Hey people,

I'm teaching my fourth graders a unit on the water cycle next week. I did a bit of research while writing this lesson, but there were a lot of different explanations out there and I wanted to make sure I had the best ones.

I've checked my facts, but I want to make sure I'm not feeding the kids any outdated or incorrect information since our school district doesn't see fit to replace the kids' textbooks until they show actual signs of decomposing.



Where is all the water?

Show the students a globe and explain that seventy percent of the Earth’s surface is water. Ask students to brainstorm different places on the earth where water can be found (naturally) and write them on the board or in their science journals.

Ask the students if their parents have ever told them not to waste water. If there’s so much water on the earth, why should we worry about wasting it?

Show students a liter bottle full of water labeled “all the water in the world.” Empty the following amounts into separate jars to represent the relative amounts.

Source Amount Percent of Total

Oceans 4 cups 97%
Icecaps/Glaciers 1.5 tablespoons 2.1%
Groundwater ½ tablespoon 0.6%
Lakes and rivers 2 drops 0.1%
Atmosphere ½ drop 0.001%



What is Water Anyway?


Water is made up of tiny particles called molecules that are always moving. These molecules can exist in three states depending on the temperature.

Show students examples of liquid water, frozen water, and gaseous water.

When you’re in a hot room, do you like having a lot of people crowded around you? Water molecules are kind of the same way. They don’t like being close together when it’s warm. Heat (usually from the sun) causes the water molecules in a puddle to move farther and farther apart. There’s a lot of extra room in the open sky, and water molecules don’t weigh very much because they’re even smaller than specks of dust. Because they’re so light, they can float right up into the air to where all that empty space is waiting for them. This is called evaporation. Evaporated water molecules move around in the air as a kind of gas known as water vapor.


When a cup of water evaporates, is the water still in the room? If so, why can’t we see it?

Tell the students that you are going to perform a magic trick for them. Dump a tiny amount of flour on your hand and make sure everyone gets a good look. Then blow on the flour as hard as possible. Did the flour really disappear? What happened to it? When students conclude that the flour is no longer visible because the particles aren’t as close together as they were, explain that we can’t see water vapor for the same reason. The heated water molecules are so far apart, they can’t be seen.

Another thing students can learn from this demonstration is that heat isn’t the only thing that causes evaporation. Wind can blow water molecules right off the surface of lakes, rivers, and oceans and into the air. That’s why puddles evaporate more quickly on hot, windy days, and why blow dryers are so useful when you want to dry your hair in a hurry!

To prove this point, put a glass of water near the window, where it can easily be reached by heat and sunlight. Place a second glass of water in a dark corner of the room and compare them throughout the week.


Can evaporated water molecules ever come back together again, or do they just float around in the sky by themselves forever?

If water just stayed in the air after it evaporated, eventually all the oceans and rivers and lakes would dry up. So we know that all those water molecules floating around in the air must have some way to come together again.


What makes the water molecules come back together?

Condensation is the opposite of evaporation. We know that air heated by the sun causes water molecules to move farther apart, but what happens to all that warm air after the sun goes down, or when the seasons change? You and I don’t like being together in a hot crowded room, but as soon as it gets cold, we all want to get closer together! Water molecules start to move closer together when it’s cold, too. When we look up in the sky and see clouds, we know that condensation has occurred. During condensation, the air becomes cold enough that water molecules don’t mind being crowded. Sometimes, two water molecules get so close, they bump into one another and stick together. Soon more and more water molecules join together until they make a little cluster. When we look at a cloud, what we’re actually seeing is sunlight shining on all those tiny clusters of water molecules!

Your body is also full of water molecules that are constantly being released into the open air from pores in your skin. Even the inside of your mouth is full of water vapor! You can breathe onto your hand and feel how moist it is for yourself—the dampness is a result of the water vapor collecting on your skin. During the winter, the water molecules from inside your body want to stick together as soon as you breathe them out into the cold air. When we see our breath during the winter, what we’re actually seeing are little clouds, which sometimes stick to the car windshield and make it really hard to see where you’re going.


If cold air causes water molecules to stick together again, why does water condense and form clouds in the bathroom when you’re taking a hot shower?

Have you ever been in a situation where you wanted to get out of a hot, crowded room, but all the doors and windows were shut? Even though you felt like you were cooking and really wanted to get away from all those people, you didn’t have a choice.

Humidity is measured by the number of water molecules in the air. Sometimes when heated water molecules are trapped in a closed space, such as a bathroom or a pot of boiling water, there isn’t enough room left for them to spread apart. As more and more water molecules are released into the air through evaporation, the space becomes more and more humid. If the number of water molecules in the air continues to increase, they eventually start getting squished together, just like people in a crowded rock and roll concert, and a visible cloud of steam is formed.

So the absence of heat isn't the only thing that causes water molecules to stick together again. Humidity plays an important role in condensation, too!


What happens if a floating water molecule touches something that’s not another water molecule?

Sometimes condensation can take place very close to the ground. When it’s foggy outside, we are walking around inside a cloud that formed in the exact same way as the clouds in the sky do. But when these little clusters of condensed water start to form somewhere close to the ground, chances are they’ll run into something that’s not another molecule, like a leaf or a rock. Splat. Eventually enough molecules will join the droplet to form a dewdrop. That’s why we see dew on the ground during cold mornings, and if it’s cold enough, the dew freezes and becomes frost. The dew point is the exact temperature at which dew starts to form.


What happens to the water droplets that are too far up in the sky to make contact with plants and rocks and things?

Even many miles up in the air, there are bits of dust and pollen floating around waiting for water molecules and water droplets to condense around them. When enough water collects around the object, it gets too heavy to float, and goes falling back to the ground as rain. Snowflakes are formed when water vapor freezes during condensation.


But what happens to the water molecules once they fall back to the ground?

They take another trip through the water cycle, which we will cover in depth next time!



Closure: We learned a lot about water today. Let’s see how much you can remember. Review guided notes with the students.

Where is the earth’s water? (in the oceans, glaciers, underground, lakes and rivers, and the atmosphere)

What are the three states in which water exists? (solid, liquid, gas)

What is evaporation? (when liquid water becomes water vapor)

What two things cause evaporation? (heat and wind)

What is water called when it is a gas? (water vapor)

What is condensation? (when water vapor becomes liquid water)

What two things cause condensation? (cold and humidity)

What is the dew point? (The exact temperature at which dew is formed)

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One nuclear bomb could wreck your whole day.

Nothing popped out to me, but then, it's not really something I try to keep current about. A few things you might consider adding though.

Condensation. Demonstrate by making a solar still. Put a cup in the bottom of a bucket. Place some sort of plant in the bucket even a few handfuls of grass will work. Place a plastic bag over the bucket with a pebble to make it a cone. Place in sunlight and check each morning. The heat will draw the water from the plants where it will condense on the inside of the bag, then drip back into the cup. maybe compare the water gained with the water lost from the cups in the evaporation experiment. Not only a lesson in condensation, but a good survival skill as well.

Condensation. We have a local meteorologist here that could best be described as a weather geek. He has a keen interest in native weather lore, in addition to the science. I wrote to him once to ask if a jet contrail could be used to predict the weather. His reply was that a visible contrail was an indication of cooler, wet air, and that the more persistent a contrail was in the air, the more likely it would be to be cloudy in the coming days. No contrail meant continued fair weather for a few days at least. I use this as a guide for planning a trip out with the scopes. It's been pretty accurate overall. Have them look for contrails and compare the observations with the weather the following week. Side effect would be planting a seed of truth about chemtrails before they get to high school.

Condensation/humidity. Dew point. This is the point as which the air is humid enough and cold enough that the water will form on any surface outside. This is why some days you need to scrape the windows at 30 degrees, and otheer days you have clear windows at 15. (or the C equiv).

Edit to add, just saw your location. The weather geek is Snowbank

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A Nerd can figure out how long it will take the original Enterprise traveling at warp 6.5 to travel from Regulus to Antares.
A Geek with think he can use that to pick up a girl in a bar.
A Dork knows he can't pick up the girl with it, but will hang around for hours anyway, just in case she asks.
She might. You never know.

Thanks Tog! The solar still is a great idea, and I'm sure the kids would find it very interesting.

I'm not sure what you mean by "the weather geek is Snowbank."

Do you think the comparison of not wanting to be crowded in a hot room and wanting to get closer together might give a nine year old the wrong impression? Technically, water molecules don't "want" anything, but I thought it would help students remember how they react to hot and cold temperatures. Some educators really frown on anthropomorphising, but I'm not sure if this would count...

I also thought of the flour demonstration myself. *beams with pride*

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One nuclear bomb could wreck your whole day.

Your location is Logan so I assumed you would know of the main weather guy on KSL out of Salt Lake. He's been known as "Snowbank" for as long as I cen recall. His son does the weather too and is often called the "Drift"

As for the crowded room, well I actually tried something similar on here once with mixed results. I got into it on post 13, then made a silly typo and it was pretty much anarchy from then on.

I like the flour demo, just keep it away from the athsmatic kid.

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A Nerd can figure out how long it will take the original Enterprise traveling at warp 6.5 to travel from Regulus to Antares.
A Geek with think he can use that to pick up a girl in a bar.
A Dork knows he can't pick up the girl with it, but will hang around for hours anyway, just in case she asks.
She might. You never know.

I checked out some of your numbers with this USGS site. I didn't check everything, but their number for total global fresh water is 2.5% where yours is about 2.8%. Not a huge difference, but still a 12% difference. It may depend on how well calilbrated the giant teaspoons are that you use to store all that water.

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Rock is dead. Long live Paper and Scissors.

Oh, geeze. Now I need to figure out how to convert that to teaspoons.

I wish we could use the metric system, but they don't learn that until sixth grade.

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One nuclear bomb could wreck your whole day.

Teach it to them early. It won't hurt.

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Keeper of the Jabberwock

Couple of things:

1. Liquid, Solid, and Gas are the three phases of matter. Frozen is technically correct, but you're out one of the phases and substituting it with a state that's typically reserved for things that are usually liquid at room temperature.

2. Clouds are formed when the temperature drops to the dew point. But clouds are also formed when the dew point increases to the temperature. Because condensation absorbs heat while removing water vapor, the dew point will always remain at or below the temperature.

3. Water molecules condense more readily on tiny, charged particles in the air than they do by themselves.

Er, correction - I see that towards the end you mentioned #3.

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I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol.

Mine: "Perception isn't reality. It's merely an abstraction thereof, and quite often not a very good one at that."

Heinlein's: "Staying young requires the unceasing cultivation of the ability to unlearn old falsehoods." "Freedom begins when you tell Ms. Grundy to go fly a kite."

and open flame--it's explosive

Dust explosions are incredible things.

Don't forget me, i am weather geek too!

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Matt


Website:
www.freewebs.com/mattweather/

Forums: http://stormshunters.informe.com/

Almost everything in the original post is at least slightly in error, technically, but I'm not sure how one would improve it and have it still be understandable to a non-meteorologist, particularly one who is in 4th grade!

The only thing that jumped out as outright wrong was the assertion that wind causes evaporation. It doesn't. The only things that affect evaporation are temperature, atmospheric pressure, and the humidity in the immediate vicinity. Wind has no effect on evaporation rates when the atmosphere is at 100% relative humidity. When the RH is lower, though, it has the effect of replacing saturated air (next to the liquid water source) with air that isn't saturated. This does increase the rate of evaporation, but the wind isn't causing the evaporation. It's just providing a better environment for evaporation to occur.

Also - so far as I recall from my training, condensation does not occur without a non-water molecule being handy (a condensation nucleus).

If you haven't already, you might want to visit the Bad Meteorology website.

So... any plan to teach sublimation?

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"I am Meteora, supreme goddess of weather" - Meteora, in The Unchained Goddess

One nice thing about being a meteorologist who also likes astronomy is that the sky is always interesting!

The OP seems to be saying that the wind can blow the molecules off the surface. That's accurate, more or less, isn't it? Blowing spray into the air, increasing surface area, etc, can cause more evaporation, no?

yeah, I think what meteora means is that the water isn't a direct cause of evaporation--aparently it can't actually blow the water molecules off the surface of the water as I'd previously thought. The wind merely blows away the water vapor directly above the surface of the water and makes room for new water vapor to form.

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One nuclear bomb could wreck your whole day.

I meant that wind doesn't directly cause evaporation. If the air is saturated, even spray will be in equilibrium (no net evaporation or condensation), so there would be no change in overall evaporation rate.

But you are sort-of right. In a non-saturated environment, the increased surface area would, in fact, increase overall evaporation. However, that is an indirect effect of the wind.

I don't think wind blows individual molecules of water off the surface. As I understand the whole process, it is driven by vapor pressure, which varies according to temperature and the pressure exerted by water vapor in the atmosphere. Wind doesn't enter into the equation - except as described above. Incidentally (for those of you who may not already know this), when water temperature reaches the temperature at which the saturation vapor pressure equals the total atmospheric pressure, the water begins to boil. That's why water boils at lower temperatures at higher elevations - and why higher boiling points can be achieved with pressure cookers.

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"I am Meteora, supreme goddess of weather" - Meteora, in The Unchained Goddess

One nice thing about being a meteorologist who also likes astronomy is that the sky is always interesting!

One thing to consider: you can't actually show water in the gaseous state. Steam is invisible/transparent. So is gaseous water vapor. Many people see condensed water droplets (clouds, contrails, or the wisps of vapor we sometimes call steam) and think that's gaseous water, but it's not -- it's the liquid state in the form of fine droplets.

I'm not sure if that's too fine a distinction for your fourth graders.

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Bring back Firefly!

"It is quite clear that Occam's razor does not sharpen in your pyramid." (Nicolas)

"Still, a man hears what he wants to hear and disregards the rest." (Paul Simon)

I wonder if there's a dye for water that had a molecular charge that would remain "attached" to the water molecule, without being bound to it, in such a way that one could see the water vapor?

Probably not, as water is one of the most polar molecules out there, and if it has enough energy to break that bond and become steam, it's unlikely there's a more polar, light-absorbing molecue out there.

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I am Mugs, of the Alien clan of Usa, Nordamerica, a Terran, of Sol.

Mine: "Perception isn't reality. It's merely an abstraction thereof, and quite often not a very good one at that."

Heinlein's: "Staying young requires the unceasing cultivation of the ability to unlearn old falsehoods." "Freedom begins when you tell Ms. Grundy to go fly a kite."