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Weather Glossary
Thunderstorms and Lightning
Thunderstorms are one of the most interesting aspects of weather. They spawn the some of the most
dangerous elements of our weather. These being Hail, Tornado’s, Flash floods, Lightning and Hurricanes.
It’s hard to belief that Thunderstorms start the formation from Cumulus clouds. The picture on the left
shows Cumulus clouds over Ladybower. Of course
not all Cumulus clouds develop into Thunderstorms
as several factors are needed to allow the continued
building of the cumulus clouds.
For the Storm to form there needs to be unstable air
and heat or some type of forcing mechanism. I’ll deal
with the basics first. First of all as the Sun warms the
earth surface heating the air around it. As the air
rises due to it being lighter than cool air this forms
what is known as a up draft. If there’s enough
moisture a cloud called a Cumulus will form. If the
heating is strong enough the cloud will continue to form and grow larger.
Eventually the Cumulus gets large enough enough water condenses out to form rain and raindrops fall
as the rising air can’t keep them aloft. Cool air now
enters the cloud and starts to descend creating an
down draft making it rain.
Once the cloud has reached maturity as it now
known as a Cumulonimbus a Thundercloud. The
picture on the left shows a Thundercloud. Note also
other Cumulus cloud in the picture which haven’t
grown due to lack of warm air rising.
Lightning starts to occur as Hail created in the higher
part of cloud collide with other crystals as they rise
and fall. The lower part of the cloud becomes
negatively charged while the upper becomes
positively charged.
The lightning heats the air causing it to expand
which you hear as Thunder.
The storm will continue to develop until the
downdraught becomes to strong preventing
formation of rain droplets and the storm will
dissipated from the bottom to the top.
The wedge shape you see in the above photo is
due to the warm air reaching a level in the
atmosphere where it can’t go any higher so
spreads outwards creating a cirrus anvil.
Another feature associated with Thunderstorms is a gust front. See the picture on the right by Sarah
Major. This forms as cooled air from the Thunderstorm descends due to either drier air entering the
cloud forcing the rain to evaporate and the resultant cooling allows the air to descend and or the falling
rain causes a drag on the air forcing it downwards. The net result is a sudden increase and possible
change in wind direction and a line of threatening cloud as can be seen from the picture on the right.
There are several types of lightning associated with Thunderstorms. Sheet lightning which is where
lightning see picture provided by the met office, is reflected in the
cloud or seen from a distance lightning the horizon up. Forked
lightning which travels from the ground to the cloud forming several
forks as least line of resistance is found. A crawler this is like forked
lightning but isn’t travels across the cloud or across clouds instead.
The very rare Ball lightning the mechanism which are not properly
understood and are very rare. The most dangerous lightning is a
positive cloud to ground strike although rare this one is the one that
causes the most damage. Sprites are upper level lightning which
occur above the Thundercloud. It is believed they are caused by the
positive lightning. Blue Jets project from the top of the
Thunderstorm and don’t seem to be triggered by Positive lightning. The mechanism how they formed
isn’t fully understood as of yet but they seem to be associated with intense hail storms. Sprites and Jets
were first observed by airline pilots and later observed from the space station and the shuttles.
Super cells , see picture on the right, are the big daddy of Thunderstorms. Very are very rare in this
country but very common in places like the states. The are storms and not always thunderstorms either,
that form updrafts that rotate due the shear in the
environment when the storm begins to build. The
nature of storm often produce damaging winds,
large Hail, Heavy rain, and Tornado’s. The nature
of the rotating updraft allows the Super cell to
overcome the self limiting mechanism that limit
the life of a standard storm. Super cells can also
self maintain themselves overcoming
environments that wouldn’t normally allow
development of storms. Cells of this type may
identified on the radar by the hook shape at the
rear side of the Storm.
Another type of Thunderstorm is the high level storm which forms at high altitude approximately 2400m
or higher. The main notable feature of these storms are the lack of rain as this tends to evaporate before
it reaches the ground and the spectacular lightning which is easier toos ee due to the lack of rain.
A further term you may here is MCS or (Mesoscale Convective System) This is where a collection of
Storms act as a system. A variation of this is MCV (Mesoscale Convective Vortex) This where a low
pressure within an MCS pulls the winds into a circulating pattern. Where these move into tropical waters
such as the Gulf of Mexico or of the coast west Africa these may eventually lead to tropical storms or
Hurricanes.
I mentioned earlier there are several aids and factors to the formation of Thunderstorms. Most of all
you need Cape (Convective available potential energy). This is the amount of energy a parcel of air would
have if lifted a certain distance vertically in the atmosphere. This is the positive buoyancy of air and a
indication of atmospheric instability. This is measured in J/kg. The higher the values the more potential
for storms.
CIN (Convective inhibition)is basically the opposite of cape and is also measured in J/kg. If CIN is present
it must be eroded by surface heating or mechanical lifting.
Capping inversion or CAP. This is an elevated inversion layer that caps a convective boundary layer.
Where this is present Storms won’t form until until temperatures rise higher enough to break through
this layer. An observer can at times see this quite easily as clouds build only to spread out at a certain
height. Only when the updraughts get strong enough will this layer get broken.
Li or lifted Index is rising parcel of air temperature when reaches the 500 millibars level subtracted from
the actual temperature of the environment at that level. IF the Lifted Index is a large negative number it
means the parcel of air is much warmer than the surrounding air and will continue to rise. This is a good
guide of the atmosphere ability to produce severe storms.
Storms may form along weather fronts normally cold fronts although it can happen with other types of
front. This where air is forced up by cooler air undercutting the main air mass thus providing the lifting
mechanism needed to produce storms.