Types of Thunderstorms
Single cell
As the name implies, there is only one cell with this type of thunderstorm. Also called a "pulse" thunderstorm, it consists of a one time updraft and one time downdraft. In the towering cumulus stage, the rising updraft will suspend growing raindrops until the point where the weight of the water is greater than what can be supported.
At which point, drag of air from the falling drops begins to diminish the updaft and, in turn, allow more raindrops to fall. In effect, the falling rain turns the updraft into a downdraft. With rain falling back into the updraft, the supply of rising moist air is cut-off and the life of the single cell thunderstorm is short.
Multi-cell
Although there are times when a thunderstorm consists of just one ordinary cell that transitions through its life cycle and dissipates without additional new cell formation, thunderstorms often form in clusters with numerous cells in various stages of development merging together.
While each individual thunderstorms cell, in a multi-cell cluster, behaves as a single cell, the prevailing atmospheric conditions are such that as the first cell matures, it is carried downstream by the upper level winds and a new cell forms upwind of the previous cell to take its place.
The speed at which the entire cluster of thunderstorms move downstream can make a huge difference in the amount of rain any one place receives. There are many times where the individual cell moves downstream but addition cells form on the upwind side of the cluster and move directly over the path of the previous cell. The term for this type of pattern when viewed by radar is "training echoes".
Also called "back building" thunderstorms, with careful observation you can count the number of individual thunderstorm cells in the cluster that pass your location. Take note of the direction from which you first hear thunder. The thunder's volume will increase as the cell approaches your location. Then, after it passes and the volume decreases, you will hear more thunder from the next cell, increasing again, coming from the same direction as the previous cell.
Often these storms will appear on radar to be stationary. However, if the new development is vigorous then the thunderstorm cluster appears to move upwind.
Training thunderstorms produce tremendous rainfall over relatively small areas leading to flash flooding.
Squall Lines
Sometimes thunderstorms will form in a line which can extend laterally for hundreds of miles. These "squall lines" can persist for many hours and produce damaging winds and hail.
While tornados occasionally form on the leading edge of squall lines they primarily produce "straight-line" wind damage.
This is damage as a result of the shear force of the down draft form a thunderstorm spreading horizontally as it reaches the earth's surface.
Long-lived strong squall lines after called "derechos" (Spanish for 'straight'). Derechos can travel many hundreds of miles and can produce considerable widespread damage from wind and hail.
Often along the leading edge of the squall line is a low hanging arc of cloudiness called the shelf cloud.
This appearence is a result of the he rain cooled air spreading out from underneath the squall line acts as a mini cold front. The cooler dense air forces the warmer, less dense air, up. The rapidly rising air cools and condenses creating the shelf cloud.
Supercell Thunderstorms
Supercell thunderstorms are a special kind of single cell thunderstorm that can persist for many hours.
They are responsible for nearly all of the significant tornadoes produced in the U.S. and for most of the hailstones larger than golf ball size. Supercells are also known to produce extreme winds and flash flooding.
They are characterized by a rotating updraft (usually cyclonic - above left) which results from a storm growing in an environment of significant vertical wind shear. Wind shear occurs when the winds are changing direction and increasing with height.
The most ideal conditions for supercells occurs when the winds are veering or turning clockwise with height. For example, in a veering wind situation the winds may be from the south at the surface and from the west at 15,000 feet (4,500 meters).
Beneath the supercell, the rotation of the storm is often visible as well (above right). The lowering in the photograph (right) represents the wall cloud. The wall cloud is sometimes a precursor to a tornado. If a tornado were to form, it would usually do so within the wall cloud.
Supercells are divided into two main groups: Low precipitation (LP) and High Precipitation (HP). As indicated by their name, the amount of rain and/or hail determines the difference between the two.
LP supercells are most easily identifiable in that their "barbershop pole" appearance on the updraft is readily apparent. And because of low amount of precipitation, tornadoes are rather easy to spot.
HP supercells, on the other hand, are the most dangerous. Often, due to the abundant moisture, and large amounts of rain and hail, tornadoes are often obscured by rain.