ÐÓ°ÉÔ­´´

A low heat

Question: I’ve always understood that hot air rises and cold air sinks. But
on a recent visit to Death Valley in the US, I learned that it is the hottest
and driest area in North America despite being the lowest point in the western
hemisphere. Why is this?

Answer: The apparent paradox of warm air lying beneath cold air is not
confined to Death Valley. All over the globe, air temperatures tend to decline
with height, up to the bottom of the stratosphere—witness the presence of
permanent snow on high mountaintops. This is partly because most of the
radiation from the Sun passes through the air without warming it. It heats
instead the land and sea surfaces that lie below, which in turn warm only the
air that is immediately above them.

So why does the hot air in Death Valley not push through the cooler air above
and continue to rise to very high altitudes? As the Sun’s rays heat the bare
rock, the hot air above it does begin to rise, initiating convection currents.
But as this warm air rises, the pressure drops and it expands. This increase in
volume uses up energy and consequently the air cools— this is a process
known as adiabatic cooling.

Because the air in Death Valley is very dry, this adiabatic cooling takes
place fairly rapidly, and the rising air quickly cools to the same temperature
as the surrounding air. At this point, it ceases to rise.

The cooler air that flows down from above to replace it as it rises is itself
heated adiabatically—pressure increases and volume decreases as it sinks
into the valley.

It is the lack of cloud cover, along with this overall atmospheric stability,
that produces Death Valley’s high temperatures.

Things are different in places where the air is warm and humid. In the
tropics, for example, the air is close to saturation with water vapour. When
rising convection currents cool, this water vapour condenses out and releases
latent heat. Consequently, the rate of adiabatic cooling is much slower than in
the dry air that is found in desert regions.

Depending on the conditions, it may be that the rising thermals do not cool
enough to reach the same temperature as the air that they are rising through.
They continue to rise and can reach great heights, sometimes as high as the
bottom of the stratosphere—which is 10,000 metres or more above sea
level.

The enormous amounts of liquid water and latent heat that are released into
the air result in violent thunderstorms which drop prodigious amounts of rain.
This is clearly all very different from the dull, dry heat that is found in
Death Valley.

Michael Ghirelli

Buckingham

The hole story

Question: As I crossed the road yesterday it struck me that most manhole
covers in the street are round. Yet the ones covering the drains in my backyard
are rectangular. Is there a reason for the different shapes?

Answer: I have learned from infuriating personal experience why manhole
covers come in different shapes. The traditional round manhole cover is round
for one very specific reason: to stop it falling through the hole.

Most manhole covers are designed to support the weight of a truck and
consequently are extremely heavy. If you drop the cover down the hole while
trying to put it on, not only will it do a lot of damage, it will also be
extremely difficult to get back up again (as I have discovered to my cost).

Making manholes and their covers circular avoids this problem. No matter
which way you turn it, the cover can’t fall into the hole. Rectangular and
square covers can fit through their respective holes if tilted at a suitable
angle, although it is probably best to avoid experimenting with them simply to
discover this.

The other benefit of round manholes is that the absence of corners makes the
covers much easier to replace. Round covers are less easily damaged than covers
that have a sharply pointed corner.

Dalin Brinkman

Camarillo, California

Answer: Most manhole covers are round for the simple reason that this
prevents the lid from being dropped down the hole. A square shape can be turned
to the diagonal and slipped through the hole.

The existence of rectangular openings in your backyard is probably due to two
factors: the drains will not be as deep and the consequences of dropping the lid
will be far less dramatic, coupled with the benefit and convenience of the
larger access area which a rectangle affords.

Matthew Payne

Hampton, Middlesex

This week’s questions

Sociable junkie: Is the Internet addictive? My mother always seems to be
online chatting with her friends. If it is, does this mean that you can become
addicted to your social life?

James Quartly

Addlestone, Surrey

Raindance: I’ve noticed that early in the morning, if it has just rained,
seagulls will stamp their feet on the wet grass. What are they up to, and why
haven’t I seen any other birds doing the same?

Neil Jackson

Margate, Kent

Hail capsules: During a recent hailstorm in Wales, I was surprised to see
that all the hailstones were conical. Each one had an apex angle of about
75°, with a flat or slightly curved base about 4 millimetres across.
What caused this? Is this shape common, and is its similarity to an
Apollo re-entry capsule coincidental?

Alan Wills

Manchester