GRADE: Seventh

CONTENT STANDARD: Earth and Space Science

CONTENT TOPIC: Structure of the Earth

CONCEPT: The earth's weather is an interaction of many parts of the environment.

CONTENT OBJECTIVE: 7I4 To discover how clouds are formed through the condensation of water vapor

INSTRUCTIONAL OBJECTIVE: The learner will

4.01 define climate
4.02 name two conditions that are used to describe climate
4.03 understand what causes climate change
4.04 explain feedback process
4.05 explain the greenhouse effect
4.06 describe how clouds can affect weather and climate
4.07 define condensation
4.08 define precipitation
4.09 explain how clouds form

I. Climate
A. Causes of climate change
B. Feedback process
II. Cloud affects on climate
A. Greenhouse effect
B. Water cycle
III. Cloud formation

TN COMPONENT OF SCIENCE: Process of Science

TN GOAL:

To enable students to demonstrate the processes of science by posing questions and investigating phenomena through language, methods and instruments of science.

1.4 ANALYZING - Data should be examined to find patterns and relationships that may suggest cause and effect or support inferences and hypotheses.

1.4a Data should be examined to find patterns and relationships.

BENCHMARK: Information can be organized into simple tables and graphs to identify relationships.

1.4c Scientific investigation may not produce concrete solutions.

BENCHMARK: There may be more than one good way to interpret a given set of findings.

TIME REQUIRED:

2 days

Beaker, safety goggles, hot plate, water, tongs, ice cubes, saucer, hot pad, incense, matches, clear plastic 2 liter soda bottle, tea kettle, large world map, postcards, magazine photos, or travel brochures that represent a variety of climates

We are going to discover that clouds, formed by the condensation of water vapor, affect weather and climate. Some do so by reflecting much of the sunlight that reaches Earth from the sun, while others hold heat energy emitted from the Earth's surface. Formation of clouds also affects precipitation, which in turn affects climate.

If you could choose a spot to spend an all-expenses-paid winter vacation, where would you go? Would you base your choice on the climate of that area? Don't you choose to go to the Rocky Mountains to snow ski because of the snowfall and cold weather, and don't you choose to go Florida to water ski because of the warm, sunny weather?

(Provide a large world map on a bulletin board. Also provide postcards, magazine photos, or travel brochures that represent a variety of climates, or ask students to bring these in.) Match the photos to their location on the map and mount the photos. Can you find patterns in climate? Can you relate the climate to the latitude or altitude?

For a given location, climate describes typical weather patterns over a period of years. The two conditions that are used to describe climate are patterns in temperature and precipitation. Did you know that both temperature and precipitation can be changed by clouds?!?

What causes climate change? Incoming energy from the sun usually balances outgoing energy from Earth. If the energy in or the energy out changes, Earth's temperature and its climates also change. Changes in the Earth's atmosphere may affect climate and this includes changes in the clouds.

Earth's atmosphere may affect the energy balance. In 1816, people wore winter coats to July fourth parades, and snow still lay on the ground in June. This unusual summer happened the year after a volcano erupted. Some volcanic gases form acid droplets in the atmosphere. The droplets and dust may have blocked the sun's energy.

In a feedback process, a change results in an effect that then causes more change. Feedback processes may cause climate changes. Compare Earth's temperature to the money in a bank account. If deposits (solar energy) increase or withdrawals (energy given off by Earth) decrease, the money in the account increases. Feedback effects are like interest on a bank account" you get back more than you put in.

What happens as sunlight hits a closed car on a hot day? (Visible light waves enter the glass windows and hit the seats and carpet, which in turn emit infrared waves; the infrared waves cannot pass back out of the glass and the car's interior heats up.) Can you compare the car with the atmosphere? (Sunlight passes through the atmosphere; rocks, soil, and plants change the light into infrared radiation; the carbon dioxide and water vapor in the air prevent the infrared from escaping into the upper atmosphere.)

Sunlight, on its way to the earth's surface, passes through the atmosphere. Most of the infrared waves radiated by the earth, however, cannot travel back into space. Infrared radiation is absorbed by water vapor and carbon dioxide in the lower atmosphere. These gases trap the heat, keeping it near the earth's surface. In this way, the atmosphere acts as a blanket, keeping the earth's heat from escaping into space. The trapping of heat by carbon dioxide and water vapor in the atmosphere is called the greenhouse effect.

Clouds also have an effect in keeping the earth's surface warm. Clouds increase the greenhouse effect by absorbing some of the infrared radiation that would normally be lost into space. The clouds radiate some of the heat back to the surface.

Why are cloudy nights often warmer than clear nights? (Infrared radiation is absorbed by water vapor and carbon dioxide as it is given off by the earth's surface. Water vapor in clouds absorbs infrared radiation and radiates the heat back to the earth's surface. Air also cools as heat radiates from it into space. For example, air cools more quickly on clear nights than on cloudy nights. Clouds trap heat and keep it from radiating into space. On clear nights, the infrared that the earth radiates can escape into space.

Clouds play an important part in the water cycle, and therefore the climate of a region. The earth's water is continuously changing forms through evaporation and condensation. These changes help to move water between the surface and the atmosphere. This continuous movement of water is called the water cycle. The sun provides the energy needed to evaporate water. Global winds then carry the water vapor through the troposphere. When the air cools, water vapor condenses into tiny water droplets, which form clouds. Eventually, the water in clouds may fall to the earth's surface as rain or snow. In time, some of this water flows into rivers, lakes, oceans, or the ground. Evaporation continues the cycle.

In order for clouds to form in the atmosphere, moist air must be cooled below its dew point. Water vapor condenses in the air after the air has become saturated. When you "see your breath"; on a cold day, you are seeing a cloud. The water vapor in the warm air you exhale condenses as it meets the cold outside air.

We are now going to do an activity that will demonstrate condensation.

Caution: Put on your safety goggles.

1. Fill a beaker one-third full with water. Place the beaker on a hot plate and bring the water to a boil.

2. Place three or four ice cubes on the saucer. Use the tongs to hold the saucer above the boiling water, as shown. Hold the saucer there for about two minutes.

3. Observe what happens between the bottom of the saucer and the beaker. Record your observations.

4. Turn off the hot plate. Use the hot pad to pick up the beaker and pour out the hot water.

(Analysis and Conclusion) You made a model of a general process that occurs on earth. What is it? (condensation) Explain what happened to cause the results you observed. (The water vapor hit the cooler saucer and condensed to a liquid.)

As I have said before, water vapor condenses in the air after the air has become saturated, but it only condenses on surfaces.

(Teacher Demonstration: Have incense burning as a source of smoke. Pour 50 ml of warm water into a clear plastic, 2 L soda bottle. Cap the bottle and shake it for 30 seconds. Then squeeze the bottle and quickly remove the cap to equalize the pressure. Repeat this procedure three times. Quickly remove the bottle cap below the smoke source. Squeeze the bottle, and then release the sides to suck smoke into the bottle. Replace the cap, shake the bottle, squeeze, and release the pressure as before.)

What is forming inside the bottle? (a cloud)

How did the smoke help to form the cloud? (Water vapor condensed around the smoke particles.)

In the atmosphere, these surfaces include specks of dust, smoke, or salt crystals from oceans. These particles around which water vapor condenses are called condensation nuclei. Eventually, each condensation nucleus collects enough water to become a tiny droplet. The droplets are so small that they float in the air. A large number of the droplets forms a cloud. If the air is cooled below 0'C , clouds will also form from tiny ice crystals, which are much larger than the droplets. Water vapor condenses only on the surface.

Most clouds pass overhead without producing rain or snow. The water droplets or ice crystals of these clouds stay small enough to remain suspended by air currents. Sometimes water droplets are heavy enough to fall, but evaporate before they reach the ground. For a cloud to produce precipitation, it must contain a very large amount of moisture.

(Teacher Demonstration: Position a beaker on a stand over the spout of a boiling teakettle. Keep the beaker over the steam until water droplets collect on the outside of the beaker. Then move the beaker away from the steam. Moisture will collect at and fall from the bottom of the beaker.)

Why do some clouds produce precipitation while other cloud do not? (A cloud must contain a very large amount of moisture in order to produce precipitation. Why might an airplane be flying through rain or snow, but it is not raining or snowing on the area over which the plane is flying? (Sometimes water droplets are heavy enough to fall from clouds, but the droplets evaporate before they reach the earth.)

Precipitation is another important condition that describes climate. Latitude affects precipitation. Recall that clouds most often form in rising, low-pressure air. Sinking high-pressure air is generally clear and dry. Notice that many deserts, areas with less than 25 cm of precipitation yearly, have latitudes of about 30'N and 30'S. At these latitudes, sinking air means that few clouds form and little rain falls.

Land forms can affect precipitation. Mountains force air to rise and cool. Clouds form, removing moisture form the air. As the air sinks and warms on the other side of the mountains, it becomes even less humid.

Applying Science: In order to produce rain over dry areas, scientists have experimented with "cloud seeding." In this process, silver iodide crystals are sometimes used to provide more condensation nuclei for larger water droplets to form in clouds. Another method involves the use of dry ice crystals to cool the moist air in clouds to help increase the speed at which water vapor condenses around nuclei.

climate- average weather conditions over a period of years

condensation nuclei- particles around which water vapor condenses

condensation- process in which a gas changes into a liquid; the moisture that appears during the process

dew point- temperature in which air becomes saturated with water

feedback process- a condition in which a change results in an effect that then causes more change.

greenhouse effect- effect of earth's atmosphere trapping heat from the sun

precipitation- any form of moisture that falls to the surface of the earth. Snow, rain, sleet, and hail are types of precipitation.

troposphere- the portion of the atmosphere included between the earth's surface and the stratosphere and in which the temperature falls as the altitude increases, cloud formation takes place, and air currents are active

water cycle- continuous movement of water between Earth and the atmosphere

water vapor- the gaseous form of water in the air

This is the time this file has been accessed since 04/02/98.

The University of Tennessee at Martin is not responsible for the information or views expressed here.

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