Atmospheric Processes — Convection

In class, you've learned that the earth's surface absorbs solar radiation, heats up, and transfers some of that heat to the air. The air then circulates, carrying the heat with it, by a process called convection. Convection currents are found in many places and on many scales, such as huge convection currents in the atmosphere, oceans, and even in the earth's interior. Smaller convection currents can be found in a cup of hot cocoa or a fish tank. Meteorologists usually use convection to refer to up and down motions of air. Heat gained by the lowest layer of the atmosphere from radiation or conduction is most often transferred by convection. Convection currents in the atmosphere carry the sun's heat from the warmest places on earth (like the tropics) to colder places (like the polar regions).

In the first part of this activity, you will be working with convection currents in water. It is important to remember that water and air circulate heat by convection in the same way. In fact both water and air can be thought of as fluids, the main difference being that water is a much more dense fluid than air is. In the second part of this activity, you will investigate the fluid nature of air.

PART 1: Currents in Water

Materials (per team)

• Clear plastic plant saucer 8 to 10 inches wide
  
  
• Pitchers or jugs for hot and cold water
  
  
• Food coloring
  
  
• Small container for food coloring (a small cup would work fine)
  
  
• Medicine dropper or pipette
  
  
• Four Styrofoam cups
  
  
• Data sheet

Procedure

1. According to your teacher's instructions, form a student team and gather the materials from the list above.
   
   
2. Place three Styrofoam cups upside down on a piece of paper.
   
   
3. Place the plastic plant saucer on top of the cups as shown. The cups should be near the outer edges of the saucer and evenly spaced.
   
   



4. Fill the plastic saucer three-quarters full with cool water. To make certain the water is still, let it sit before the experiment. Be careful not to bump the desk or table at any time during the experiment.
   
   
5. Using a dropper, slowly release a small amount of food coloring at the bottom of the saucer of water. Slowly remove the dropper, taking care not to stir the water.
   
   
6. Observe and record on the data sheet what the drop does as it sits in the tray. Draw what happens.
   

Variations

Repeat the experiment with the following variations. Record and draw your observations after each variation. Make certain you start each trial with a clean saucer of water.

For the following three trials, place a cup of hot water under the center of the saucer as shown. Fill the cup almost to the top.

Trial A: Place a drop of food coloring on the bottom of the saucer in the center, over the cup of hot water. Take care not to stir the water.

Trial B: Place a drop of food coloring on the bottom of the saucer about halfway between the center and the side. Take care not to stir the water.

Trial C: Place two drops of food coloring on the bottom of the saucer, one halfway between the center and side of the saucer, the other in the center. Take care not to stir the water.

Observations and Questions for Part 1

In your lab books or on a separate piece of paper, complete the following.

1. For each trial, draw your observations from an overhead view, including a caption for each drawing that explains what is happening. Now draw the same thing from a side view.
   
   
2. What effect does the hot water in the center under the plant saucer have upon the currents?
   
   
3. What type of heat transfer is taking place? How do you know?
   
   

PART 2: Air is a Fluid

Materials

• Baking soda
  
  
• Vinegar
  
  
• 500 ml beaker or glass jar of similar size
  
  
• Candle
  
  
• Matches
  
  
• Strip of poster board or cardboard about 12" by 3"

Procedure

1. Fold the poster board or cardboard lengthwise.
   
   
   
   
2. Place the candle on a plate and light the candle.
   
   
3. Put about a tablespoon of baking soda in the glass jar or beaker.
   
   
4. Pour about 1/4 cup of vinegar in the jar or beaker.
   
   
5. When the fizzing subsides, hold the poster board "funnel" at an angle so that one end is near the candle flame and the other end is slightly higher.
   
   
   
   
6. "Pour" the gas in the beaker or jar down the funnel. What happens to the flame?
   
   

Observations and Questions for Part 2

Answer these in your lab book or on a separate piece of paper.

1. What happens when the vinegar and baking soda are mixed?
   
   
2. Explain how the flame was extinguished.
   
   
3. In this demonstration, how did the gas act as a liquid?

When you're finished with the activity, click on Back to Teacher Guide at the top of the page.