[narrator] We have
uncovered a remarkable planet,
a world of extreme weather,
alien landscapes,
and bizarre phenomena.
It's the most complex system
we know of in the universe.
That planet is Earth...
...and we're only beginning
to unlock its secrets.
It seems like
a most familiar sight,
but, in fact,
it's one of the strangest
phenomena in the universe,
and humans
couldn't live without it.
[man] The water
that is in our bodies right now
has been around
for millions of years,
and all of us probably
have water
that passed
through Albert Einstein,
George Washington.
[narrator] It's called rain.
Created by powerful
but little-known forces,
it teems with mystery
and even sometimes with life.
Rain decoded
on Secrets of the Earth.
Even from space,
it's clear that Earth
is a special planet.
It's unique
in this solar system
because it has massive oceans
and it rains water
from the sky.
[thunder]
[man] When you look
down at the Earth from space
the two things that jump
out at you are
the white--the clouds--
and the blue--the oceans.
[narrator]
Canada's Vancouver Island,
the wettest place
in North America.
Here it can average
an astonishing
256 inches of rain a year.
[man] The oceans, the clouds,
and even the water
on the moss around me here,
it's all part of
the same water cycle,
the cycle that sustains life
on the planet.
This cycle includes everything
from my breath
adding moisture to the air
to the water hanging
on this moss
to a river like this
flowing towards the ocean.
Or this.
[narrator] Even
the world's driest deserts,
like Death Valley,
are shaped by rain.
[man] Here in Death Valley,
even though
it's an extremely dry place,
everywhere we look,
we see evidence
of the work of water
that it sculpts the landscape.
It creates these fantastic
shapes that we see here,
and this arroyo
that's located behind me
that's deeply carved, deeply
incised into this hillside is
caused by torrential rains
that flood down the slope,
carve into the landscape,
and move
those sediments downstream.
[thunder]
[narrator] Rain may seem simple,
but few people
on the planet understand
how it actually works,
how it relies
on visitors from outer space
and strange creatures
from Earth
to make it fall from the sky.
The secret life of rain
begins with a basic fact--
that across most of the Earth,
rain starts out not
as a liquid but as a solid.
[Donner] Here in the rainforest
on Vancouver Island,
it's raining on me right now,
but these rain droplets
didn't actually start as rain.
They started
in the clouds as snow,
which is a combination
of ice, air,
and little droplets of water.
Most rain on the planet
actually starts as snow.
[narrator]
High in Earth's atmosphere,
it's far below 32 degrees,
and as every child
on Earth is taught,
water freezes at 32 degrees,
except that it doesn't.
[man] It wants to freeze
because it's below the
freezing point, but it can't.
It needs an impurity,
an impurity around
which ice crystals can form
and then a solid can take place.
[narrator]
And this planet's atmosphere
is full of tiny impurities
that play this vital role.
[Quiring] The particulates
that we find in the atmosphere
come from many
different places--
from sea spray from the ocean,
from forest fires and the smoke
that they produce,
and even here in Death Valley
in a sand storm,
it's gonna pick this sand up,
toss it into the air,
and carry it into the clouds.
[narrator]
And some rain-making particles
actually zoom in
from outer space.
Every day, 6,000 pounds
of tiny micrometeorites
crash into Earth's atmosphere.
[Kaku] It's hard to believe,
but it's estimated
that the Earth may get heavier
by perhaps 10,000 tons
or so every year
because it's sweeping
through this dust cloud
of tiny micrometeorites.
[Donner] They're very tiny.
A micrometeorite is
less than the width
of a piece of human hair,
and what that means
is that there's
not that much friction
as they're falling.
So they don't burn up
in the atmosphere
like a big meteorite would.
[narrator] This micrometeorite,
once part of the tail
of an ancient comet,
hits the atmosphere.
It slows down
and eventually drifts into
a cloud full of microdroplets.
They're below freezing,
but they can't crystallize
without coming into contact
with an impurity
like this tiny space invader.
[Quiring] They encounter
one of these particles
in the atmosphere
and crystallize
around a single particle.
[narrator] This happens
billions of times a day,
and the proof
is everywhere on Earth.
You can actually look
for micrometeorites yourself.
All you want to do
is collect some rainwater.
You get yourself
a really strong magnet.
You take your magnet and just
swish it around in the water
for a couple minutes
to make sure you pick up
everything that's in there.
If you look through them
under a microscope,
anything
that's really spherical--
there's a good chance that
it's actually a micrometeorite.
[narrator]
Now that a water droplet
has turned into an ice crystal
around the micrometeorite,
the crystal itself creates
an even larger solid surface
for other tiny droplets
to freeze onto as well.
[man] It turns out ice
is the best template for ice.
So once you initiate freezing,
you can very readily form
many, many crystals
from that template.
[narrator] The tiny crystal
grows into a snowflake
heavy enough to begin to fall,
but its journey
is far from over.
[Quiring] The closer
it gets to the surface,
the warmer the temperatures,
and as it passes through
that warm air,
that warm air works to add
energy to that ice crystal.
[Donner] The snow begins
to melt and turns into rain.
[narrator] Here in Vancouver,
it finally reaches the ground.
[Donner] So on a rainy day
like today
if you reach out
to catch a raindrop,
you might actually catch
a little micrometeorite.
[narrator] But take a close
look at that raindrop.
There might be
something else inside.
Instead of being
formed by a tiny speck of sand
or micrometeorite,
it might have been formed
by a living being.
If so, that would hardly be
the strangest kind of rain
to fall on the planet Earth.
For years, scientists
on Earth believed
that all rain was caused
by water condensing
on solid particles in the air,
but it turns out that
some rain is actually caused
by living creatures
trapped in the sky
and desperate
to get back to solid ground.
[Christner]
We've known for a long time
that many microbes use
the atmosphere to get around.
Within a cubic meter of a cloud,
there may be as many
as 100,000 microbes.
[narrator]
Every day across the planet,
warm air on the surface
rises into the atmosphere,
pulling two million tons
of bacteria along with it.
One of these bacterial species
is a remarkable creature
known as Pseudomonas syringae
that lives on plants,
including those
that mankind grows for food.
[Christner] On a typical
plant leaf like this,
a cubic centimeter could contain
100,000 individual
microbial organisms.
[narrator] Farmers
hate Pseudomonas syringae
because it has
a bizarre ability
to create frost on crops
at temperatures where water
wouldn't usually solidify.
How does it do this?
Incredibly, it has evolved
a protein that forces water
to take on the same
solid shape as an ice crystal.
Cold water then locks
onto the crystal,
forming additional ice.
[Christner]
Here in southern Louisiana,
we don't see
freezing very often,
but when it does happen,
it can decimate crops.
So, for example,
this field of sugar cane
can all be wiped out
in less than an hour.
Pseudomonas syringae
and other ice nucleators
which are present
on crop surfaces
are responsible
for most of the damage
which occurs during freezing.
We can demonstrate what
Pseudomonas syringae might do
right here in the laboratory.
Here we have some liquid water
that's been cooled down
to about 20 degrees Fahrenheit.
You'll note
that it isn't frozen.
I'm going to add a sample of the
Pseudomonas syringae culture...
...and it freezes
instantaneously.
[narrator] This secret talent
for turning water into ice
isn't just used
for causing frost on plants.
As syringae floats into a cloud
and comes into contact
with a cold microdroplet,
it forces that droplet
to crystallize into ice.
The droplet quickly grows into
a snowflake, melts into rain,
and falls back to Earth.
This microscopic creature
is carried back home,
riding in the raindrop
it created.
[Christner]
They will incite precipitation
and make their way
back to surface environments
that they call home.
[narrator] But syringae is
hardly
the strangest secret of rain.
Even stranger is
the fact that every rainstorm--
and every human body,
including yours--
contains
ancient water molecules
that once existed
in every dinosaur
and every other person
who once walked the Earth.
Since humans
are largely made of water,
the cycle of rain
even includes the human body.
Up to 60% of the human body
is water.
The human brain is 70% water.
Human lungs are
nearly 90% water,
and that water
has been endlessly recycled
from the days it first
rained down on early Earth
in frozen comets
to the age of dinosaurs
to the rise of early man
and all throughout
recorded history.
[Quiring] The water
that is in our bodies right now
has been around
for millions of years,
and all of us right now
probably have water
that passed through
Albert Einstein,
George Washington.
[narrator]
For it to rain on Earth,
water droplets in the sky
need to freeze
around something solid,
like a micrometeorite
or a microbe,
but how does the water get into
the sky in the first place?
It's a hot afternoon
in a North American suburb.
From the surfaces
of the houses,
lawns, roads, and buildings,
liquid water is warming,
evaporating and entering
the air as water vapor.
Near the surface,
water molecules
in this typical puddle absorb
enough heat to make the leap,
the sudden transformation
from liquid to gas.
In a split second,
they separate
from their fellow
water molecules in the puddle
and rise into the air.
[wind]
[Quiring]
As air parcels become warmer
than the surrounding
air mass, they become unstable.
They're less dense.
They're going to rise
like a helium balloon
up into the atmosphere
and carry that moisture
with them.
[narrator] Across the planet,
92 quadrillion gallons of
water rise up from the oceans
and land every year.
[Quiring]
The average residence time
for a water vapor molecule
in the atmosphere is
about ten days.
So over the course
of ten days,
this average water molecule can
travel thousands of kilometers.
[narrator] At any given moment,
there are 25 million billion
pounds of water in the sky.
If it all fell out at once, it
would cover the whole planet--
both oceans and land--
in one inch of water.
As a water molecule rises,
it also cools
at a constant rate
of 5.5 degrees
for every 1,000 feet.
As the cooling air gets denser,
the water molecule
starts slowing down.
Suddenly,
it hits a critical moment.
As it bumps
into another molecule,
they latch together and
condense back into a liquid.
If enough of them do the same,
the whole mass becomes visible
to the humans on the ground.
A cloud is forming.
[Quiring]
We begin to be able to see
those water droplets
coming together.
So there's enough condensation
so that it reflects the light,
and we can see it as a cloud.
[narrator] In the solar system,
clouds themselves are not
that unusual.
On Venus, a perpetual cloud
of deadly sulfuric acid
powerfully reflects
sunlight back into space.
[Kaku] That's why
it shines so brightly,
and that's probably why Venus
was named after
the goddess of beauty and love,
because it shines so brightly,
but it's due to sulfuric acid.
[narrator] The clouds of Mars
are wisps of carbon dioxide.
Jupiter's
are thick with ammonia.
In Earth's clouds,
microdroplets of water
are far too small and light
to fall to the ground.
Millions will have to lock
together into an ice crystal
to begin their journey
back to the Earth's surface,
and to do that, they need
to form around something solid.
This is where particles
from billowing sand storms,
forest fires,
erupting volcanoes,
micrometeorites,
and even microscopic creatures
come into play...
[thunder]
...but sometimes
on planet Earth,
even stranger things
fall from the sky.
The watery rain of earth is
unique to this blue planet...
[thunder]
...but the universe
is full of strange rain.
[Kaku] On Venus,
it rains sulfuric acid.
We think on Titan,
a moon of Saturn,
it rains some kind of ethane,
which is very similar
to methane.
[narrator] But earth also has
its share of strange
and bizarre forms of rain.
Throughout history,
people have reported
frogs, lizards, fish, and eels
pouring from the skies.
[Kaku] Believe it or not,
stories about it's raining frogs
and it's raining fish
are actually true.
[narrator] There were
600 published reports
in the 20th century,
including widely reported
incidents in Australia in 1906,
and St. Petersburg, Florida,
in 1952,
when fish poured
from the skies;
Buckinghamshire,
England, in 1969,
when frogs rained down;
and Connecticut in 2003,
when tiny gelatinous frogs'
eggs pelted the town of Berlin.
The secret culprit
of these strange rains
is a surprisingly
common phenomenon...
the waterspout.
Essentially a tornado
over water,
a waterspout can spin
at up to 200 miles an hour.
Anything within
several feet of the surface
can be pulled up
into the vortex.
[Kaku] It will suck
into the funnel
many of the sea life,
and it will rain down on people.
[narrator] Even when
the waterspout stops spinning,
the life within can travel
up to 100 miles,
buffeted up and down
by powerful updrafts.
Eventually, though,
the winds calm down,
and the Earth's strangest
rain pours onto the land...
...but rain itself--
the watery kind--
has held on to its secrets
for millennia.
Only now are humans discovering
how water really makes
the astounding leap
from solid to liquid to gas,
a feat that remains
unique to Earth;
how rain is dependent not just
on temperature and pressure
but on volcanoes and
forest fires and dust storms,
on invaders from space,
and even invisible
forms of life;
and most of all,
how rain has turned
this third planet from the Sun
into the most watery place
we know.