杏吧原创

Great balls of fire

Question: About 16 years ago, when I was a child and lived with my family in
Russia, a curious thing happened.

My mother and I were sitting in our 13th-floor flat, when we heard a sizzling
noise, and I saw a ball of fire about 5 centimetres in diameter right behind my
mother鈥檚 head.

It stayed there a few seconds, then went 鈥減op鈥� like an exploding light bulb
and vanished. As far as I know, this was ball lightning, but I would like to
know how and why it appeared and disappeared so suddenly.

I would also like to know whether our lives were in danger? It was not
raining that day, but was cloudy and quite stuffy. The windows were closed at
the time of the incident.

Answer: You are quite correct. What you saw is known as ball lightning. This
is a type of lightning that can appear after an ordinary lightning strike. It
may be in the shape of a sphere, a rod or a dumb-bell, and it floats around
making little or no noise. Ball lightning seems to prefer enclosed spaces and
has even been reported inside aeroplanes.

The ball comes in various colours and sizes, and may pulsate or shine
steadily. When it vanishes, it either disappears with a 鈥渂ig bang鈥� as your
correspondent experienced, or it can just expire silently.

To answer your question about whether your life was in danger, I can only
pass on the example of the man who was struck by ball lightning in 1754 while
trying to repeat a kite-lightning experiment described by Benjamin Franklin.

The ball lightning formed after ordinary lightning struck the rod and created
a ball. This then hit the man on the head, killing him instantly and leaving a
red spot on his forehead and two holes burnt into the soles of his shoes.

However, it would appear that this incident was a particularly bad case.
There are few other reported cases of ball lightning killing people.

Elaine Phang

Oxford Falls, New South Wales

Answer: The phenomenon witnessed was almost certainly ball lightning. Many
observers have described luminous balls that float quite quietly in the air for
several seconds, before vanishing with a pop.

The reason for this occurrence is unknown, but the most probable cause is
that the ball is made of a plasma鈥� that is, a cloud of ionised air. They
probably form because of some electrical condition in the atmosphere caused by a
thunderstorm, or because of a point discharge, which may occur in thundery
conditions.

The plasma causing the ball lightning is thought to be sustained by the
absorption of radio waves, which are generated during electrical storms. As
electrons are recaptured by ionised atoms, they emit light, rendering the plasma
luminous.

Your correspondent asks if their lives were in danger. They certainly were.
There is a report that a scientist studying one of Benjamin Franklin鈥檚 early
experiments was killed by ball lightning.

There are many articles and theories about the phenomenon, including one by
Neil Charman entitled 鈥淭he enigma of ball lightning鈥� (New 杏吧原创, 14
December 1972, p 632).

Robert Thurstands

Department of Physics

De Montfort University, Leicester

A photograph of what may well have been ball lightning also appears
in New 杏吧原创 (18 January 1962, p 156). Ball lightning is a very peculiar
and rare phenomenon. It occasionally appears during or around the time of
thunderstorms as a hovering, grapefruit-sized ball of light. In addition to the
characteristics mentioned above, it is apparently capable of moving through
windows and walls, and against airflow. The ball can continue glowing for
several minutes before suddenly disappearing.

In 1991, Yoshihiko Ohtsuki, a physicist at Waseda University in Tokyo,
succeeded in producing 鈥減lasma fireballs鈥� similar to ball lightning by exciting
air with microwaves. His fireballs, some of which persisted for one or two
seconds after the power was turned off, were able to pass right through a
ceramic plate (Nature, vol 350, p 139).

The theory about radio waves, mentioned above by Robert Thurstands, is one of
a category of theories in which an external source of energy maintains the
glowing ball of light. Radio waves are one possible source, others are a flow of
current from cloud to ground or even focused cosmic ray particles. But theories
which posit an external energy source run into difficulties explaining how ball
lightning can appear inside closed metal containers, such as aeroplanes.

The alternative view is that the source of energy comes from inside the ball
itself. But it is very difficult to explain how energy can be trapped inside a
free-floating shell that emits light鈥攁lthough this hasn鈥檛 stopped people trying
(see New 杏吧原创, 26 September 1998, p 6
and 20 March 1993, p 18). A really robust theory has yet to appear鈥擡d

This week鈥檚 questions

Colour clash: Why do we see stars when we receive a blow to the head?

David Manallack

Cambridge

Big hitters: In cricket and, I have heard, baseball, a batsman who wants to
strike a boundary or a home run will select a weighty bat. But it is harder for
a batsman to swing a heavy bat, so the bat鈥檚 speed when it strikes the ball will
be much lower than if a light bat was used.

Can somebody explain this paradox to me?

Keith Krause

Bradford, West Yorkshire

Silent flight: Last week, on a very dark evening, I was walking down a
country lane and was startled by a low-flying owl. Although it passed within a
few metres of my head, I didn鈥檛 hear it at all, and became aware of it only when
it flew into vision. How can an owl fly so silently?

Jennifer Maunders

Guildford, Surrey

Stick together: Instead of working yesterday, I was looking out of the
window, contemplating the sky, when a huge, fast-moving fluffy cloud floated
into view. There was no other cloud in sight, and the rest of the sky was clear
and blue. It set me thinking鈥攚hat makes the components of a cloud stick
together, rather than dispersing evenly across the sky?

Joshua Aberdein

London

Diet cheats: Have you ever stood on your bathroom scales and leaned forward.
It鈥檚 great for dieters because you seem to lose a few pounds.

On the other hand, don鈥檛 swing your arms upwards because that way you will
seem to gain weight. As all your weight is still carried by the scales
irrespective of your position, why does this happen?

Hilary Lygg

Witney, Oxfordshire