The water cycle | Ecology | Khan Academy

Summary

Highlights

• Evaporation from oceans, rivers, and lakes turns water into vapor, starting the water cycle. 💧
• Cloud formation occurs when water vapor condenses around tiny dust particles. ☁️
• The cycle includes processes like transpiration, sublimation, and groundwater percolation. 🌿

Key Takeaways

• The water cycle involves evaporation, condensation, and precipitation, occurring in a continuous loop. 🌧️
• Freshwater constitutes only 2.5% of Earth's total water, mostly locked in glaciers and permafrost. ❄️
• Water molecules can remain in glaciers for up to 10,000 years, showing their long residence times. ⏳

Overview

Chapters

• 00:00 - 00:30: Introduction to the Water Cycle The chapter introduces the concept of the water cycle, emphasizing its continuous and omnipresent nature in our lives. It begins by describing the evaporation process, where water from surfaces like oceans, rivers, and lakes transforms from liquid to gas as molecules gain energy and escape into the air.
• 00:30 - 02:00: Evaporation and Condensation The chapter discusses the concepts of evaporation and condensation, particularly focusing on water transitioning into a gaseous state, known as water vapor. It explains how water vapor rises with heated air near the Earth's surface due to solar energy. The chapter also notes the complex dynamics involved in this process and describes how as the temperature cools, the water vapor condenses.
• 02:00 - 03:00: Cloud Formation and Precipitation This chapter explains the process of cloud formation and precipitation. It begins with water vapor in the atmosphere, which condenses into tiny droplets around microscopic particles of dust. This condensation forms clouds as the droplets come together, interacting more as they transition from vapor to liquid form. The chapter highlights the role of these dust particles in providing a surface for water vapor to condense upon, leading to the formation of visible clouds.
• 03:00 - 05:00: Runoff and Groundwater The chapter 'Runoff and Groundwater' discusses the formation of water droplets and ice crystals that make up clouds. It highlights the role of transportation in the movement of clouds by wind, resulting in the movement of water droplets. Once these droplets become heavy enough, they precipitate back down, potentially returning to their originating source like the ocean.
• 05:00 - 06:00: The Role of Plants and Sublimation The chapter titled 'The Role of Plants and Sublimation' explores how precipitation behaves in certain conditions, particularly in mountainous areas where the temperatures may cause it to remain as snow or ice. If temperatures rise, this snow or ice melts, resulting in snowmelt runoff. This process exemplifies the sublimation and the natural water cycle in various weather conditions.
• 06:00 - 10:00: Human Interaction and Freshwater Facts The chapter discusses how precipitation not cold enough to be snow primarily becomes rain, which mostly percolates into the soil and thus, a significant amount of water resides underground. This contrasts with popular perception, as people often notice visible water bodies like rivers and lakes, overlooking the vast quantities beneath the surface and in oceans.
• 10:00 - 14:30: Residence Time of Water Molecules The chapter discusses the natural movement and storage of water, particularly focusing on underground aquifers and lakes. It explains how lakes form when the ground is saturated or the rock types allow water to be contained. It also touches on the formation of rivers, highlighting snowmelt runoff as a common source, and notes that creeks and rivers swell during rain, signaling active water flow in the vicinity.

The water cycle | Ecology | Khan Academy Transcription

• 00:00 - 00:30 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 Sur off of that surface water molecules that were in their liquid state liquid state they just have enough energy to bounce away and and go into
• 00:30 - 01:00 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 the air that has been heated on the surface due to the Sun and there's other uh more complex Dynamics at play but as it rises and as the overall temperature cools that water vapor will condense
• 01:00 - 01:30 into little droplets and 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 this is little droplets little droplets so the water is actually back to the liquid form they're not individual water molecules anymore they're now getting they're they're now able to interact with each other and they they're condensing around these little microscopic dust particles and to
• 01:30 - 02:00 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 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
• 02:00 - 02:30 you could go onto a mountain here and since that and that 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 might warm up or they might not warm up but when they if they do warm up well then they would melt and then they would they would be snow melt runoff and that's what you're seeing there if that rain is falling in this area
• 02:30 - 03:00 so and let's say it's not cold enough for it to be snow uh so that we are talking about rain well most of that water is actually going to percolate down into into the soil so most of it goes down we look around us and we see these rivers and lakes and we say oh 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 and so you have these you have all of this water that forms in these Underground
• 03:00 - 03:30 these underground Aqua fires here but some of it also ends up in these lakes and these Lakes are usually in a situation where uh the the ground is either already saturated water or there's the right top types of rocks so it can contain the water up here and similarly rivers are formed by runoff it's you know snowmelt runoff can famously form rivers and in general if you see a a creek or a river near your house that especially when it rains it fills up that's a good indication that
• 03:30 - 04:00 the groundwater is 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 other actors at play you have things like plants plants will take up water from the upper soil as far as the plants roots go and it will use that water to transport nutrients down from the soil up into the leaves it also uses
• 04:00 - 04:30 that water as part of the photosynthesis process that we've studied in in many videos and a lot of that water gets transpired gets transpired out so uh once again this is transpiration is essentially evaporation out of the leaves of the water over here you see this word sublim sub 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 you well it's cold and it's it's very very very very very dry
• 04:30 - 05:00 uh and and you have in general low pressure and so right when instead of going into the liquid state right then the water molecules just start just leaving as water vapor now and of course you know I said we're part of it well how are we part of it well we will drink some of this fresh water our bodies are actually mostly water the cells in our bodies are 70% water we you know everything we study in biology water is a is a key uh environment for all of
• 05:00 - 05:30 these things to occur and then we use that water and then we will um uh we will get that water out of our body and then it continues on is part of this water cycle now one thing that that I find really interesting as a 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 this water that's in the fresh water in this lakes or In this River or we might dig Wells so that we can get the water out of these aquifers it actually turns
• 05:30 - 06:00 out that very little of the overall water in the in 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 and a half% is freshwat and even of that 2 and 1 12% fresh water very little of it is what we
• 06:00 - 06:30 traditionally associate fresh water the lakes and the rivers when I think of fresh water I mean to say oh let me go to a lake or a river that that's stuff that we could potentially drink but most of it is actually in glaciers and permanent snow cover so it's 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 drink dig Wells so we're talking about we ground water includes soil moisture
• 06:30 - 07:00 swamp water and permafrost but 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 saw this chart now another really interesting thing is how long on average water molecules might stay in different parts of this water cycle so you know going back here you can imagine that a water molecule could stay for a very long time in the ocean especially you know it's it's going to be moving around depending on
• 07:00 - 07:30 ocean currents and temperature and all of that but you could imagine it could 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 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 could
• 07:30 - 08:00 stay as groundwater anywhere from 2 weeks to 10,000 years I guess depending on how isolated that groundwater 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 to within an or 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 1 and A2 weeks and once again these are averages it doesn't mean that every water
• 08:00 - 08:30 molecule will stay exactly one and a half weeks in the atmosphere but it's a pretty interesting thing to think about it 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