and also from the bottom when the surface
is no longer heated by the Sun’s rays.
With all this in mind, when fog first
starts to form right next to the ground,
it creates a really good insulation layer,
preventing the air above it from losing its
heat as efficiently and cooling to saturation.
If there’s a small breeze, mixing can loft
the fog layer and replace it with the air
from above, allowing that to cool and form
fog, which grows up from the ground.
Usually.
If you’re up a mountain, fog can happen
just because a normal cloud bumps into
you. It still counts as fog, even though the
process for making it is entirely different
(seriously, it should be “cog”). Fog can also
be blown around – or advected – from
place to place, so you can still end up with a
fog even if the conditions aren’t quite right.SOUND & LIGHT SHOW
Only cumulonimbus can make thunder
and lightning, because the phenomenon
requires specific processes that only occur
in the explosive updrafts associated with
the cloud. (Exploding volcanoes can also
create lightning, but that’s one for the
vulcanologists to explain.)
Lightning is a wildly scaled-up static
shock such as you might get after rubbing
your shoes on carpet. That’s dealing with
puny charges in the order of 20,000 volts,
whereas lightning can deal out a potential
one billion volts per bolt. Electrons are
stripped from their molecules by some
sort of collision, and when you have great
updrafts keeping things in the air for a long
time, collisions are a given – particularlyin
the middle levels of the cloud where you can
have a mix of supercooled water, ice crystalsand hail. Typically, lighter raindrops and
small ice particles are lifted upwards and
encounter heavier, falling hail. That’s
where things get a little hazy (forgive the
forecaster’s pun), as meteorologists aren’t
yet entirely convinced on a single method by
which those charges form and gather. But
the result is well known: charge separation
across the cloud, with a positive charge at
the top and negative at the bottom.
That negative charge in the underside
of the cloud also induces a positive charge
on the ground, and that’s when the fun
starts. Air is a great insulator, and it takes
a lot of charge build-up to overcome the
resistance. Once the charge is large enough,
a catastrophic discharge of electrons occurs
to try to equalise the charge differential
(because nature doesn’t only abhor a
vacuum, but also a sufficiently strong
gradient), with a massive current heating
the air into plasma as it passes through.
The charge equalisation can happen
either between the top and the bottom
of clouds (cloud-to-cloud or intra-cloud
strike), the bottom of the cloud and the
Earth’s surface (cloud-to-ground strike),
and occasionally between the positively
charged cloud-top and a distant negatively
charged area of ground (also cloud-to-
ground, but called positive lightning – the
amount of charge needed to overcome the
air’s resistance is so huge that these bolts
are particularly dangerous and can travel
many kilometres away from a storm cell).Thunder is a side effect of the heat of
the energy ripping through the air. The air
expansion along the bolt’s path is so fast
that it breaks the sound barrier: thunder.
When the electricity is turned off, the air
cools rapidly, causing more reverberations.
Bounce that sound around a bit, plus allow
for the several strikes that actually make
up one complete bolt of lightning, and you
end up with a roll of thunder.
There’s also a wide array of lightning we
don’t get to see from the ground but which
has been observed by pilots and people
in orbit. They have some great names- sprites, blue jets and tortured ELVES
(Emission of Light and Very Low Frequency
perturbations due to Electromagnetic
Pulse Sources) among them – but little is
known of these phenomena.
Some wish for clouds to go away, many
of us wish for more – but either way, it’s
hard to deny that these enormous, heavy
and complicated structures are fascinating,
despite often passing overhead completely
unnoticed. And after all of that, we’ve
barely scratched the surface, which if you
were ever to try to do, would show even
more cloud underneath. It’s kind of the
nature of the beast.
NATE BYRNEis a meteorologist,
oceanographer,ABCpresenter and
all-round science nerd.FROM LEFT TO RIGHT, BROOK MITCHELL, NICK RAINS, JOHN CRUX PHOTOGRAPHY
WEATHER ZEITGEIST
Issue 86 COSMOS – 103