raises the humidity of the air below its
parent cloud. If the rain evaporates before
it hits the ground, it leaves visible streaks
that slowly vanish called “virga”.
Cumulus clouds tend to cause showers- on-again-off-again rain that reflects the
cellular nature of the clouds, but with heavy
raindrops because of the strength of the
updrafts that create them. Stratus clouds’
individual drops are usually smaller, since
there aren’t turbulent winds to make them
collide, but they usually cause steady falls
that last a long time, which can result in
large totals on the ground.
Inside a cumulonimbus cell, huge
updrafts keep the drops in the air for a
long time, allowing them to grow very
large, often taking them above freezing
altitude to create hail. (This is a different
process to the formation of snow, which is
usually driven by water vapour attaching
to pre-existing ice crystals, growing until
they are too heavy to be kept aloft.)
When these large raindrops finally fall
and start to evaporate, they cool the air
around them – much like sweat cools you
when a breeze is applied – and this can
have dangerous implications. If the air is
cooled sufficiently it can become much less
buoyant than its surroundings and crash
rapidly to earth, causing microbursts
with erratic winds blowing hundreds of
kilometres an hour.
CLOUD. ON. GROUND
On a foggy morning, when you step out the
front door, you are walking into a cloud.
Fog is just cloud on the ground (which
makes me want to rename it “cog” – if
there’s a petition, let me know). To create
“cog”, the ingredients need to be just right.
First of all, you want the air to have
some moisture – the more the better,
because it will take less effort to get it
to saturation. Next, you want the air to
cool down. A clear, cloudless night is
usually the easiest way (surface-level high-
pressure systems are best for this, because
they help push air downwards, clearing out
any clouds). Lastly, you want the faintest
of breezes (more on that in a moment).
From there it’s fairly straightforward –
cool the air near the ground to saturation,
cloud droplets form. Voila, fog.
But there’s a problem. The atmosphere
heats and cools from the bottom up,
and liquid water in the air acts as a great
insulator. Air is fairly translucent to
much of the sun’s radiation (certainly the
wavelengths that provide the bulk of the
heating), which is good for those of us who
want to see. This means that heating in the
atmosphere primarily works by sunlight
getting absorbed by the Earth’s surface,
then slowly passed to the air immediately
above it. Air has a lot of thermal inertia (and
water has even more), so it too cools slowly,RAIN DANCES
When it comes to rain, size matters. While
a fairly small cloud can hold large amounts
of water, a good, average-sized small
cumulus can easily yield a volume in the
cubic kilometre range. The liquid water
density of a fair-weather cumulus (a fluffy
white cottonball that won’t rain) is around
0.5 grams per cubic metre (g/m3), and that
represents roughly 500 tonnes of water
in the air. In a cumulonimbus, the density
can be as much as six times that, with the
cells reaching potentially 20 kilometres
into the atmosphere with a footprint many
kilometres across. The amount of water
these clouds hold can be incredible – one of
20km^3 could hold 9600 Olympic swimming
pools of water.
Tiny cloud droplets have a fair
distribution, measuring anywhere from
one to 100 microns in size, and are easily
kept aloft like motes of dust. In fact, every
raindrop starts as a mote of dust, or a grain
of salt, a particle of pollution, a bacterium,
a fungal spore or some other similarly
sized particle in the atmosphere. Water
doesn’t like to condense out of clean air; it
prefers a surface of some sort, which is why
you can have misty windows in a clear car.
The droplets bump into each other on
air currents, combining as they collide. Like
a steamy bathroom mirror, they remain
suspended until they get large enough that
gravity forces them downwards, 100 to
1000 times larger than when they formed.
Once a drop leaves the cloud, it finds
itself surrounded by air with relative
humidity less than 100%, so it starts
to evaporate. If you’ve ever noticed a
raincloud with a shaggy underside, you’re
seeing a cloud that has formed as the rainZEITGEISTWEATHER
102 – COSMOS Issue 86