The Water Cycle      


Body of Lesson
Body Continued

Lesson Plan


The Disappearing Water Mystery

Think back to the last time it rained. Afterwards the streets were wet and puddles were everywhere. What happened to those puddles? They dried up, yes, but where did the water go?

My Mom once left water boiling on the stove and forgot to check it? Later, when she remember it was still on the hot stove, almost all the water was gone. What happened to the water? It boiled away, yes, but where did the water go?



Today we're going to find out what happens to the water in puddles when it dries up, or what happens to the water on the stove when it boils away.

We'll test the hypothesis that water "goes somewhere" by attempting to bring water back again from wherever it goes when it dries up or boils away.


When puddles dry up, or when water boils away, the water changes from water into water vapor. When this happens we say the water evaporated. Or, we say evaporation has occurred.



Later, the water vapor in the air is cooled and water droplets form. When the water droplets bump into each other they combine and form bigger and bigger drops. When these drops get big enough and heavy enough they fall as raindrops.

Click to see an enlargement of the Water Cycle.


Body of Lesson Continued:


What causes the water vapor in the air to change back into droplets? Two things: temperature and dewpoint.

Dewpoint is the point on the temperature scale at which water vapor turns back into liquid water. Unlike the freezing point, which is always 32F, the dewpoint varies because it is effected by relative humidity, temperature and air pressure. Calculating the dewpoint is important in predicting fog.

The amount of water that can remain evaporated in the air increases as the temperature increases. The relative humidity is the amount of evaporated water in the air compared to the amount the air could hold at that temperature. So, 70% relative humidity means the air is holding seven tenths of the amount of moisture that it could hold at that temperature.

If the temperature drops suddenly, as it often does at night, the evaporated water in the air will be forced to turn back into water drops when the temperature reaches a certain temperature. That point in temperature is called the dewpoint.


Ever notice the beads of water that form on the outside of your cold soda can on a hot summer day?

That moisture isn't coming from inside the can. It is condensation forming on the outside of the can as the vapor in the air touches the ice-cold can.

Contact with the can forces the water vapor in the air to turn back into water again. The surface of the can is below the dewpoint for the surrounding air. Pretty cool, huh?

In addition to being a liquid and a vapor, water can also be a solid. For more about ice click Ice Info.



Instructor describes the setup of the rain-in-a-jar experiment briefly without telling them what the result will be.

Students write a hypothesis and make predictions about what will occur in the experiment. It is important that they explain the basis for each prediction.

Instructor demonstrates the Rain Cloud in a Jar experiment with the students observing closely.

Students observe and record what happens in the experiment.

Next, they compare the results to their predictions and try to explain why what they observed occurred.


Rain Cloud in a Jar instructions and hand out.



Students report their results and compare them to their predictions. The students attempt to explain what the results mean in terms of the original hypothesis.


Instructor confirms or corrects the conclusions of the students. The ideal conclusion being that water does change into water vapor and will condense back into liquid water when the temperature drops sufficiently.

Next week we'll learn about thunder and lightning.