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xpect rain. Those
two simple words
can ruin picnic
plans or herald
rescue for drought-
stricken crops. Few
things in our lives
are as universal as
the weather.
“It’s what’s going on in the atmosphere
all around us all the time,” says Russ
Schumacher, Colorado State climatolo-
gist and director of the Colorado Climate
Center. “Storms and all the other inter-
esting things that Earth’s atmosphere
brings us have this big effect on our daily
lives in a lot of ways.”
But even though we tune in to local
news stations or check apps to find out
what the weather will bring, we don’t
always trust the forecasts. You’ve prob-
ably heard the joke: Meteorology is the
only occupation where you can be wrong
all the time and still get paid for it.
In reality, weather forecasts have
improved in leaps and bounds in just the
past few decades. And meteorologists in
pursuit of an ever-more-perfect forecast
continue to push what’s possible toward
its theoretical limit.
MAKING THE WEATHER
Before we can predict the weather, we
have to understand where it comes from.
To do that, we must look to the sky.
Earth is enveloped in an atmosphere
of mostly nitrogen, oxygen and water
vapor. This air, like liquid water, behaves
as a fluid. As air flows from one place to
another, it carries its properties with it,
changing the temperature, humidity and
more. Weather is simply the byproduct
of our atmosphere moving heat from one
place to another.
Cooler air is dense and can’t hold
much moisture; warmer air is less dense
and can hold more water. When regions
of air with different temperatures and
densities meet, the boundary is called a
front. Sometimes these cloudy clashes
can cause rain, as the cooling warm air
is forced to drop its water.
It’s not just fronts that can make it rain;
convection can also drive precipitation.
As warm, moist air rises, it also cools,
and its water condenses onto airborne
particles such as dust. These droplets
are carried aloft by rising air, growing
larger and larger until they become too
heavy and fall back to Earth. When that
happens, grab your umbrella.
Once a storm has formed, if there’s
nowhere for it to get more moisture from
the ground or the air, it will peter out as
it lumbers along. If it finds more warm
air and moisture — like a hurricane does
as it moves across the ocean — it will
grow and grow.
FORECASTING BASICS
With so many factors involved, it may
seem impossible to predict what weather
is on the horizon. But that’s far from the
case. “Weather forecasting is one of only
a few fields where we can accurately
forecast the evolution of a system. We
cannot do that in economics or sports,”
says Falko Judt, a research meteorologist
at the National Center for Atmospheric
Research in Boulder, Colorado.
Doing so depends on reliable obser-
vations. Scientific weather observations
began in the Renaissance, when barom-
eters and thermometers were invented.
European scientists of old, like Galileo,
used these instruments to take the
types of measurements that would one
day explain weather events. By the late
1800s, rudimentary weather maps had
come into common use.
But early forecasts were limited and
relied on persistence, or the assumption
that a system’s past would dictate its
future behavior. “If a storm system is in
Kansas one day and Missouri the next,
then by persistence you can say it’ll be
in Illinois the next day,” explains Bob
Henson, a meteorologist who writes for
Weather Underground.
Persistence is an OK way to predict the
weather when conditions are constant
— when a storm trundles along with-
out breaking up or the local climate
changes little day to day, say, in Southern
California. But this simple technique
doesn’t account for changing condi-
tions, such as storms that form quickly
through convection (typical for thunder-
storms) or moving fronts that change the
temperature.
Luckily, we have newer, better ways
to predict the future. Today’s weather
forecasts aren’t made by people looking
at weather maps and yesterday’s highs
and lows — they’re made by machines.
MODERN WEATHER PREDICTION
Meteorologists use a process called
numerical weather prediction to create
forecasts by inputting current conditions
— which they call the “nowcast” — into
computer models. The more current
and accurate information available to
these models, the better the forecast will
be. Ground radar, weather balloons,
aircraft, satellites, ocean buoys and
more can provide three-dimensional
observations that a model can use. This
allows meteorologists to simulate what
the atmosphere is currently doing and
predict what will happen in the next few
days or, for some models, hours.
Weather models divide a region, say
a single state or even the whole globe,
into a set of boxes, or cells. The size
of these cells — the resolution of the
model — affects its forecasting accu-
racy. Large boxes mean poor resolution,
or the inability to tell what’s happening
over small areas, but a broad picture of
large-scale weather trends over long
timelines. This big-picture forecast is
helpful when you want to know how a
big storm will move across the U.S. over
the course of a week.
Smaller boxes mean higher resolution,
which can forecast smaller storms. These
models are more expensive in terms of
computing power, and only run to the
one- or two-day mark to tell people