• Frontal Thunderstorms: are formed as a fast moving cold front creates instability at the boundary of two air masses. Numerous thunderstorms develop very quickly along the edge of the front with little separation.
• Orographic Thunderstorms: are created as a result of moist unstable air been forced up a mountain.
• Cold Stream Thunderstorms: are caused by a mass of cold air flowing over a warm surface. This generates instability in the lower layers as the cooled air mixes with the warmer surface air.
• Nocturnal Thunderstorms: form over the warm ocean at night. The tops of clouds cool by radiating their heat out into the atmosphere. The bottoms of the clouds stay warm due to their close proximity to the warm ocean. A steep environmental lapse rate causes unstable conditions, creating large cumulus clouds and thunderstorms to develop.
• Air Mass Thunderstorms: are more isolated compared to frontal thunderstorms. They are characterised by the lifting mechanism that triggered their development.

Thunderstorms form when there is significant condensation, resulting in the production of a wide range of water droplets and ice crystals. Thunderstorms occur in an atmosphere that is unstable and supports deep, rapid upward motion. They require three conditions: sufficient moisture accumulated in the lower atmosphere, reflected by high dewpoint temperatures, a significant fall in air temperature with increasing height, known as a steep lapse rate; and a force such as mechanical convergence along a cold front that will focus the lift.

There are four main types of thunderstorm, single-cell, multi-cell, squall line and super-cell. Which type forms depends on the instability and relative wind conditions at different layers of the atmosphere ("wind shear"). Vertical wind shear is a critical factor in the determination of thunderstorm types and potential storm severity. Vertical shear, or the change of winds with height, interacts dynamically with thunderstorms to either enhance or diminish vertical draft strengths.

Single Cell Thunderstorm

The ordinary or single cell thunderstorm is nothing more than ephemeral bursts of convection. Each burst creates a towering cumulus or "cell" of convection. Once the cell gets large (and tall) enough, it is classified as a cumulonimbus, or thunderstorm. The updraft and downdraft of this small cloud interfere with each other, resulting in a fairly short-lived cell, but the downdraft from one single cell may give birth to a new cell nearby. This "parent-daughter" effect allows an ordinary thunderstorm to pulse with convection for up to one hour.

When a single cell thunderstorm does form and produces severe weather, it is called a "pulse" severe storm. These often form in a more slightly unstable environment, and tend to have a more intense updraft. If you ever get caught under a pulse severe storm, one can expect lightning, marginally severe hail and brief microburst. Winds can be completely storm dependent, but a drier sub-layer would increase confidence of wind potential.

Because of the poor organisation of these storms the forecaster may find it very difficult to forecast when and where they may occur. Most storms of this type occur when CAPE (Convective Available Potential Energy) values are between 1500-2000 J/kg and the vertical shear in the atmosphere is less than 30 knots.

Multi-Cell Cluster Thunderstorm

This is the most common type of thunderstorm. The name multi-cell simply means that there are many cells that grow to form a group of cells that move together as one unit. A multi-cell cluster thunderstorm results from a vigorous parent-daughter effect. In this case, an ordinary thunderstorm creates neighbouring storms via the downdraft and gust front. If the surrounding atmosphere is unstable and moist enough (and with adequate low-level vertical shear), new cells will be created around the old one, giving birth to a cluster of active thunderstorms.

Each cell will be in a different phase of the life cycle. Each individual cell will have its turn to be the dominant one. New cells tend to form on the western or south-western edge of the cluster, while the more mature cells are usually found in the centre. The dissipating cells are often found on the eastern or north-eastern edge of the cluster. The multi-cell cluster storms last a bit longer than ordinary or single cell, storms and cover a larger area. Each cell in a multi-cell cluster storm usually lasts about 20 minutes, but the cluster of storms itself can persist for several hours.

A mesoscale convective complex is a run-away multi-cell thunderstorm. It lasts at least 6 hours (sometimes much longer) and cover over 100,000 square kilometres. A Multi-Cell Cluster is large enough to actually alter the mesoscale or even synoptic scale environment. By far, the greatest threat posed by a Multi-Cell Cluster is the rain... counties or states under the Multi-Cell Cluster typically experience both volume and flash flooding.

Because of multiple storm interaction it is rare to get extreme weather (large hail or tornadoes) from a multi-cell cluster thunderstorm, but heavy rain, strong winds, and small to medium sized hail are still a threat.

Squall Line Thunderstorm

These types of storms tend to form in long lines with a well-developed cold front at the leading edge of the line. Oftentimes there will be gaps or breaks in this line. Typical lines of thunderstorms tend to have the strongest most, mature storms in the north or northwest end of the line. Moderate storms still not fully mature typically are found in the middle of the line and the dying variety is found in the south or southeast section of the line. On RADAR, bow echoes can be found denoting areas of strong straight-line winds.

As the gust front moves forward, the cold outflow forces warm unstable air into the leading edge (usually on the eastern side) into the updraft edge of the storm, with the heaviest rain and largest hail just behind (west) the updraft. As these cells die they will produce lighter rains behind the main squall line, so one often sees a broad region of stratiform rain behind it.

Squall lines are prolific downburst producers. Occasionally an extremely strong downburst will accelerate a portion of the line forward causing it to form what is called a bow. On the leading edge of this bow formation is where you will find the most damaging winds. Tornadoes will sometimes form just to the north of this bow, or on the line's southernmost cell, which is called the anchor cell. Cells that form on the southern ends of squall lines tend to inherit super-cell characteristics since there is nothing to the south of them to disrupt air flow being entrained into them.

Super-Cell Thunderstorm

Finally, the epitome of thunderstorms is the super-cell. This type of storm is the classic cumulonimbus tower with the anvil top and on occasion the overshooting updraft tower. These storms are notorious for producing damaging straight-line winds, frequent lightning, flash floods, large hail, and violent tornadoes.

This is a relatively small stand-alone entity that is so well organised it actually supports itself and enhances its own growth. The base of the storm may be only 20-50 km across, but the anvil aloft can stretch for many hundreds of kilometres. Conditions have to be just right in the atmosphere in order for super-cells to form. This may make them somewhat easier to forecast than others. These storm types exist with CAPE values of 2500+ J/kg and vertical wind shear values in excess of 50 knots.

The super-cell is unique because the updraft actually rotates; this rotating updraft is called a mesocyclone. As mentioned earlier, organisation within the storm is incredible. The updraft and downdraft regions do not interfere with each other like in the ordinary thunderstorm; instead, they collaborate to prolong the life of the storm. The inflow, outflow, updrafts, and downdrafts all work together like a living, breathing creature. A super-cell can exist in a quasi-steady state for hours, moving along the ground with the ambient wind (unlike an ordinary storm that was a result of regenerating cells).

In order to form a super-cell, three ingredients are necessary: high thermal instability, strong winds in the middle and upper troposphere, and veering of the wind with height in the lowest kilometre. As precipitation is produced in the updraft, the strong upper-level winds blow the precipitation away from the updraft, allowing little or none of it to fall back down through the updraft core. In weaker wind environments, the precipitation actually "chokes" off the updraft, causing the cell to die quickly. Keeping the precipitation away from the updraft is what allows the super-cell to thrive for many hours, capable of producing violent weather throughout its lifetime.

Light rain will begin as the super-cell approaches. As the updraft area comes closer, the areas to the south and east will see an increase in rain, hail and near the updraft itself (typically toward the rear) is where most severe weather (tornadoes) will occur.

Microburst

A downburst (known as a Microburst or Macroburst) is a column of sinking air outwards that is capable of producing damaging straight-line winds of over 130 kt, similar to, but distinguishable from tornadoes which are convective air inwards. Downburst damage will radiate from a central point as the descending column spreads out when impacting the surface, whereas tornado damage shows a pattern consistent with rotating winds.

Downbursts are particularly strong downdrafts from thunderstorms. Downbursts in air that is precipitation free or contains virga are known as dry downbursts; those accompanied with precipitation are known as wet downbursts. Most downbursts are less than 2 nautical miles in extent, these are called Microburst. Downbursts larger than 2 nautical miles in extent are sometimes called Macroburst. Sometimes downbursts are larger, in the extreme case, can cover a huge area of more than 175 nautical miles wide and cover 870 nautical miles long, lasting up to 12 hours or more. They are associated with some of the most intense straight-line winds, but the generative process is somewhat different than for most downbursts.

The formation of a downburst starts with hail or large raindrops falling through dryer air. Hailstones melt and raindrops evaporate -- this is an endothermic process that demands a lot of energy (in form of latent heat) so the air is cooled. Cooler air has a higher density than the warmer air around it, so it falls as a "cold air balloon" (compare to hot air balloon, which rises because hot air has a lower density than the surrounding air). As the cold air balloon hits the ground, it spreads out, and a mesoscale front can be observed as a gust front.

A special, and much rarer, kind of downburst is a heat burst, which results from precipitation been evaporated and the air compressed creating heat as it descends from very high altitude. Heat bursts are chiefly a nocturnal occurrence, can produce winds of up to 85 kt, are characterised by exceptionally dry air, and can suddenly raise the surface temperature up to 49 degrees Celsius, sometimes persisting for several hours.

A Microburst is a localised column of sinking air from the base of a large cumulus cloud or thunderstorm producing damaging straight-line vertical winds similar to, but distinguishable from tornadoes, they also can be powered from the high speed winds of the jet stream deflected to the surface in a thunderstorm. Microburst is often associated with squall lines and may be sometimes identified with virga that is capable of generating vertical wind speeds higher than 500 ft/s.

Danger to Aircraft: The scale and suddenness of a microburst makes it a great danger to aircraft, particularly those at low altitude which are taking off and landing. Sometimes the downburst can exceed the climbing capabilities of the aircraft, resulting in the aircraft been forced to the ground. Fatal crashes have been attributed to Microburst in the vicinity of airports.