LAYERED:If your cloud is created gently by
cooling it down to saturation, or by slowly
pushing it up in the atmosphere, it takes on
a much more uniform appearance. Those
days when clouds look flat and stripy – that’s
when you’re dealing with stratiform clouds.LUMPY AND LAYERED: Sometimes it’s
really hard to tell exactly what’s going on- you’ve got something that looks like a
layered cloud, but there are some features,
or maybe your cumuliform clouds are
spreading out and joining; that’s when to
just mix the two and call it stratocumulus.
Next, think about how high your cloud is.
This can be tricky, because the depth of
the troposphere (the lower levels of the
atmosphere where almost all of the weather
happens) changes depending on where
you are on the planet. At the poles the
atmosphere is significantly smaller than at
the equator, largely because warm air takes
up more space.
Clouds in the lower regions – from
the surface to about two kilometres – are
generally made of liquid water, and get the
straight names: cumulus and stratus.
Mid-level clouds are a mix of liquid
water (often supercooled below 0°C) and
ice crystals. These get the prefix “alto” – so
altostratus and altocumulus.
In the high levels, it’s all ice, which
makes for a much more wispy appearance.
To our meteorological forebears these
clouds looked like mares’ tails, which may
explain their Latin hair-curl-inspired name
“cirrus”. They are often difficult to clearly
define, so cirrus is a cloud in its own right,
along with cirrostratus and cirrocumulus.
Sometimes clouds extend through
several layers – “nimbus”. Nimbostratus
is a wall of featureless cloud that heralds a
very wet day, while cumulonimbus is the
cloud most people tell me they love – a
thunderstorm.
Put all that together and we’ve got
the 10 genera that all clouds fit into. But
there’s a whole range of other features that
can give the species in the classification. Is
it towering, are there waves or lobes? If you
want to explore the cornucopia, the World
Meteorological Organisation’s International
Cloud Atlas is your one-stop-shop.
Now that you can name your nearest
cloud, I hope you’re wondering: What can
it actually do?Everyone has their favourite, and you
may already know its name – cumulonimbus
or cirrus – but knowing why it’s called that
can tell you a bit about what caused it, and
what it might go on to do. There are a few
basic guidelines and several methods of
attack when trying to tie your cloud down.WHAT CLOUD IS THAT?
Clouds are the weather we can see, and they
can tell us a lot about what’s happening in
the atmosphere. Where clouds form, air is
(almost always) moving upwards. That’s
something air doesn’t generally like to do –
sideways is relatively easy, but up and down
are much more difficult, because you’re
either fighting gravity or trying to push
towards an area of higher pressure. When
you do get the air to move upwards, though,
you bump into the good old combined gas
law: P1V1/T1 = P2V2/T2.
Higher in the atmosphere the pressure
is lower, so any air that pushes up there
expands and the temperature drops. Air
holds water vapour, and when you drop the
pressure or the temperature, the saturation
point also drops. When it’s high enough (or
cool enough), the air reaches 100% humidity
and excess water vapour condenses into
water droplets. That’s the start of a cloud.
There are two ways you can get air to
move upwards – either give it a sharp kick
from an external force, or gently sweep it.
This informs what kind of cloud you end
up with, and that’s how I like to start my
classification, with the age-old question: Is
it lumpy or layered?LUMPY: When air gets pushed upwards
explosively – perhaps by heating it or
by forcing it up a mountain range like a
skateboarder on a ramp going for some epic
air – eddies and currents around the airmass
cause the cloud particles to take on structure.
It can look lumpy or fluffy like a scoop of ice-
cream or cotton wool. It’s all kind of piled
up, hence their name: cumuliform.Nate’s Fab Five
Choosing a favourite cloud is as hard
as picking a favourite child, but here
are a few stand outs:MAMMA
Often called mammatus clouds, these
are ominous, pendulous lobes that
can be found hanging underneath
a cumulonimbus in the right
circumstances. They often indicate a
severe storm cell.FLUCTUS
An accessory cloud that gets physicists
excited. Curled wave formations called
Kelvin-Helmholtz waves are caused
when an airmass interacts with another
above it that’s moving at a different
speed or in a different direction.
Nothing significant weather-wise, but
a rare delight to spot.PILEUS
When a rising airmass is creating a
cloud, it can force a layer of air above
it to rise, creating a small stratus above
the upwelling cumulus, like a cloud
with a whimsical hat.HOMOGENITUS
Clouds created by human activity, like
cumulus forming above a smokestack
or cirrus left behind a jet in a contrail
(ice particles formed in the wake of the
jet by a pressure disturbance).FLAMMAGENITUS
Clouds formed by the heat rising from
a fire. The one you’ll hear about most is
commonly called pyrocumulonimbus,
but its official designation is
cumulonimbus flammagenitus.
These clouds are incredibly difficult
to forecast and can cause widespread
havoc with sudden changes of wind
direction and lightning.Nimbostratus CumulonimbusRIGHT PAGE ANDREW MERRY, LEFT JOHN GREIM
WEATHER ZEITGEIST
Issue 86 COSMOS – 101