Welsh 
Thunderstorms
PART 1
Thunder is the loud rumbling or crashing noise which follows a flash
of lightning. It is caused by the expansion of rapidly-heated air and follows
lightning because the speed of sound is less than the speed of light
Lightning is seen virtually instantaneously, but sound takes three seconds
to travel a kilometre (five seconds to travel a mile). The distance of
a lightning flash can, therefore, be estimated by timing the interval between
seeing the flash and hearing the thunder. Thunder is normally audible within
five or six miles of the flash (8-10 km) - sometimes twice that distance
- and may be of quite long duration, particularly when the sound echoes
from neighbouring hills. Lightning can be seen much farther away - at night
from 50 or more miles away.
Lightning is produced by discharges of electricity, from cloud to cloud
or from cloud to ground. A large positive charge builds up in the upper
part of a thunder cloud and a negative charge builds up near the base
of the cloud. When the potential difference between the charged areas
becomes large enough, electrical energy is discharged and a flash of
lightning occurs. In fair weather, the electric field or potential
gradient can exceed 100 volts per metre near the ground. On a thundery
day, there can be a potential of a few hundred volts between a person's
head and toe! It isn't noticeable, though, because air has such a large
resistance that the resulting current is only a few millionths of an
amp.
The potential at the top of a thunder cloud may be as high as 400,000 volts.
Huge quantities of electricity are discharged in lightning flashes (typically
20 coulombs per flash) and intense heat is generated in discharges (temperatures
of 30,000°C or more). No wonder lightning is dangerous!
When (in 1752) Benjamin Franklin flew a kite in a thunderstorm, he risked
his life. He luckily came to no harm and lived to invent an important
safety device, the lightning conductor, which is a metal rod or wire
through which electrical discharges are led harmlessly to earth. Without
these devices, church towers and spires and other tall buildings would
be damaged by lightning.
It is not safe to stand under a tree during a thun-derstorm or to make
yourself a target for lightning when out in the open. Lightning tends
to strike the highest point, and that point does not have to be much
higher than the surrounding places to be a target. Few people survive
the experience of serving as a lightning conductor. To maximise
safety in a thunderstorm, avoid high ground, golf courses and other
open spaces - and remember that you are not safe in a shed or tent or
under an umbrella, especially one with a spike pointing heavenwards!
Keep at least 30 metres away from wire fences and do not touch such
a fence immediately after a storm, as it may have stored a lethal charge.
A car is a safe place to be in a thunderstorm because it serves as a
'Faraday cage', a metal screen through which electricity passes to earth.
If rain is falling, so much the better, as the wet surfaces of the car
and tyres help conduct the lightning to earth.
St
Elmo's Fire is a corposant (from the Old Spanish corpo
santo, meaning 'holy body'). It takes the form of a luminous electrical
discharge (typically blue or green) and is sometimes seen on lightning
conductors, masts of ships, wing-tips of aircraft and other elevated
objects. It occurs when the electrical field in the vicinity of the
object becomes very strong. Though St Elmo's Fire is not in itself dangerous,
its glow and the crackling sound that it makes may frighten the passengers
in an aircraft and cause radio communications to become difficult, if
not impossible.
PART 2
For
thunderstorms to occur, cumulonimbus clouds are required. These are heavy,
dense, towering clouds with tops shaped like anvils or vast plumes
In addition to lightning, these clouds bring other hazards in the form of hail, torrential rain, tornadoes and downbursts
It
is unpleasant at best, dangerous at worst, to fly aircraft through thunderstorms,
as lightning, hail, severe turbulence, vigorous updraughts and strong downdraughts
all occur inside them. An aircraft can be wrecked by flying from a powerful
updraught into an adjoining downdraught. Even the biggest of aircraft can
be flung about the sky by the violent winds of vigorous cumulonimbus clouds.
Pilots therefore try to fly around such clouds if they can. As the clouds
are typically only 10-12 km across, this is normally possible.
Inside
cumulonimbus clouds, upcurrents tend to be concentrated in 'chimneys',
with compensating currents of sinking dry air close by. Rain falls into
the dry air, where it evaporates, thereby cooling the dry air, making it
heavier. These downdraughts may become 'downbursts', blasts of cold air
which are travelling with a speed of 40 m/s or more when they reach the
ground. The most severe types, called 'microbursts', can be very dangerous.
When
downbursts reach the ground, they spread out radially. Aircraft land and
take off into wind. When there is a downburst nearby, an aircraft may land
or take off into wind but then, almost immediately, fly into the rapidly-sinking
air of the downburst and a tail wind. A loss of lift therefore occurs
and the aircraft may crash before enough speed can be gained to create
the lift needed to stay above the ground. Downbursts occur in many parts
of the world and have caused several plane disasters at or near airports,
with many lives lost. Most of the disasters have occurred in the United
States, where determined efforts have been made in recent years to protect
against the menace of downbursts. At and around many airports in the USA,
sophisticated wind-detection systems have been installed.
People
on aeroplanes are not the only ones endangered by microbursts. The strong,
dry, gusty winds of downbursts can quickly turn a minor forest fire into
an inferno. Microbursts can also overturn large boats. On 7 July 1984,
for example, one overturned an excursion boat 27 metres long on the Tennessee
River near Huntsville, Alabama, USA, killing eleven of the eighteen aboard.
PART 3
Earth
is quite an electrical planet! About 44,000 thunderstorms occur on it each
day, with about 2,000 in progress at any moment. The number of lightning
discharges on Earth per second is about 100.
There is fossil evidence that lightning occurred 250 million years ago. Fulgurites are
rocky substances formed of sand fused or vitrified by lightning. In the
British Isles, the oldest examples are on Arran. Fulgurite is
derived from fulgur, the Latin for lightning.
In the UK, the average number of lightning fatalities per year is about
four, so the chance of being hit by lightning is roughly 1 in 14 million,
which is about the same as the chance of winning the jackpot in the National
Lottery! The most thundery parts of the British Isles are the East Midlands
(where the average number of storms per year is about 20) and inland East
Anglia (where it is about 16). The least thundery parts are the north of
Scotland, south-east Ireland and the western side of Wales. Here, the average
number of storms per year is about four.
The
most thundery places in the world are Indonesia, south-east Asia, central
Africa and the north-west of South America. In each of these regions, thunder
occurs on more than 200 days per year.
The
hailstones that fall from cumulonimbus clouds are usually small (diameter
less than 1 cm) but occasionally reach grapefruit size (diameter 12 cm
or more). Among the biggest are those which fall in north-east India in
the period mid-April to early June. These may weigh as much as 1 kg, and
when they hit the ground (or a person) their fall-speed is considerably
more than 50 m/s. It is not surprising, therefore, that hailstones kill
people and damage property.
INFORMATION
For an analysis of weather as a factor in aviation accidents, see "Weather-influenced
accidents to UK transport aircraft"
by L.Symons, published in Weather, December 1996, Volume 51, pp.419-425.