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.
