7. The Atmosphere and Electricity
It is thought that, when the Earth was young, after it had cooled from
a molten mass of condensing gases and a crust had formed, it was
entirely covered in water. In those early days there was great heat loss
and the Earth was cooler. The lower part of the initial atmosphere
was composed of water vapour evaporated from the vast ocean, with
a contribution of other gases emanating from volcanic eruptions.
Because of its high specific heat^1 and its capacity to retain heat, the
water vapour gradually absorbed the heat of the Sun, thus raising the
average temperature. Heat losses were kept to a minimum at night
because water absorbs infrared heat. It was these qualities of water
that allowed the greenhouse effect to take hold. Otherwise the Earth
would have remained cold, lifeless and barren.
Of all liquids, water has the greatest ability to store heat. It
absorbs heat slowly, releasing it slowly. Water vapour was thus an
ideal medium for conserving heat on the Earth's surface, enabling
life to gain hold and, once it was established, water became the
medium for complex life forms to develop.
What makes water different from all other liquids is its so-called
'anomaly point' or 'point of anomalous expansion,' which will be
discussed in more detail in Chapter 9. Contrary to the behaviour of
other liquids, the volume of water does not decrease continually
with increasing cold; below a temperature of +4°C (39°F) it starts
to expand again, and on freezing expands still further.
Pure water will freeze only at a temperature of around -40°C
(-40°F) or in clouds at about -10°C (14°F), which again is fairly
important, as we shall discover later. Compared to absolute zero
(-273.15°C), supposedly the lowest temperature found in the Uni-
verse, the temperature of 0°C (32°F), or freezing point, is relatively
warm. The normal human living environment, between approxi-
mately -10°C (14°F) and +40°C (104°F), is not a large range.
At a height of about 22 km (14 miles) above the Earth's surface,
water vapour becomes so thin and unsubstantial that it is dissoci-
ated into its constituent atoms of oxygen and hydrogen through the
action of strong ultraviolet radiation. As it is the heavier element,
- THE ATMOSPHERE AND ELECTRICITY