Most thunderstorms are associated with towering
clouds known as cumulonimbus. The right conditions
for the formation of a thunderstorm are (a)
unstable air and (b) a mechanism for causing
air to rise.
Air is said to be unstable when a 'parcel'
of air continues to rise of its own accord after
being given an upward impetus. This instability
is the result of a rapid fall of temperature
with height as well as a considerable amount
of moisture. The mechanism may be provided by
a sufficiently warm surface; the air near the
surface being forced to rise over higher ground;
or instability in the large-scale ascent within
a front.
As an example, on a summer's day, the land
is warmed by the sun, and as the air just above
becomes warmer it starts to rise. As it rises
it cools, and, if cooled sufficiently, cumulus
clouds form at the condensation level. These
small, white puffy clouds grow larger and larger
as the temperature of the ground increases,
causing more warm air to rise.
After a time, the top of the cloud turns to ice (usually
below a temperature of -20 °C) and streams away
in the winds at the level of the cloud top, giving
it a characteristic anvil shape.
Lightning
Lightning is a large electrical spark caused
by electrons moving from one place to another.
Electrons cannot be seen, but when they are
moving extremely fast, the air around them glows,
causing the lightning flash. The actual streak
of lightning is the path the electrons follow
when they move.
An atom consists of three basic parts, a proton (which
has a positive charge), a neutron (which has no charge)
and an electron (which has a negative charge). Electrons
cling to the positively charged centre of the atom
because they have a negative electrical charge. During
a thunderstorm, some of the atoms in the cloud lose
electrons while others gain them.
When a cloud is composed entirely of water droplets,
there is very little transfer of electrons. As a storm
cloud grows in height, the temperature of the water
droplets higher up falls. They continue in the liquid
state below 0 °C as supercooled water, but
eventually they begin to turn to ice, usually at a
temperature below -20 °C. These ice particles
often collide and the smaller particles lose an electron
to the larger, thereby gaining a positive charge.
The small particles are propelled towards the top
of the cloud by strong internal winds while the larger
particles start to fall. This causes the top of the
cloud to develop a strong positive charge.
The larger, negatively charged, ice particles begin
to 'capture' supercooled water droplets, turning
them instantly to ice and thereby growing, some reaching
a sufficient size to start falling.
This leads to the base of the cloud becoming negatively
charged which, in turn, induces a positive charge
on the ground below. In time, the potential gradient
between cloud and ground, or between adjacent clouds,
becomes large enough to overcome the resistance of
the air and there is a massive, very rapid transfer
of electrons, which appears as a lightning flash.
There are several types of lightning, all of which
are made up of different parts and none of which are
alike. Lightning that shoots from the cloud to the
ground is made up of four main parts: a stepped leader,
upward streamers, return strokes and dart leaders.
As negative charges collect at the base of the cloud,
they repel the electrons near the ground's surface.
This leaves the ground and the objects on it with
a positive charge. As the attraction between the cloud
and the ground grows stronger, electrons shoot down
from the cloud. The electrons move in a path that
spreads in different directions - like a river delta.
Each step is approximately 50 metres long and the
branching path is called a stepped leader. Further
electrons follow, making new branches. The average
speed at which the stepped leader cuts through the
air is about 270,000 miles per hour.
As the stepped leader approaches the ground, positive
electrical sparks rise from tall objects such as trees
and buildings. These sparks are known as upward streamers.
When the stepped leader meets the upward streamer,
the lightning channel is completed. When the lightning
channel is complete, the electrons in the channel
rush towards the ground. This is the return stroke
which lights up the channel. The first electrons to
reach the ground light up the bottom of the channel.
The upper part of the channel glows as the electrons
move rapidly down it. Therefore, the light from the
flash starts at the ground and moves upwards. The
branches of the stepped leader are also lit up, but
not as brightly as the main channel as there are less
electrons present. The lightning flash ends when there
are no electrons left in the channel.
If lightning flickers, it is probably because there
has been more than one return stroke. Following a
lightning flash, the lightning channel is momentarily
empty and it is then possible for electrons from another
part of the cloud to enter it. The movement of the
electrons into the channel is called a dart leader.
It causes another return stroke to occur. The repeated
return strokes and dart leaders make the lightning
appear to flicker because of the great speed at which
they occur.
Thunder
The word 'thunder' is derived from 'Thor', the Norse
god of thunder. He was supposed to be a red-bearded
man of tremendous strength; his greatest attribute
being the ability to forge thunderbolts. The word
Thursday is also derived from his name.
Thunder is the sharp or rumbling sound that accompanies
lightning. It is caused by the intense heating and
expansion of the air along the path of the lightning.
The rumble of thunder is caused by the noise passing
through layers of the atmosphere at different temperature.
Thunder lasts longer than lightning because of the
time it takes for the sound to travel from different
parts of the flash.
How far
away is the thunderstorm?
This can roughly be estimated by measuring the interval
between the lightning flash and the start of the thunder.
If you count the time in seconds and then divide by
three, you will have the approximate distance in kilometres.
Thunder is rarely heard at a distance of more than
20 km.
Are
thunderstorms dangerous?
Many people are frightened by the crackles and rumbles
of thunder rather than the flash of lightning. However,
thunder cannot hurt anybody, and the risk of being
struck by lightning is far less than that of being
killed in a car crash. Ninety per cent of lightning
discharges go from cloud to cloud or between parts
of the same cloud, never actually reaching the earth.
Most of the discharges that do strike the ground cause
little or no damage or harm. Lightning takes the shortest
and quickest route to the ground, usually via a high
object standing alone.
Days of thunder annual average 1971-2000
Lightning strikes lone trees on high ground -
don't shelter here!
You are safe inside a car. The electricity is
carried through the metal of the car itself and
to the ground through the tyres
If you get stuck in the open, make yourself low
by crouching down, or run for shelter
Lightning strikes aircraft, but the people inside
are safe because it runs around the outside, though
it can make a hole in the superstructure
Lightning strikes tall buildings, but they have
lightning conductors to carry the electricity harmlessly
to the ground
Facts and
figures
Number of thunderstorms occurring at any given
moment: 2,000
Number of lightning strikes every second: 100
Number of lightning strikes a day: 8 million
The average flash would light a 100 Watt light
bulb for three months
The average lightning stroke is six miles long
A typical flash of folk lightning lasts for about
0.2 seconds
The temperature of lightning's return stroke can
reach 28,000 °C. The temperature on the surface
of the sun is around 6,000 °C
The Empire State Building in New York has been
struck by lightning as much as 48 times in one day!
Quick test!
Here are a few beliefs about thunder and lightning.
Test yourself with the following statements - are
they true or false? Some of the answers can be found
in the text.
It is dangerous to leave doors and windows open
during a lightning storm.
When caught out in the open during a thunderstorm,
take shelter under a tree.
You are safer in the city than in the countryside
during a storm.
When a thunderstorm occurs while you are driving
your car, it's best to get out and away from the
car.
