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- [Voiceover] Let's talk a little bit

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

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

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every moment of our lives.

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We might not fully appreciate it.

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So let's just jump in the cycle.

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I'll start with evaporation.

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So we could start with the[br]surface of the ocean here,

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or this river, or this lake.

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And at any given moment,

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

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Water molecules that were[br]in their liquid state,

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liquid state, they just have[br]enough energy to bounce away

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and go into the gas state.

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And water in a gaseous state[br]we call vapor, water vapor.

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Water, water vapor.

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And so that water vapor,[br]it is going to rise,

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likely with the air that has[br]been heated on the surface,

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

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at play, but as it rises, and[br]as the overall temperature

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cools, that water vapor will[br]condense into little droplets.

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It'll condense around[br]little, tiny air particles,

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little particles of dust[br]that you can't even see

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

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

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

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

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and they're condensing[br]around these little,

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microscopic dust particles[br]to form these water droplets.

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

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

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And we see here, they're[br]talking about transportation,

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you can have these clouds, we[br]obviously, if you look outside

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

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And so they could be moving[br]all of those droplets

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with the overall wind.

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And when those droplets get heavy enough,

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they will precipitate,[br]they will precipitate down.

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Now they could precipitate[br]back into where we started,

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they could go back to the ocean there.

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Or you could go onto a mountain here,

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and since if the air is cold enough,

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and if you have the right conditions,

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that precipitation might be snow,

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and it might stay snow right over there.

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Or ice, but then eventually[br]things might warm up,

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or they might not warm up,[br]but if they do warm up,

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well, then, they would melt,

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and there would be snow melt run-off.

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And that's what you're seeing there.

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If that rain is falling in this area,

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

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we are talking about rain.

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Well, most of that water[br]is actually going to

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percolate down into the soil.

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So most of it goes down.

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We look around us and we[br]see these rivers and lakes,

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

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

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inside the ground and[br]obviously, in the ocean.

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

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So you have all of this water that forms

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in these underground aquafiers here.

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But some of it also ends up in these lakes

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and these lakes are usually[br]in a situation where

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the ground is either already[br]saturated with water,

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or there's the right types of rocks,

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so it can contain the water up here,

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and similarly, rivers[br]are formed by runoff,

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

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And in general, if you[br]see a creek or a river

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near your house, especially[br]when it rains it fills up,

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that's a good indication[br]that the ground water's

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already saturated and so[br]things are running off

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into that river.

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And so that in general is the water cycle.

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You have evaporation, it[br]condenses into clouds,

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it eventually precipitates,[br]and it keeps going,

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round and round and round.

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

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You have things like plants.

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Plants will take up water[br]from the upper soil,

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as far as the plant's roots go.

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And it will use that water[br]to transport nutrients

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down from the soil up into the leaves.

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It also uses that water as part of

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the photosynthesis[br]process that we've studied

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in many videos.

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And a lot of that water[br]gets transpired out.

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So once again, this is[br]transpiration, essentially,

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evaporation out the leaves of the water.

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Over here you see this word sublimation.

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

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into the gas form of[br]water, or water vapor.

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And this will happen in[br]situations where it's cold

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

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And you have, in general, low pressure.

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So instead of going into the liquid state,

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right then the water[br]molecules start just leaving

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as water vapor.

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

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Well, how are we part of it?

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We will drink some of this fresh water,

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our bodies are actually mostly water.

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The cells in our bodies are 70% water.

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Everything we study in biology,[br]water is a key environment

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for all of these things to occur.

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And then we use that water,

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

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

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Now one thing that I[br]find really interesting,

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as an organism that is[br]dependent on fresh water,

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when people say fresh water,

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we're talking about water without salt,

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as opposed to salt water.

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So we really need the[br]fresh water in this lake,

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or in this river, or we might dig wells,

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

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It actually turns out that[br]very little of the overall

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water in the world is fresh water.

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And so let me show you[br]this chart over here.

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I always knew that, but[br]I didn't fully appreciate

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how little was fresh water.

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So of all the water on our planet,

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

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Only 2-1/2% is fresh water.

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

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very little of it is what[br]we traditionally associate

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fresh water, the lakes and the rivers.

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

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let me go to a lake or a river,

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that's stuff that we[br]could potentially drink.

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But most of it is actually in glaciers

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and permanent snow cover.

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

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And it makes you think[br]about what would happen

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if this stuff were to melt.

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And then you also have ground water,

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which we could have access to.

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

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well, ground water includes soil moisture,

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swamp water, and permafrost.

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

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which I personally find fascinating.

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

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Now another really interesting thing is,

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how long, on average,[br]water molecules might stay

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in different parts of this water cycle.

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Going back here, you can[br]imagine that a water molecule

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can stay for a very[br]long time in the ocean,

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

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depending on ocean[br]currents and temperature

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and all of that, but you can imagine,

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it could stay in that[br]liquid form in that ocean

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for a very long time.

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

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And people have actually studied this,

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which I find fascinating.

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I'd be curious to figure[br]out how they actually

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got this data.

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But this is the average residence[br]time for water molecules.

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And you can see here that[br]water can stay in glaciers

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and permafrost for a very long time,

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

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and these are all rough numbers.

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It can stay as ground water[br]anywhere from two weeks,

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to 10,000 years, I guess,[br]depending on how isolated

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that ground water is.

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It could be in the oceans[br]and seas as salt water

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

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all the way, within living organisms,

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it'll stay about, on[br]average, a water molecule

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will last about a week in the atmosphere,

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

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precipitating down, on[br]average, one and a half weeks.

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And once again, these are averages.

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It doesn't mean that every[br]water molecule will stay

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exactly one and half[br]weeks in the atmosphere,

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

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and it gives you a[br]little bit more sense of,

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well, one, where all the water is,

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and how it all works together[br]with the water cycle.