When the public has the opportunity to ask meteorologists those
questions that have troubled their minds, several are most frequently
asked: Why is the sky blue? Does lightning ever strike the same place
twice? and Is it ever too cold to snow? Another frequent question is:
"If water drops are heavier than air, why do clouds float?" With lovely
forms of cumulus clouds popping up all around me on this summer's day,
that is the question I will tackle this month.
The steady visage of clouds gives us the appearance that the cloud is floating gently on air.
the beginning to that question states: "If the water droplets that make
up a cloud are heavier than air...." Yes, liquid water is denser than
an equivalent volume of air by about a factor of 1000. But density is
not the only factor pertinent to this discussion. But first, let's look
at the weight of a cloud. (I consider only purely water clouds here.
While some clouds are made of ice and many are a mix of ice and water
droplets, the same principles apply to them as pure water clouds.)
a warm summer's afternoon, I see a typically small cumulus growing over
the western hills. I guess its volume to be about a cubic kilometre —
many that size are in this range — and its base lies at an altitude of
about 3500 metres above the ground. It is not difficult to calculate
the total mass of that volume and it weighs in at about one billion
kilograms. Within that mass are billions of cloud droplets comprised of
liquid water and a dash of solid materials that were used as
condensation nuclei. They account for about one million of those
kilograms, thus giving the liquid cloud itself a weight approximately
that of 700 standard-sized automobiles, though the droplets are spread
across the whole volume. The cloud may look rather dense because each
droplet scatters, reflects, refracts and diffracts the light rays
passing through the cloud mass. The sum total of all this cloud–light
interaction delineates the visible mass.
We know liquid
water is heavier than air, otherwise spilling a glass of it would not
wet the floor but would wet the walls. From the above estimates, we
have seen there is a lot of mass in the liquid cloud, so what keeps the
cloud afloat? Why doesn't gravity pull the cloud down to earth?
saying a cloud is floating on air is a bit of a misnomer. When we think
of floating in a technical sense, we think of an entity kept up within
the medium by buoyancy forces due to density differences such as how a
piece of wood floats on a lake, or how a hot-air balloon floats in the
air. The cloud does not "float" in the air, because its water droplets
are heavier than the surrounding air. The second question in the
preceding paragraph is in fact where the error lies. Gravity does pull
the cloud down to earth. The reason the cloud we are watching does not
seem to fall to the ground is the result of two other mechanisms acting
on those cloud droplets.
The most important of these is
that the cloud, particularly our afternoon cumulus, has been formed
within a rising current of air. There are many causes for this rising
air, I wrote about several elsewhere (making clouds and updrafts).
A rising air current is one of the important steps in the formation of
a cloud. Rising air in a cloud core continues until the cloud reaches
old age and begins to dissipate. Cloud droplets within this rising air
are thus continually pushed upward at a greater rate than the rate at
which gravity pulls them downward. The net result is that the cloud
droplets are rising within the updraft, which you can see by watching
the tops of that cumulus. If the cloud is not dissipating, you will see
growing and changing bumps of white at the top of the cloud, an
indication of updrafts and continued growth.
Now put aside
thoughts of the updrafts for a moment. We next look at what happens to
those cloud droplets under the influence of gravity. Now we know that
gravity wants to bring everything down to earth, and as Galileo showed
us, the pull of gravity is independent of the object's mass. That is
mostly true, because the objects he dropped off the Tower of Pisa were
of approximately equal size and shape though not of equal mass. As a
result, the forces of air resistance on the two objects were about the
same, and the distance of fall not that great. Galileo's experiment
works to perfection when objects fall through a vacuum, but we do not
live in one nor do clouds.
Our cloud droplets are falling
through air which puts up some resistence to any object falling through
it. This is technically known as aerodynamic drag, and its
value depends on the object's mass, shape and size. Cloud droplets are
more or less spherical (ice crystals have a wide variety of shapes that
may increase their drag effect) and thus have a smaller drag effect
than a less symmetric shape — say a feather. They also have a very
small size, 12 micrometres or less in diameter.
object falls through the air (or through water, for that matter), the
aerodynamic drag force counters the force of gravity, and after a
time/distance, the falling object reaches its terminal velocity.
This is the rate at which the falling object thereafter descends toward
the surface. The time it takes to reach the ground can be calculated
from the terminal velocity.
For the typical cloud droplet
of 10 micrometres (it takes about 15 million cloud droplets to form the
typical raindrop), its terminal velocity is around 0.3 cm/s or about 10
metres per hour (30 ft/h). To fall from the cloud base at 3500 m at
this rate would take 350 hours! An object moving that slow would appear
to us as being stationary unless we observe it closely for some length
of time. Only when the droplets congregate to form rain drops, 300 or
more times larger, does the fall time drop to minutes.
small fall velocity is countered by the rising air surrounding it so
that many cloud droplets are actually rising at a net rate of perhaps
tens of centimetres per second. Of course, a droplet caught in a
downdraft will descend at the rate of the downdraft plus its terminal
velocity. Now should a cloud droplet be in a region of descending air
heading toward the surface, it must pass through the cloud base on its
way to the ground. In passing, nature does a bit of a magical trick and
makes the droplet disappear.
Ah, but no hocus-pocus here,
just old reliable physics. You see, the cloud base is also at the
condensation level for the atmosphere surrounding the cloud and its
neighbours. Above this level (height), rising air will have cooled to its
condensation point, and liquid water will be produced from the rising
water vapour within the air. That is why all cloud bases are more or
less at the same altitude. But reverse the process and the condensation
level becomes the evaporation level, and liquid water within the
descending air will return to the vapour state...and that makes it
disappear to our sight.
The combination of slow descent
of cloud droplets and their quick evaporation below the cloud base
gives us the appearance that the cloud is floating gently on air. Ah
the tricks of Mother Nature!
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