The water cycle | Ecology | Khan Academy

- [Voiceover] Let's talk a little bit

about the water cycle, which
we're all familiar with.

In fact, we're all part
of the water cycle,

every moment of our lives.

We might not fully appreciate it.

So let's just jump in the cycle.

I'll start with evaporation.

So we could start with the
surface of the ocean here,

or this river, or this lake.

And at any given moment,

there's going to be water
evaporating off of that surface.

Water molecules that were
in their liquid state,

liquid state, they just have
enough energy to bounce away

and go into the gas state.

And water in a gaseous state
we call vapor, water vapor.

Water, water vapor.

And so that water vapor,
it is going to rise,

likely with the air that has
been heated on the surface,

due to the sun, and there's
other more complex dynamics

at play, but as it rises, and
as the overall temperature

cools, that water vapor will
condense into little droplets.

It'll condense around
little, tiny air particles,

little particles of dust
that you can't even see

with your eye, and
that's what forms clouds.

So this is little droplets,
so the water's actually back

to the liquid form, they're
not individual water molecules

anymore, they're now able
to interact with each other,

and they're condensing
around these little,

microscopic dust particles
to form these water droplets.

And if it's cold enough,
they might also form

small ice crystals, and
that's what clouds are.

And we see here, they're
talking about transportation,

you can have these clouds, we
obviously, if you look outside

and you see clouds, those
clouds are moving with the wind.

And so they could be moving
all of those droplets

with the overall wind.

And when those droplets get heavy enough,

they will precipitate,
they will precipitate down.

Now they could precipitate
back into where we started,

they could go back to the ocean there.

Or you could go onto a mountain here,

and since if the air is cold enough,

and if you have the right conditions,

that precipitation might be snow,

and it might stay snow right over there.

Or ice, but then eventually
things might warm up,

or they might not warm up,
but if they do warm up,

well, then, they would melt,

and there would be snow melt run-off.

And that's what you're seeing there.

If that rain is falling in this area,

so let's say it's not cold
enough for it to be snow,

we are talking about rain.

Well, most of that water
is actually going to

percolate down into the soil.

So most of it goes down.

We look around us and we
see these rivers and lakes,

and we say wow, there's
a lot of water there.

But it turns out, there's
actually a lot more water

inside the ground and
obviously, in the ocean.

And we'll talk about that in a little bit.

So you have all of this water that forms

in these underground aquafiers here.

But some of it also ends up in these lakes

and these lakes are usually
in a situation where

the ground is either already
saturated with water,

or there's the right types of rocks,

so it can contain the water up here,

and similarly, rivers
are formed by runoff,

it's snow melt run-off
can famously form rivers.

And in general, if you
see a creek or a river

near your house, especially
when it rains it fills up,

that's a good indication
that the ground water's

already saturated and so
things are running off

into that river.

And so that in general is the water cycle.

You have evaporation, it
condenses into clouds,

it eventually precipitates,
and it keeps going,

round and round and round.

Now of course, there's
others actors at play.

You have things like plants.

Plants will take up water
from the upper soil,

as far as the plant's roots go.

And it will use that water
to transport nutrients

down from the soil up into the leaves.

It also uses that water as part of

the photosynthesis
process that we've studied

in many videos.

And a lot of that water
gets transpired out.

So once again, this is
transpiration, essentially,

evaporation out the leaves of the water.

Over here you see this word sublimation.

That's going straight from
the solid form of water, ice,

into the gas form of
water, or water vapor.

And this will happen in
situations where it's cold

and it's very, very, very, very dry.

And you have, in general, low pressure.

So instead of going into the liquid state,

right then the water
molecules start just leaving

as water vapor.

And of course, I said, we're part of it.

Well, how are we part of it?

We will drink some of this fresh water,

our bodies are actually mostly water.

The cells in our bodies are 70% water.

Everything we study in biology,
water is a key environment

for all of these things to occur.

And then we use that water,

and then we will get that
water out of our body,

and then it continues on as
part of this water cycle.

Now one thing that I
find really interesting,

as an organism that is
dependent on fresh water,

when people say fresh water,

we're talking about water without salt,

as opposed to salt water.

So we really need the
fresh water in this lake,

or in this river, or we might dig wells,

so that we can get the water
out of these aquafiers.

It actually turns out that
very little of the overall

water in the world is fresh water.

And so let me show you
this chart over here.

I always knew that, but
I didn't fully appreciate

how little was fresh water.

So of all the water on our planet,

97.5% is salt water, for the
most part, in our oceans.

Only 2-1/2% is fresh water.

And even of that 2-1/2% fresh water,

very little of it is what
we traditionally associate

fresh water, the lakes and the rivers.

When I think of fresh
water, I'm gonna say oh,

let me go to a lake or a river,

that's stuff that we
could potentially drink.

But most of it is actually in glaciers

and permanent snow cover.

So it's ice, snow, that
is just not melting.

And it makes you think
about what would happen

if this stuff were to melt.

And then you also have ground water,

which we could have access to.

That's why people dig wells,
so we're talking about,

well, ground water includes soil moisture,

swamp water, and permafrost.

Very little of the water is
actually in lakes and rivers,

which I personally find fascinating.

It wasn't obvious to me before
I, frankly, saw this chart.

Now another really interesting thing is,

how long, on average,
water molecules might stay

in different parts of this water cycle.

Going back here, you can
imagine that a water molecule

can stay for a very
long time in the ocean,

especially, you know it's
going to be moving around,

depending on ocean
currents and temperature

and all of that, but you can imagine,

it could stay in that
liquid form in that ocean

for a very long time.

And maybe it spends a shorter
amount of time in a cloud.

And people have actually studied this,

which I find fascinating.

I'd be curious to figure
out how they actually

got this data.

But this is the average residence
time for water molecules.

And you can see here that
water can stay in glaciers

and permafrost for a very long time,

we're talking it could
be up to 10,000 years,

and these are all rough numbers.

It can stay as ground water
anywhere from two weeks,

to 10,000 years, I guess,
depending on how isolated

that ground water is.

It could be in the oceans
and seas as salt water

for 4,000 years, and we
can look at all of these,

all the way, within living organisms,

it'll stay about, on
average, a water molecule

will last about a week in the atmosphere,

so that's getting water
vapor, turning into a cloud,

precipitating down, on
average, one and a half weeks.

And once again, these are averages.

It doesn't mean that every
water molecule will stay

exactly one and half
weeks in the atmosphere,

but it's a pretty interesting
thing to think about,

and it gives you a
little bit more sense of,

well, one, where all the water is,

and how it all works together
with the water cycle.