The water cycle | Ecology | Khan Academy

Summary

This educational video script delves into the water cycle, highlighting its significance to all life forms. It explains the process from evaporation to condensation, forming clouds, and precipitation, including the roles of temperature and air currents. It touches on sublimation, transpiration, and human interaction with water, emphasizing the scarcity of fresh water. The script also discusses the average residence time of water molecules in various parts of the cycle, from glaciers to the atmosphere, providing a deeper understanding of the water cycle's complexity.

Takeaways

• 🌊 The water cycle is a continuous process involving all living beings and includes evaporation, condensation, precipitation, and collection.
• 💧 Evaporation is the process where water changes from a liquid to a gaseous state, forming water vapor that rises into the atmosphere.
• ☀️ Solar energy heats the Earth's surface, causing water to evaporate and contributing to the formation of water vapor.
• 🌤️ Water vapor rises and cools, leading to condensation around tiny particles, forming clouds which can include both liquid droplets and ice crystals.
• 🌀 Clouds are transported by wind, and their water droplets can precipitate as rain, snow, or other forms of precipitation, depending on temperature and conditions.
• ❄️ Precipitation may fall as snow in cold conditions and can remain as ice or snow until temperatures rise, leading to melting and runoff.
• 🌱 Plants play a role in the water cycle by absorbing water through their roots and releasing it into the atmosphere through transpiration.
• 💦 A significant portion of the Earth's fresh water is stored in glaciers and permanent snow cover, highlighting the importance of these for the water cycle.
• 🚰 Fresh water is essential for human consumption and is primarily sourced from lakes, rivers, and groundwater, which make up a small percentage of the Earth's total water.
• 🕰️ Water molecules can reside in different parts of the water cycle for varying lengths of time, from weeks in the atmosphere to thousands of years in glaciers and permafrost.

Q & A

What is the water cycle?

The water cycle is the continuous movement of water on, above, and below the surface of the Earth. It involves the processes of evaporation, condensation, precipitation, and collection.

How does evaporation occur in the water cycle?

Evaporation occurs when water molecules at the surface of oceans, rivers, or lakes gain enough energy to transition from a liquid state to a gaseous state, becoming water vapor.

What causes water vapor to rise?

Water vapor rises due to being heated by the sun and often moves with the air that has been warmed at the surface. As it rises, it cools and condenses into droplets.

What forms clouds and how?

Clouds form when water vapor cools and condenses around tiny particles of dust in the air, forming visible water droplets or ice crystals.

How does transportation of water occur in the water cycle?

Transportation of water in the water cycle is facilitated by wind, which moves clouds and the water droplets within them across the sky.

What is precipitation and how does it relate to the water cycle?

Precipitation is the process by which water droplets in clouds become heavy enough to fall back to the Earth's surface as rain, snow, sleet, or hail.

How does water return to the ocean or become part of the land?

Water returns to the ocean or becomes part of the land through precipitation. It can also percolate into the soil, forming underground aquifers, or accumulate in lakes and rivers.

What is the role of plants in the water cycle?

Plants play a role in the water cycle by absorbing water from the soil through their roots and releasing it into the atmosphere through a process called transpiration.

What is sublimation and how does it fit into the water cycle?

Sublimation is the process where water transitions directly from its solid state (ice) to its gaseous state (water vapor) without becoming liquid first. This occurs in very cold and dry conditions.

How do humans participate in the water cycle?

Humans participate in the water cycle by consuming fresh water, which makes up about 70% of our body's cells, and then releasing it back into the environment through various means such as waste and evaporation.

What percentage of the Earth's water is fresh water, and where is most of it located?

Only about 2.5% of the Earth's water is fresh water. Most of this fresh water is found in glaciers and permanent snow cover, with a smaller amount in groundwater, lakes, and rivers.

How long do water molecules typically stay in different parts of the water cycle?

The average residence time of water molecules varies: in glaciers and permafrost up to 10,000 years, in groundwater from two weeks to 10,000 years, in the oceans and seas as saltwater about 4,000 years, and in the atmosphere about one and a half weeks.

Outlines

00:00

🌊 The Water Cycle Explained

This paragraph delves into the water cycle, a natural process integral to life on Earth. It begins with evaporation from bodies of water like oceans, rivers, and lakes. Water molecules gain enough energy to transition from liquid to gaseous state, becoming water vapor. This vapor rises, often with warm air, and as temperatures decrease, it condenses into tiny droplets around microscopic particles, forming clouds. Clouds, composed of these droplets or sometimes ice crystals in colder conditions, are moved by wind, facilitating the transportation of water. When droplets become heavy, they precipitate, potentially returning to the ocean or other locations, and can also fall as snow in colder climates. The discussion also touches on the melting of snow and the role of soil in water percolation, highlighting the vast amounts of water stored underground in aquifers and the formation of rivers and lakes through runoff. The water cycle is a continuous process, with additional components like transpiration in plants and sublimation in cold, dry environments. Human involvement is also noted, as we consume and return water to the cycle.

05:02

💧 Freshwater: A Precious Resource

The second paragraph emphasizes the importance of freshwater, which is water without salt, contrasting it with the majority of the world's water, which is saltwater. It points out that only 2.5% of the Earth's water is freshwater, with the majority of this being trapped in glaciers and permanent snow cover, not readily accessible as lakes and rivers. The paragraph also discusses the practice of digging wells to access groundwater, which includes soil moisture, swamp water, and permafrost. The concept of residence time of water molecules in different parts of the water cycle is introduced, indicating that water can remain in glaciers and permafrost for up to 10,000 years, while in the atmosphere, it lasts about a week and a half on average. This section underscores the scarcity of freshwater and the various durations water resides in different environmental compartments, providing a deeper understanding of the water cycle's dynamics.

Keywords

💡Evaporation

Evaporation is the process by which water changes from a liquid state to a gaseous state, known as water vapor. In the video script, it is described as the starting point of the water cycle, where water molecules from bodies like oceans, rivers, or lakes gain enough energy to transition into the gaseous state. This process is crucial as it initiates the movement of water through the atmosphere, which is a central theme of the video.

💡Water Vapor

Water vapor is the gaseous state of water, which is formed when water evaporates. The script explains that water vapor rises, often with the heated air from the Earth's surface, and is a key component of the water cycle. It is highlighted as the form that water takes when it moves from the surface of the Earth into the atmosphere, illustrating the continuous transformation of water in nature.

💡Condensation

Condensation refers to the process where water vapor in the air cools and changes back into liquid form, forming droplets that can be seen as clouds. In the context of the video, condensation is depicted as a critical phase in the water cycle where water vapor turns into visible water droplets, often around tiny dust particles, which is essential for cloud formation and the subsequent stages of the cycle.

💡Clouds

Clouds are visible masses of water droplets or ice crystals suspended in the atmosphere. The script describes clouds as being formed from condensed water vapor, which can also include small ice crystals if the temperature is low enough. Clouds are significant in the water cycle as they are the medium through which water is transported across the sky by wind, eventually leading to precipitation.

💡Precipitation

Precipitation is the process by which water falls from the atmosphere to the Earth's surface in forms such as rain, snow, sleet, or hail. The video script mentions precipitation as the stage where water droplets in clouds become heavy enough to fall back to the Earth, which can occur as snow or rain depending on the temperature and other conditions. This is a vital part of the water cycle, as it returns water to the Earth's surface.

💡Aquifers

Aquifers are underground layers of water-bearing permeable rock or unconsolidated materials from which groundwater can be extracted using a water well. The script explains that some of the water from precipitation percolates down into the soil and forms aquifers, which are significant reservoirs of fresh water. This concept is important as it relates to the availability of fresh water for human use and the natural storage of water underground.

💡Transpiration

Transpiration is the process by which plants release water from their leaves into the atmosphere. The video script mentions that plants take up water from the soil and release some of it back into the atmosphere through transpiration, contributing to the water cycle. This process is essential for the movement of water within ecosystems and plays a role in the overall balance of water on Earth.

💡Sublimation

Sublimation is the transition of a substance directly from the solid to the gas phase without passing through the intermediate liquid phase. In the context of the video, sublimation is described as occurring in very cold and dry conditions where ice can turn directly into water vapor without melting first. This concept is relevant to understanding the different ways water can move through its cycle, especially in polar or high-altitude environments.

💡Fresh Water

Fresh water refers to water with low concentrations of dissolved salts, making it suitable for drinking and other uses without desalination. The script emphasizes the importance of fresh water for human consumption and the scarcity of fresh water in comparison to salt water. It points out that most fresh water is found in glaciers and underground, highlighting the importance of sources like lakes and rivers for human use.

💡Residence Time

Residence time, in the context of the water cycle, refers to the average duration that water molecules remain in a particular part of the cycle before moving to another. The video script discusses the varying residence times of water in different states, such as in glaciers, groundwater, and the atmosphere, providing insight into the dynamics and longevity of water in different environmental compartments.

Highlights

The water cycle is a continuous process that we are all part of.

Evaporation is the first stage of the water cycle, where water turns from liquid to gas.

Water vapor rises and cools, leading to condensation around dust particles to form clouds.

Clouds are made up of water droplets or ice crystals, depending on the temperature.

Wind transports clouds and the water droplets within them.

Precipitation occurs when water droplets become heavy enough to fall from clouds.

Precipitation can be in the form of rain or snow, depending on the conditions.

Snow can remain as ice or snow if the conditions are cold enough, or it can melt and run off.

Most rainwater percolates into the soil, contributing to underground aquifers.

Lakes and rivers are formed by water that does not infiltrate the soil and runs off the surface.

Plants play a role in the water cycle by taking up water and releasing it through transpiration.

Sublimation is the process where ice turns directly into water vapor without becoming liquid.

Humans are part of the water cycle, as we consume and release water through various processes.

Fresh water is essential for human consumption, but it makes up a very small percentage of the world's water.

97.5% of the Earth's water is salt water, with only 2.5% being fresh water.

Most fresh water is found in glaciers and permanent snow cover, not in lakes and rivers.

Ground water, including soil moisture and swamp water, is a significant but often overlooked part of the fresh water supply.

The average residence time of water molecules varies greatly between different parts of the water cycle.

Water molecules can stay in glaciers and permafrost for up to 10,000 years.

In the atmosphere, water molecules typically stay for about a week on average.

Transcripts

- [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 aquifers 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 aquifers.

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.