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Guide to...... Storms

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  • Location: Rugby, Warks
  • Weather Preferences: Dangerous
  • Location: Rugby, Warks

What makes a Thunderstorm?

Heat and moisture are key to the formation of thunderstorms. In order to produce the thunder and lightning, you need a cloud tall enough to pull moisture up into the sub-freezing level over 5km/3miles high. These clouds are called cumulus they have a cauliflower like appearance. Cumulus clouds can form any time of the year as long as there is warmth below and coolness above. The process is called convection. Once cumulus clouds reach a height where ice crystals form they become cumulonimbus clouds. The upward motion gradually slows. The stronger upper level winds spread the cloud out at the top, and this formation is called an anvil. As the ice crystals form, they collide with each other and with still-unfrozen water droplets. Electrical charges are produced and eventually the cloud becomes an electric field. If the field becomes more intense lightning is produced.

Single-cell Thunderstorm

The single-cell thunderstorm (meaning a single updraught) develop and die quickly on summer afternoons. They do not last very long because lackadaisical upper winds which keep them becalmed. Once the rain begins, it cools the air below and cuts of the storms energy in less than half an hour. More single-cell storms can form along the cool air outflowing from older ones or several cells may form together to create the more developed Multicell.

Multicell Thunderstorm

Multicellular storms consist of a series of evolving cells. At low levels, cooler air diverging from the downdraft intersects the inflowing air along a gust front, creating a region of strong low- level convergence favorable for new updrafts. It is the presence of vertical wind shear that results in the "tilting" of the updraft and downdraft. Because of the tilting, the less buoyant downdraft air will not destroy the updraft and deprive itself up supersaturated updraft air. In any case, the movement of multicell storms systems is determined by combining the new cell development with the mean winds. Each individual cell typically moves with the mean winds, while new cells develop where the inflow meets the outflow in the region of strongest surface, or low-level, convergence.

The multicell thunderstorm can drop small hail and produce heavier rain. When a strong cold front is marching through, a squall line may form. This band of connected cells moves through quickly with strong wind, heavy rain, small hail and perhaps even a small tornado.


Supercell Thunderstorm

The supercell is the biggest of the storm world. These powerful beasts only form when instability is quite strong and, typically, when upper-level winds strengthen with height. This keeps the storm moving and keeps the top of the storm ventilated, so that warm, most air is pulled in from below . We define a supercell as a thunderstorm with a deep rotating updraft (mesocyclone). The major difference between supercell and multicell storms is the element of rotation in supercells. The supercell produces many elements of the strongest thunderstorms which include: Torrential rain, large hail, hurricane-force wind and violent tornadoes. The lifespan of a supercell can reach beyond six hours.


Mesoscale Convective Systems (MCS)

These are the largest thunderstorm groups on earth. These are a collection of storms - typically organised as a cluster or squall line which can span 100-200 miles and last for more than twelve hours! Partly due to the size of MCSs, they can produce huge amounts of precipitation (250mm/10 inches of rain can fall). MCSs can also generate vast amounts of lightning (Over 10 000 strikes per hour, or about 3 strikes per second). They are more known more known for the spider lightning they produce which stretches from horizon to horizon. MCSS favour the moist heat of the warm season across the mid-latitudes and tropics. In many places, they peak during overnight hours, as smaller storms merge and nocturnal low-level jet streams intensify.

If an MCS forms or moves over an ocean, it can serve as the nucleus of a tropical cyclone.

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