as someone with a british accent i am
contractually obliged to bring up what
peculiar weather we're having today at
least once for conversation
today i've come to the blue ridge
mountains of north carolina to find out
how weather prediction actually works
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i'm going to begin today by introducing
you to my friend and personal lawyer
dart the blood sucking leech
leeches don't do too well during
flooding so have evolved the ability to
detect the small decrease in atmospheric
pressure indicating an oncoming storm
when storms approach they will climb up
and out of the water in order to seek
shelter
realizing their weather detecting
capabilities and with medical leeches in
ready supply in 1850 dr george
merriweather designed and built one of
these
a leech-powered tempest prognosticator
the leech would reside within a couple
of centimeters of water at the bottom of
a glass jar
when storms approached it would then
climb upwards activating a whalebone
tripwire pulling on a cord and ringing a
spherical belt
after lengthy study of his leech buddies
merryweather determined a few things
occasionally one of the leeches would be
having a bad day lazing around at the
bottom of its container and unable to
predict the weather
leeches enjoyed a communal work
environment but at the same time would
also occasionally eat each other
in response merryweather modified the
prognosticator to house 12 leeches in
separate glass jars they could see each
other but also wouldn't eat each other
this meant that each could individually
predict the weather the more frequently
the leech bell chimed the more certain
meriwether could be of an oncoming storm
the thing did work and was able to
predict storms some three days in
advance
in fact it impressed the british
government so much that it was briefly
considered for adoption by the royal
navy
however that honor eventually went to
the much more pretty although also
entirely useless fitzroy stormglass
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as good as leeches are at predicting
storms their predictions understandably
lack some detail for that we need
something a bit more scientific and a
lot more precise
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in order to predict the future we first
need to really understand the present
and the way we do that is with weather
stations
there are hundreds of thousands of these
scattered across the world this one at
the blue hill observatory is one of
america's oldest constructed back in
1885.
this massive spidery looking piece of
equipment is actually just a regular old
rain gauge like the ones which you may
have in your garden
you record the amount of rain that falls
in a set period of time empty out the
bucket
and then reset it
the only difference is that these metal
sheets prevent wind from blowing in
range from the side and interfering with
the measurement
modern automator stations switch out the
cylinder for a seesaw upon which is
mounted a miniature bucket and magnetic
counterweight
rain fills the bucket the seesaw tips
and a magnetic reed switch increments
the counter
the most readily recognizable piece of
weather monitoring equipment are both
the old and new systems has got to be
that involved with the wind
this wind vane points in the direction
of the oncoming breeze while the
cut-based anemometer measures its
velocity
we're able to record how fast it spins
or in what direction we're pointing by
using a series of magnets and
magnetometers
to help gain them protected from the
elements our remaining sensors are held
within these corrugated boxes allowing
air to flow through without the
precipitation
while the outsides may look similar the
insides are nothing alike
as expected we use a bunch of capacitors
and resistors in order to report
humidity
but for the old school example we use
actual human hair which expands or
contracts as humidity changes
regardless of where when or how this
collection is done eventually our data
streams make it to central repositories
like the world data center for
meteorology in asheville
here we cross the streams allowing us to
do some pretty cool analysis
if you've ever seen the weather report
then you're probably familiar with one
of these
it's called a synoptic chart
it works by combining data from lots of
different weather stations
showing features such as pressure
temperature humidity and rainfall which
are of interest when understanding the
weather
in the past the way that we would
predict weather is by looking at the
current synoptic chart and comparing it
to a database containing all the
synoptic charts generated in the past
when we found one that looked pretty
similar to the current version then we'd
look forward in that history's future
which is still our past and therefore
predict what the future of our future is
actually going to look like
since weather patterns will move and
morph over hugeland
this method is only reliable for about a
day or so into the future
however its comparative success proved
that there was at least some underlying
logic to the weather
in 1904 norwegian physicist wilhelm
beigness formulated a set of seven
equations which could be used to
mathematically model the climate
he suggested splitting the world into a
grid of one degree by one degree
segments within which atmospheric
properties were considered homogeneous
starting with some initial conditions
you would then integrate each of these
equations over a time step of an hour
and then feed the results of each
segment into its immediate neighbors
to find out what the weather is going to
be this time next week all you need to
do is solve for these seventy six
million two hundred four thousand eight
hundred equations without making a
single mathematical error
good luck
the blueprint for computer-based weather
modelling came from british
mathematician lewis fry richardson in
his seminal work or the prediction by
numerical method however back in 1922
his idea for computer was actually 64
000 mathematicians who would manually
crank out the required calculation
with this in mind lewis designed a
spherical auditorium where each
mathematician was situated to represent
the real-world coordinates of one of the
grid cells and would be responsible for
all the calculations of that cell a
conductor wielding different coloured
torches would then tell individual
mathematicians to speed up or slow down
in order to keep the entire global
calculation in sync
from a practical standpoint this whole
thing is ridiculous but once silicon
based computers with serious number
crunching power came along it was a
simple matter to convert these
suggestions into actual usable code
today we use a mix of low resolution
global models and high resolution local
ones adding in an extra dimension and
altitude in order to make things a bit
more realistic
physics-based equations known as
parameterizations link larger scale
results of the grid to smaller scale
local phenomena
pioneered by dr duran simpson in the
1960s these equations for cloud
formation allow us to consider effects
which would otherwise be missed
improving both the accuracy and
usefulness of our predictions
our final obstacle brings us back to
bugs not leeches this time but
butterflies
the story goes this in 1961 american
meteorologist edward lorenz was
re-running a simulation on his computer
going out to grab a coffee upon his
return he found some rather interesting
results
the alberts from his two simulations
started identical but after about a week
of simulation time the two started to
differ
at first just by a trailing decimal
point but over more and more time they
were completely unrecognizable
at first edward panicked thinking that
he'd fried his computer during the
second simulation
looking in more detail however he saw
the real problem was with his inputs
in the first round he used a humidity
value of 0.506127
but in the second he rounded it off
2.506 thinking nothing of it however as
it turns out the tiny difference in
initial conditions eventually grew to
dominate the solution
this chaotic phenomena has since been
dubbed the butterfly effect with the
idea being that a flap of a butterfly
swing in brazil
could eventually settle for tornado in
texas
predicting the weather as a chaotic
system is mathematically impossible you
might as well go right back to leeches
which is actually sort of what we do
today
like how merryweather used a council of
12 in order to predict oncoming storms
even if one or two of his leeches were
having a bad day
today we use ensemble forecasting
essentially we run up to 50 simultaneous
simulations with slightly different
input conditions
we then take the average of their
results in order to produce a forecast
if in one simulation we have a hundred
percent of the area is going to be
covered in rain and in the other zero
percent then together we get a 50 chance
of rain
on the other hand if both simulations
say 50 rain cover then together they
will still give this value of 50 chance
of rain even the day that we experience
will be very different
from a council of leeches to a dome of
mathematicians and an ensemble of
simulations predicting the weather can
be almost as extreme as the weather
itself
brought about rather than just
anticipating what's to come instead we
could control it
dial up a downpour or clear the skies
with a wave for your hand
it's going to be some extreme
engineering but we'll need to wait for
another episode
until then this has been james dingley
from the atomic frontier
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