Friday, March 8, 2013

The water cycle

Last week we began talking about the water cycle, particularly as it relates to cloud formation. The students brought up many excellent questions for us to explore as we begin our spring semester on the topic of water. Here are just a few:

·      How are clouds made?
·      How is water made?
·      How does water get clean?
·      How does water evaporate? What is evaporation?
·      What is water vapor?
·      How does water get in the pond?
·      How does water stay in a pond?
·      How and when does rain happen?
·      How can animals live in water?
·      Why do we need water?

I was also thrilled to hear they were thinking about experiments that they want to try:

·      Will snow melt faster in the sun or shade?
·      Where is the best spot to collect the most water?
·      How long does it take water to evaporate?
·      Will snow melt faster than ice?
·      Which melts faster when you add water to it, ice or snow?
·      What melts faster, packed or unpacked snow? 

In order to introduce the water cycle we drew a circular diagram with arrows depicting land with a pool of water (rivers, ocean, lakes), the sun and clouds, and from the clouds, rain drops. The students were already aware of the terms: collection,  precipitation, condensation, and evaporation but not precisely what they meant. So we decided to go through each of the cycle terms together. 

For visualizing evaporation we each put a small squeeze of hand sanitizer onto our hands and talked about how it felt: "wet, squishy, sticky, cold". Then some of us waved our hands through the air to simulate wind and students described how our hands felt: "dry!". They decided that the hand sanitizer had evaporated from their hands. And we talked about how the sun does a similar process, as it warms the water that is collected on the land (in our rivers, oceans, lakes) it causes some portion of the water to evaporate and that means that water vapor - gas that we can't see - is able to rise up into the sky.  So what happens to water vapor as it rises into the atmosphere, I asked them. At a certain level in the sky the air temperature changes (many students were familiar with this thanks to being well-seasoned travelers; having seen water condensation on the windows of an airplane in flight) and at that high altitude, those vapors turn into tiny water droplets (and with dust) collect to form clouds. 

This came back to one of their original questions - How and when does rain happen? We talked about how on some days it is cloudy but also sunny and the shape and color of those clouds may appear fluffy and white, while on other days, clouds may appear quite grey even black.  Intuitively is seemed, they decided that on the grey-cloud days it was more likely to rain. I explained that as the clouds get more and more full of tiny droplets of water, they may appear grey at the bottom, at some point the cloud can't hold anymore water droplets, and it rains (thank you gravity). 

In order to visualize condensation we ventured to carry out by now universally famous, Make a Cloud in a Bottle Experiment. For those who are not familiar, we poured a small amount of warm water into a 2 liter plastic bottle and, in order to produce dust debris, we lit a match and let the particles/smoke fall into the bottle. We quickly closed the bottle off with the cap and began squeezing the bottle in the middle. By adding particles such as smoke and by squeezing the bottle we caused the air pressure to drop, and created a cloud!

Finally we made our way out to a nearby field with our clipboards, paper, and pencils, to look at the sky. We were lucky to find, on that chilly but beautiful winter morning, the sky was absolutely full of clouds. The color and shape of the clouds varied widely across the sky. 

One second grader knew the names of all the different types of clouds and he wrote down every name and definition. Many students actually sketched the clouds they saw, some chose those in the distance while others picked clouds right overhead. 
Here is a Kindergartner working on her sketch.

They made special care of the shading - for grey clouds that looked to be just about to empty on us, they used darkened pencil marks. 

Next week we plan to talk more in depth about gases that we cannot see and we have a very cool experiment that will help us to visualize water vapor.

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