METR 104:
Our Dynamic Weather
(Lecture w/Lab)
Lab Exploration #6:
Moisture, and Cloud Formation
Dr. Dave Dempsey,
Dr. Oswaldo Garcia,
& Denise Balukas
Dept. of Geosciences
SFSU, Fall 2012


(Lab Section 1: Wed., Nov. 28; Lab Section 2: Fri., Nov. 30)

Objectives. During and after this lab exploration, students should be able to:

Background Knowledge:

Materials Needed. To complete this exploration, you will need:

I. Introduction.

Water vapor is water (H2O) in the form of a gas. It is an invisible and odorless, and there is always at least a little bit present in the air.

In contrast, when clouds are present they are quite visible. A cloud consists of a large collection (aggregate) of tiny water droplets and/or ice crystals, (virtually) suspended in the air. Cloud droplets and cloud ice crystals are individually too small to see but collectively they scatter enough visible light for us to see the aggregate.

Clouds reflect sunlight back to space, contribute to the greenhouse effect, and exhibit a wide range of forms, extent, and altitude. They tell something about the state and motion of the atmosphere, and can inspire in us feelings ranging from awe to delight to fear to melancholy. Some (though not most) clouds are capable of producing precipitation.

In this lab, you'll explore some of the conditions and mechanisms that can lead to the creation of most kinds of clouds in the atmosphere.

II. Instructions. Your instructor will assign you to a group of 3-4 people. Your group will perform the experiments described below.

For each of the experiments, observe and carefully record what you see, including both qualitative (descriptive) results and quantitative results (measurements). For some things your instructor will assign tasks, while for other things you'll assign tasks to yourselves within your group. In some cases you'll carry out the experiment or parts of it, while in other cases you'll be observing and recording results.

Before you carry out each experiment, read all of its instructions so you know what sequence of actions you're about to take, and decide in advance exactly who will be doing what and that each person understands how to perform their assigned task(s).

Although you won't turn in anything at the end of this lab, later (out of class, via iLearn) you'll be asked to refer to some of your observations from this lab and from demonstrations performed in lecture (plus information in Reading #6) to confirm (support) or disconfirm possible explanations for what you see happening. Hence, you need to record detailed and accurate notes of what you observe, for later reference.

  1. Temperature and pressure inside of a bottle.

    You need to know:
    • how to read a thermocouple (electronic thermometer) graph
    • how to pump air into a bottle using a vacuum bulb
    • how to read a digital scale and set it to read zero (weight) with an object on the scale

    The experimental setup: a one-gallon, narrow-necked glass bottle; a rubber stopper and a vacuum bulb, connected by a flexible rubber or plastic tube; a thermocouple threaded through a hole in the stopper with the sensor inside the bottle; a cup with a little water in it; and a digital scale.

    • Does air have weight?

      1. Make sure that the rubber stopper is inserted firmly into the mouth of the bottle, so it is air tight. (Moistening the stopper slightly first can improve the seal.)

      2. Make sure the digital scale is plugged in and turned on. Set it so that it records in grams (not ounces). Place the bottle on the digital scale. Hold the rubber tube firmly against the table or countertop surface so that the portion of the tube between the stopper and the point where you're holding it won't move (but not so firmly that you pinch the tube closed).

      3. "Zero" the weight scale (press the "tare" or "Z" button) to get it as close to zero as you can. When the weight reading is stable, record it.

      4. Using the vacuum bulb, pump as much air into the bottle as you can, while one person continues to hold the rubber tube firmly against the table or countertop and another person prepares to prevent the stopper from flying too far should it pop off.

        Everyone not directly involved should step back, and those directly involved should use an arm or hand to shield their face against a flying object. If the person pumping gets tired, someone else should take over. [Does the difficulty of pumping change as you pump air into the bottle? Think about why or why not.] Continue to note the weight indicated by the scale. Record the weight at the point where it differs the most from the starting weight (assuming that the rubber tube between the bottle and the point where you're holding against the table or countertop hasn't moved).

    • How does changing pressure affect temperature in a bottle?

      1. Plug the thermocouple into the USB port of one of our Mac laptops, and start the Logger Lite software (green notebook icon on the "Dock"). Insert the stopper firmly into the bottle, with the thermocouple sensor inside.

      2. On the laptop, click on the big, green "Collect" button at the top of the Logger Lite window. When the temperature inside the bottle is relatively steady, record it.

      3. While one person holds the rubber stopper firmly in place, a second person should shine a flashlight through the bottle from the side. While the stopper continues to be held firmly in place to prevent air from leaking (and the stopper from popping off), a third person should use the vacuum bulb to pump air into the bottle until they can't pump any more in.

      4. Carefully loosen the stopper until it pops off, but hold it loosely (a) to prevent it from flying away and hitting someone, and (b) to keep the thermocouple sensor inside the bottle. Immediately reinsert the stopper afterwards. If you see that anything inside the bottle has changed, record the observation.

      5. After a short time (less than 30 seconds), click on the big, red button at the top of the Logger Lite window to halt plotting of the temperature. To adjust the vertical scale on the temperature plot and make it easier to read, click on the "Scale" icon on the tool bar along the top of the Logger Lite window. (The Scale icon has a big blue "A".) Record the change in temperature, if any, that occurred between the time just before you opened the stopper and just after you popped it off. (If there was a change, think about what might have caused it.)

      6. Loosen the stopper and pump some air into the bottle to flush out the air inside and replace it with new air. When the temperature inside returns to the value roughly where it started in Step (1) above, remove the stopper and the thermocouple sensor, tip the bottle a little, and pour in just enough water to cover the bottom of the bottle. Set the bottle upright, insert the thermocouple sensor and firmly insert the stopper. (You'll come back to the bottle later.)


  2. Measuring dew point temperature and relative humidity

    You need to know:
    • how a sling psychrometer works and how to use it to measure dry bulb and wet bulb temperatures
    • how look up the dew point temperature and relative humidity in a table, given the dry bulb and wet bulb temperatures

    The setup: a sling psychrometer, with the cloth sack over the bulb of one of the thermometers moistened with water.

    1. Note the temperatures recorded by the wet bulb and the dry bulb on the sling psychrometer. (Are they different? If so, why do you think so, given that the water used to moisten the wet bulb had essentially the same temperature as the air in the room?)

    2. If you were to sling the psychrometer around for a 30-60 seconds, what do you think would happen to the wet bulb and dry bulb temperatures? Why? Try it and find out.

    3. Read and make note of the dry bulb and wet bulb temperatures.

    4. Sling the sling psychrometer around for another 30 seconds, and read the dry and wet bulb temperatures again.

    5. Keep repeating Step (4) until the temperatures of both bulbs are steady. Record the final temperatures.

    6. Using the dew point and relative humidity table(s) provided, look up the dew point temperature and the relative humidity corresponding to the dry bulb and wet bulb temperatures that you measured. (Note that some tables use the "wet bulb depression" instead of the wet bulb temperature. The wet bulb depression is just the difference between the dry bulb and wet bulb temperatures.)


  3. Temperature, pressure, and moisture inside of a bottle (continued).

    You need to know:
    • Same things as before.

    The experimental setup: a closed, one-gallon, narrow-necked glass bottle with a shallow layer of water at the bottom; a rubber stopper and a vacuum bulb, connected by a flexible rubber or plastic tube; a thermocouple threaded through the stopper with the thermocouple sensor inside the bottle; several wooden matches.

    1. [Warm-up questions to think about: What happens to liquid water left sitting around at room temperature? (For example, have you every washed dishes and set them in a dish drainer for a while?) How would the amount of water vapor in the air inside the bottle (that is, the water vapor content of that air) compare to the water vapor content of air outside the bottle? What about the dew point temperature of the air inside the bottle compared to outside? Relative humidity inside vs. outside the bottle? In each case, why?]

    2. On the laptop, click on the big, green "Collect" button at the top of the Logger Lite window. If you are prompted about whether you want to continue the previous plot or not, say no (create a new plot).

    3. When the temperature inside the bottle is relatively steady, record it.

    4. Have someone hold the rubber stopper firmly in place, and a second person shine a flashlight through the bottle from the side. While the stopper continues to be held firmly in place to prevent air from leaking (and the stopper from popping off), a third person should use the vacuum bulb to pump air into the bottle until they can't pump any more in, as before.

    5. Wait perhaps 20 seconds, continuing to hold the stopper in place.

    6. Continue to shine the flashlight through the bottle. Carefully loosen the stopper until it pops off, but hold it loosely (a) to prevent it from flying away and hitting someone, and (b) to keep the thermocouple sensor inside the bottle. Immediately reinsert the stopper afterwards.

    7. If anything inside the bottle that you can see has changed, record the observation.

    8. After a short time (less than 30 seconds), click on the big, red button at the top of the Logger Lite window to halt plotting of the temperature. To adjust the vertical scale on the temperature plot and make it easier to read, click on the "Scale" icon on the tool bar along the top of the Logger Lite window. (The Scale icon has a big blue "A".) Record the change in temperature, if any, that occurred between the time just before you opened the stopper and just after you popped it off.


  4. Temperature, pressure, and moisture inside of a bottle (continued further).

    You need to know:
    • Same things as before, plus how to light a wooden match.

    The experimental setup: same as before, plus several wooden matches.

    1. [Questions to think about: Have you ever been inside of a cloud? (For example, have you ever been in San Francisco in the summer? :-)) What does it look like through the first foot (12 inches) or so in front of your face—can you see the cloud? If there were a cloud like that inside this bottle, do you think you'd be able to see much? What does smoke consist of? Is it a cloud?]]

    2. On the laptop, click on the big, green "Collect" button at the top of the Logger Lite window. If you are prompted about whether you want to continue the previous plot or not, say no (create a new plot).

    3. Tip the bottle at an angle, making sure that the water inside doesn't get the thermocouple sensor wet. Get ready to remove the stopper briefly, but don't do it yet.

    4. Strike a wooden match to light it. Let it burn perhaps halfway, so that it will produce a fair bit of smoke when extinguished.

    5. As quickly as you can, remove the rubber stopper, gently blow out the match, and immediately drop the smoking match into the bottle, firmly reinsert the stopper, and set the bottle upright.

    6. Shine a flashlight through the bottle. [Can you see any smoke inside?]

    7. Hold the stopper firmly in place, continue to shine a flashlight through the bottle, and pump air into the bottle until you can't pump any more. [Can you see any smoke inside now?]

    8. Wait perhaps 20 seconds, continuing to hold the stopper in place. If Logger Lite has stopped recording the temperature, click on the big, green "Collect" button to restart it, adding the new measurements to the previous plot (rather than starting over with a new one).

    9. Continue to shine the flashlight through the bottle. Carefully loosen the stopper until it pops off, but hold it loosely (a) to prevent it from flying away and hitting someone, and (b) to keep the thermocouple sensor inside the bottle. Immediately reinsert the stopper afterwards.

    10. If anything inside the bottle that you can see has changed, record the observation.

    11. After a short time (less than 30 seconds), click on the big, red button at the top of the Logger Lite window to halt plotting of the temperature. To adjust the vertical scale on the temperature plot and make it easier to read, click on the "Scale" icon on the tool bar along the top of the Logger Lite window. (The Scale icon has a big blue "A".) Record the change in temperature, if any, that occurred between the time just before you opened the stopper and just after you popped it off.