Thunderstorms
Thunderstorms come in many different shapes and sizes and can form in a variety of different atmospheric environments. The internal structure of a thunderstorm is underpinned by the amount of wind shear and instability available in the atmosphere. Generally, wind shear pays the biggest role in the evolution of a thunderstorm, whereas instability changes the intensity of the storm - see                          on the role of wind shear in thunderstorms.
 
Thunderstorms can exists on their own, in groups or clusters, or in long lines. The strongest storms tend to exist on their own and have little interaction with other storms, since thunderstorms "compete" for warm, moist air at the surface and they do not want to have other storms close by which could potentially steal the moist air. Therefore, it is the discrete - isolated and alone - thunderstorms that tend to produce the most severe weather. There are also methods by which groups of thunderstorms can organise themselves into discrete shapes:
 
Mesoscale Convective System (MCS) - this is a group of well-organised thunderstorms that can form distinct shapes and move whilst maintining the same shape. They are smaller than extratropical cyclones and typically last a few hours to several days. They must produce a continuous precipitation area on the order of 100km or more in any one direction, according to the American Meteorological Society. They can produce circular patterns of precipitation, such as a tropical cyclone and a polar low, or produce linear precipitation shields, such as with a squall line. MCSs form in vastly different environments with respect to wind shear and instability. Tropical cyclones hate wind shear, and will be destroyed rapidly if they encounter any - they need an environment with calm or unidirectional winds. Squall lines require wind shear to maintain their strength and any evolution in strong wind shear will result in the squall line usually becoming more severe and lasting for longer. Other examples include lake-effect snow bands, mesoscale convective vorticies, bow echoes and derechos. 
     
A type of MCS is the Mesoscale Convective Complex (MCC), which represents a larger form of MCS organisation and has very specific critera to meet, based upon infrared satellite imagery. There must be a general cloud shield with temperatures below -32°C over an area greater than 100,000 km2, with an interior cold cloud region with temperatures below -52°C having an area of 50,000 km2. These are a large area of thunderstorms that appear as a "blob" on satellite imagery and are most common at night, when storms have had enough time to organise themselves from initiation during the day. They generally form in relatively low wind shear environments in the summer and generally do not produce especially severe weather - they are prolific rain producers. MCCs often last for 6 - 12 hours, although severe wind, hail and tornadoes can also occur during the early phases of evolution, when storms are more discrete. MCSs need a large landmass to form over, where the instability and low level heat and moisture can be generated, so are a rare visitor to the UK. They are common over the Great Plains and Midwest USA in the late spring and summer, where they produce the majority of the annual rainfall that this area sees, often continuing through the night. 
 
this section 
An MCC centered over Kanas in the central United States. This is an infrared image, with the colours indicating the temperature (and therefore height) of the cloud tops. The coldest clouds are indicated by the white in the centre of the dark red. The MCC extends over Oklahoma, Missouri, Nebraska and Iowa too, indicating its size. 
From http://cimss.ssec.wisc.edu/goes/blog/archives/15840
An MCC much closer to home - over northern France. This gives a good perspective of just how large these systems can be, since the cloud shield would cover much of England! MCCs are more common over the European continent during the summer, since there is so much more land here. They can wander over the UK on occasion after forming over France. This one developed on the 26th July 2013. 
From http://metteochannel.blogspot.co.uk/2013/09/analysis-of-thunderstorm-and-lightning.html
In terms of the large scale flow dynamics of an MCC, it is similar to a tropical cyclone. Air ascends into the storm clouds from the surface (or above the surface at night), through the updrafts and exits the system from the large anvil cloud shield, with anticyclonic outflow. At the middle heights (500 hPa), there is low pressure generated from the warm-core structure and associated cyclonic flow around it. Near the tropopause, a small area of high pressure is created due to the warm-core structure, promoting divergent, anticyclonic outflow at the top. If the MCC occurs in weak tropospheric flow, then this is the likely outflow pattern, but if the jet stream flows over the top of the MCC, it can enhance the outflow when under the right jet entrance or left jet exit (due to the promotion of ascent through the troposphere), or hinder it when under the left jet entrance or right jet exit (due to the promotion of descent through the troposphere). 
 
Types of thunderstorms that will be discussed in this section are:
 
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Other examples of mesoscale convective systems, such as tropical cyclones, polar lows and lake-effect snow, will be discussed in separate sections. 
 
 
 
Single cell or pulse thunderstorms.
Multicell thunderstorms.
Supercell thunderstorms.
Squall lines.
Bow echoes.
Derechos.
Mesoscale convective vorticies.