acts to distribute energy more evenly in the atmosphere.
Three Main Parts
Every thunderstorm cloud has a core region, a spreading anvil top, and an inflow-outflow region. The core is that part of the cloud where sustained strong updraughts of relatively warm and moist air condense to produce rain, hail and/or snow (collectively known as precipitation) and associated downdraughts (Photo 4). Underneath the core we see a rain curtain, whilst above it the tallest part of the thunderstorm can be found. The dark flat cloud base that extends away from the core (usually to the west or north) is called the flanking line (Photo 3) or rain-free base, along which air fuelling updraughts into the thunderstorm rises in successive cumulus towers.
Photo 3. A typical thunderstorm cloud with flanking line on the right and crisp anvil
above and to the left. Storm motion is towards the left foreground near Tamworth,
New South Wales. Photograph courtesy of Gordon Garradd.
Photo 4. Crisp, cumuliform anvil indicating strong updraught.
Photograph courtesy of Ian Forrest.
The anvil is a flat, often fibrous cloud sheet, above and usually ahead of the core. It consists mostly of frozen particles that have been carried away from the thunderstorm's core by the stronger winds high in the atmosphere.
The boundary between the warm air entering the thunderstorm and its cool outflow is known as the inflow-outflow region. This region can shift around over time and varies greatly from one thunderstorm system to the next. The leading edge of the outflow air is often marked by a cool, gusty wind change known as the gust front. This can spread forward far ahead of the core when surface winds are blowing roughly in the same direction as the thunderstorm motion. In other situations it moves with the thunderstorm, arriving abruptly as the core passes, or it can be left in the thunderstorm's wake as a cool breeze flowing away from the thunderstorm.
Figure 1. Diagram illustrating severe thunderstorm features.
Types of Thunderstorms
There are three general thunderstorm types: the single-cell, the multicell and the supercell. Each has a distinct structure, circulation pattern, and set of characteristics. Thinking about their differences can help you interpret the clouds you see.
A single-cell thunderstorm is one whose life cycle is limited to the growth and collapse of a single updraught pulse. The cloud forms, grows to maturity, produces a heavy downpour, then decays as downdraughts suffocate and dilute the original warm inflow. Such thunderstorms are common in mid-summer and usually last no more than an hour. They almost never produce severe weather.
The multicell thunderstorm type is the most common and consists of successive, separate updraught pulses that help maintain the system's overall strength, structure and appearance. The pulses may be very close together, causing the thunderstorm's characteristics to be quite uniform over time, or they may be widely spaced, resulting in a thunderstorm that cycles repeatedly through stronger and weaker phases. The updraught pulses can be seen from a distance by watching the spacing and growth rate of individual cumulus towers along the flanking line. Multicell thunderstorms can produce any of the severe weather associated with thunderstorms, but cause tornadoes only infrequently.
The supercell is a special thunderstorm type in which the system can maintain an intense steady state for many hours. A highly organised circulation with a continuous large updraught, control over the surrounding atmosphere and magnified size and impact make this a fascinating but dangerous cloud complex. Supercells account for most of the serious thunderstorm events we experience.
Ingredients for a Thunderstorm
To produce a thunderstorm the atmosphere needs the right ingredients. These include moisture (sometimes indicated by low clouds or haziness in the morning and/or many cumulus clouds later) (Photo 5), atmospheric instability to make the atmosphere more buoyant (often recognised by the presence of altocumulus castellanus, or turreted middle-level clouds, early in the day), and a lifting mechanism such as heating or the approach of a front or low pressure trough.
The most likely severe weather days will have two essential factors in place. First, the atmosphere will be unstable enough to permit very strong updraughts to rise rapidly into colder air aloft. Second, the winds aloft are sufficiently strong to carry much of the leftover cloud matter well downwind and out of the way of warm air entering the system from below. The variation of the horizontal wind direction and/or speed with increasing height is known as wind shear. It plays a significant role in extending the thunderstorm's life and its ability to produce strong winds, large hail and tornadoes.
Photo 5. A field of small cumulus clouds prior to strong convective
development over the Apollo Bay coastline, Victoria, 21 January 1976.
Photograph courtesy of Gordon E. Tralaggan.
Copyright Eleanor Joan Tralaggan.
A severe thunderstorm usually requires more than just surface heating to initiate its updraught. The upward 'push' of air from near the surface is usually brought on by an approaching cold front, low pressure trough, or smaller-scale disturbance. Occasionally, the atmosphere has a low-level stable layer in place, which prevents strong convection (and sometimes even small cumulus) until later in the afternoon. When surface heating reaches a critical point, or the layer is otherwise weakened, sudden and explosive cloud development can occur. Thunderstorms which break out in this abrupt fashion are more likely to be severe because all the day's ingredients and energy are available for an immediate, concentrated release.
Conditions conducive to severe weather events vary greatly across the country. In South Australia and Victoria many severe summer thunderstorms accompany cool changes sweeping in from the west or southwest, while in Western Australia, New South Wales and southeast Queensland many severe thunderstorms develop in warm, moist north to northeast flow into a surface trough. Wintertime severe thunderstorms in southern parts of South Australia and Victoria and the southwest of Western Australia are usually associated with the passage of vigorous cold fronts.
On a typical summer thunderstorm day, the sky will have only small cumulus clouds around until early afternoon. A few may become briefly larger but their tops soon evaporate. At some point the number of clouds decreases, and fewer, larger ones remain. Several may sprout tall towers (towering cumulus) or a group may form and grow larger. Then a 'boiling-over' stage begins and bigger, steadily growing cloud masses will emerge. It is during this stage that organised, sustained updraughts take over from weaker, random ones. In 10-20 minutes, the developing cumulonimbus cloud will form a short anvil and become a young thunderstorm.
Figure 2. The sequential development of a thunderstorm cell.
This first thunderstorm will probably have a single updraught at its core, but for the organised multicell thunderstorm there will already be signs of the next stage to come. As heavy rain descends to the ground, the inflow region is shifted so that the next developing cumulus tower along the flanking line becomes the dominant one. The thunderstorm has now become a separate, evolving process that survives by regeneration. Most multicell thunderstorms will maintain themselves by this process for up to an hour or more, before the cool outflow finally smothers the warm inflow, causing the system to collapse entirely. Supercells, on the other hand, maintain an orderly balance between the warm inflow and the cool outflow for much longer periods, and may exist in a steady state for many hours.
Day-to-day circumstances and local factors can alter these ideal or typical situations. For example, in eastern Australia many thunderstorms form preferentially along the ranges where lifting and heating by the sun are enhanced by the terrain. Near the coast, thunderstorms can form along the sea-breeze front, the leading edge of a cool sea breeze blowing inland away from the water. Also, the approach of a cold front or low pressure trough may bring layers of middle cloud that delay thunderstorm development or make spotting convective clouds difficult.
Movement and Change
The way a thunderstorm moves is the result of its growth pattern, in combination with the motion caused by the winds carrying the cloud along. Weaker thunderstorms travel with, or slightly to the left of the average wind in the cloud layer but stronger thunderstorms can move almost at right-angles to the flow. The deviation serves to enhance inflow into the thunderstorm and is always toward the side where new updraughts are adding to the system.
Every Thunderstorm is Unique
Whilst all thunderstorm clouds show similarities, each one is unique. As we have seen, they extend both vertically and horizontally and this produces an almost unlimited number of possible shapes and sizes to observe. The moisture content of the inflow air affects the height at which the cloud base forms, and, coupled with atmospheric instability, the depth to which the cloud system will extend upward. Also, the cloud is subjected to different winds at every level, which affect the way it leans with height. There are many variables contributing to the outcome. Changing even one of them alters the recipe and the visible result.
Next: Observing Thunderstorms