The water cycle | Ecology | Khan Academy
Summary
TLDRThis 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
🌊 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.
💧 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.
Mindmap
Keywords
💡Evaporation
💡Water Vapor
💡Condensation
💡Clouds
💡Precipitation
💡Aquifers
💡Transpiration
💡Sublimation
💡Fresh Water
💡Residence Time
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.
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