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Welcome to New Å®ÉúСÊÓƵ’s questions and answers column which will become
a regular feature of the magazine, appearing on this page every week. We
have been printing readers’ questions in the Letters section of the magazine
since 26 March. Now we begin to publish the answers, all supplied by readers.
Remember, a £10 book token goes to the writer of each answer published

Just plane weird

Q: A friend and I used to go to Los Angeles airport to watch the planes
coming in. We’d stop on the perimeter road which runs underneath the flight
path of most of the landing planes. Our attention was captured by an event
that sometimes occurred between 20 and 30 seconds after a plane flew over.
A thin tube of misty air would zap past us, apparently at high speed, accompanied
by a loud flapping sound. Sometimes the mist followed a straight path, but
often it was like a snake engaged in violent action. Can anyone explain
what the misty tubes were?

* * *

A: Any body which, when moving in a fluid, generates a rotation in the
fluid about itself, experiences a force with a component at right angles
to the direction of motion. At the extremities of the body, this rotation
continues as a pair of trailing vortexes of equal and opposite strength.
Therefore, a wing which is supporting an aircraft always trails a pair of
vortexes. Those behind a big airliner can be very strong.

At high altitude, or in very humid conditions, the low pressure at the
centre of the vortexes causes water vapour in the atmosphere to condense,
making them visible, but they are always there – this is the flapping noise
heard. Aircraft also deflect the air downwards behind them and this causes
a pressure rise on the ground, which is more concentrated during the landing
approach or takeoff. An interesting point is that vortexes are remarkably
stable; witness a smoke ring. It is only because of the viscosity of air,
which disperses the energy of a vortex, that it does not persist indefinitely.
Another theoretical point of interest is that vortexes cannot have an end
(tornadoes are continued by their reflection in the ground). In other words,
they are always produced as a closed loop, as with a smoke ring. If it were
not for viscosity, the trailing vortexes from each aircraft would continue
back to the airfield from which it had taken off, where they would join
together. The world’s airfields would therefore be littered with take-off
vortexes!

Arthur Cox Alton, Hampshire

* * *

A: At takeoff and landing when the aircraft wing has its flaps out and
is at a very high angle of attack, the drag energy creating the vortexes
is at its greatest. In a crosswind, they will drift some distance and interfere
with operations on parallel runways and taxiways.

Student pilots in light aircraft are given severe warnings to leave
two minutes’ or more delay before following a large airliner and a four-mile
separation is required for wide-bodied jets. Air traffic control will invariably
use the word ‘heavy’ in radio transmissions to alert light aircraft pilots
to such traffic, and accidents from this recognised phenomenon are still
regrettably common.

PS Helicopters have their own vortex problem where the aircraft descends
too fast into its self-generated toroidal vortex ring in still air!

John Paulson Warrington, Cheshire

Light sneeze

Q: I have noticed that many people tend to sneeze when they go from
dark conditions into very bright light. What is the reason for this?

* * *

A: Photons get up your nose!

Steve Joseph Sussex

* * *

A: I think that the answer may be fairly simple: when the sun hits
a given area, particularly one shielded or enclosed in glass, there is a
marked rise in local temperature. This results in warming of the air and
a subsequent upward movement of the air, and with it, many millions of
particles of dust and hair fibres. These particles quite literally get up
one’s nose within seconds of being elevated; hence, the sneezing.

Alan Beswick Birkenhead

* * *

A: My mother, one of my sisters and I all experience this. I feel the
behaviour is genetic and confers an unrecognised evolutionary advantage.
I have questioned many people, and we sun-sneezers seem to be in the minority.
However, as the ozone thins and more ultraviolet light penetrates the Earth’s
atmosphere it will become increasingly dangerous to allow direct sunlight
into the eye. Those of us with the sun-sneeze gene will not be exposed to
this, as our eyes automatically close as we sneeze! The rest of the population
will gradually go blind, something not usually favoured by natural selection.

Alex Hallatt

Clammy claims

Q: It is very firmly held in this country, supported by a wealth of
anecdotal evidence, that ‘damp’ cold feels colder than ‘dry’ cold. Is there
any physical basis at all for this, or is it purely psychological?

* * *

A: Water content increases the specific heat capacity of the air (and
thus its cooling capacity). Taking it to the extreme, you can die of hypothermia
in infinitely humid air (water) at 4 degree C, while dry air at this temperature
only causes a shiver. This is part of the reason why damp heat feels hotter
than dry heat (the other, important, part being that sweat doesn’t evaporate
well in humid air).

Daniel Boyd Amsterdam

* * *

A: With one very important exception, for a given temperature and wind
speed, air is less chilling for us if it is more humid. This is because,
even if we’re not noticeably sweating, we’re being cooled by evaporation
of water from the skin and respiratory surfaces.

The exception occurs when the air is carrying droplets of water. If
you’re in mist, fog or cloud, despite high humidity, the air will be much
more chilling than if it were not carrying the droplets. In this situation,
heat is constantly drawn from you by the evaporation of droplets in the
layer of air being warmed by your body.

Philip Goddard Exeter, Devon

* * *

A: I recall that Russians stationed in London during the Second World
War claimed it ‘felt colder here’ than in Moscow, when the evidence was
clearly to the contrary. Having lived for some years in New York, I would
surmise that dry cold can be kept out by warm clothing and damp cold can’t.

Lex Diamant London

This week’s questions – Strange spuds

Strange spuds: Why do boiled potatoes taste horrible the next day? Surely
having been prepared by a method of sterilisation they should not have ‘gone
off’ so soon.

S. A. Hawkins Luton, Bedfordshire

What’s the rub

What’s the rub: How does a pencil rubber work? Do all types – conventional,
plastic and putty – work in the same way?

Helen Geake York

Street fashion

Street fashion: While walking at night through streets lit by low-pressure
sodium vapour lights (that is the orange ones), I have noticed a shimmering,
horizontally oriented pattern of lines in the glow surrounding the lamp.
As I get further from the light, the pattern gets coarser. What causes this?

Timothy Ostler London