Forecasting the Future
Meteorology: Going, Going, Gone

Forecasting the Future - Background Explanatory


To measure the changes in evaporation rate of water in different forms at different temperatures.


Background Information

Water exists in three forms called phases: solid, liquid, and gas. We commonly call these phases ice, water, and water vapor, respectively. Heat energy is required to change ice to liquid water and to evaporate liquid water. The energy from the Sun reaches Earth, some of it is absorbed by the land and the water. The energy absorbed by water molecules in oceans, lakes, and streams causes these molecules to move about more and more quickly. If enough energy is absorbed, some surface water molecules break free from neighboring molecules and rise into the atmosphere as water vapor. This is the process of evaporation.

Phase changes notwithstanding, one of water's crucial aspects is its constancy. Regardless of its appearance, the total amount of water on or above the planet hardly changes. Some water vapor is lost to space and some water becomes "locked" into rocks, but to a surprising extent the quantity of water on Earth is constant. Water might change forms, appearing as ice, liquid water, or as water vapor; however, rain isn't new water. There is virtually no new water. Almost all the water that was, that is, and that will ever be has been present on this planet in one form or another for many millennia. In other words, some chemical processes aside, we drink the water the dinosaurs drank! That's right! Virtually all water has been warmed by the Sun innumerable times. It has fallen as rain over and over and over again. It has frozen into snow and ice almost as often.

The total amount of water on Earth does not change when the global climate warms or cools, but the fraction of the water that is liquid, solid or gas does change. So does the distribution of the water on the planet. For example, at times when the global climate is cooler, more water might be frozen as polar ice, leaving less liquid water in the oceans. During times when the Earth's atmosphere warms up, the rate of evaporation increases, and so does the amount of water vapor in the atmosphere. This increase in humidity results in more clouds and hence more rain. During colder periods, there is less evaporation and humidity is low, so there are fewer clouds and less precipitation.

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Introduction to Activity

a. List everyday examples of evaporation. Discuss what sorts of conditions speed up or slow down evaporation. Do clothes hung outside dry faster when the weather is hot or cold, windy or still? Why?

b. Devise strategies to speed the disappearance of a small puddle (2-3 drops) from a piece of waxed paper. Which strategies are predicted to work best? Which do?

c. Discuss the order in which water's changes in phase can occur. Draw a picture showing the three phases of water and connect the phases with arrows to show common transformations. Can vapor and solid be connected?

d. Scientists are working to understand how climate change might affect the rates of natural processes such as evaporation. In their experiments, comparatively normal conditions - generally called controls - are compared to novel, or experimental, conditions. What would the control condition be for a classroom experiment on evaporation? List good places to simulate the experimental condition of a warm climate.

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a. Decide on four classroom locations that have different temperatures (e.g., on a windowsill in the sun, on top of a bookshelf near room lights, in a dark corner, in a refrigerator, in a freezer, on the teacher's desk).

b. Label the sixteen cups: four "ice", four "snow", four "water", and four "wet earth".

c. Place one ice cube into each cup marked "ice".

d. Using the plastic bag and a hammer, crunch one ice cube. Place the resulting "snow" in one cup marked "snow". Repeat this procedure to fill each remaining cup marked "snow". Do not crush more than one ice cube at a time, as volume must be as close to identical as possible for later comparisons to be legitimate.

e. One at a time, pour one ice-tray section of water (having volume equivalent to one ice cube) onto each of the four cups marked "water".

f. Use the tablespoon to place the same amount of soil in each of the cups marked "wet earth", then add one ice-tray section of water to the soil. Mix the soil and water.

g, Group the cups into four sets of four, so that each group contains one cup of ice, one of snow, one of water, and one of wet earth. Place one set of cups in each of the location simulating control and experimental conditions. Make an initial observation of water level in each cup, and record the experiment's starting time.

h. Once an hour (or at other pre-arranged times), check how much water remains in each cup. Record how long it takes for each cup to dry out completely. Check the soil for moisture by blotting it with the filter paper and looking for signs of wetness.

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a. Which of the forms of water evaporated first? Why?

b. How might rates of evaporation be speeded?

c. What could happen to evaporation rates in the event of global warming?

d. How have ice caps at the North and South poles continued to exist in the face of past periods of warming?

e. Besides increased evaporation and resultant precipitation, what other consequences of global warming might arise?

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