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


(Lab Section 1: Wed., May 9; Lab Section 2: Fri., May 11)

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 that indicate 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. For each of the four experiments described below, 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. 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 make sure that it's clear who will be doing what.

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 (as well as Reading #6) to 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. Wind chill experiment: wind and temperature

    You need to know: The experimental setup: an electric fan, with several thermometers mounted in front of it and behind it.

    1. Read the thermometers and record their temperatures.
    2. Predict what will happen to the temperatures measured by the thermometers after the fan is turned on. The temperature will:
      1. go up a lot (say, a few degrees or more)
      2. go up a little (say, half a degree to a degree)
      3. change little or not at all (say, less than half a degree)
      4. go down a little (say, half a degree to a degree)
      5. go down a lot (say, a few degrees or more)
    3. Turn the fan on "low" and wait a minute.
    4. Reread the thermometers and record their temperatures.
    5. Turn the fan on "high" and wait a minute.
    6. Reread the thermometers and record their temperatures.
    7. Compare your prediction to what you observed and discuss what might be going on.

  2. Bike tire experiment: pressure and temperature

    You need to know: The experimental setup: an inflated bike tire, several thermometers, and a bike pump

    1. With a thermometer, measure and record the air temperature in the room.
    2. We can't easily measure the temperature of the air inside the bike tire, but given that (a) the bike tire has been sitting around in the room for a while, (b) the air temperature in the room hasn't been changing much, and (c) knowing how conduction works, what would you say about the temperature of the air inside the tire compared to the room temperature (higher, about the same, or lower)? Why?
    3. Answer the same question about the valve on the bike tube.
    4. With your fingers, briefly feel the metal part of the valve on the bike tube. Does it feel warm, neutral, or cool? Why, do you think? [Note: when something feels warm or cold, what you're actually feeling is your own skin temperature, because that's where the nerves are that are responsible for detecting and signaling temperature change. Similarly, a thermometer really only measures its own temperature. If your skin is warmer than it's "normal" temperature (around 91°F), your skin will feel warm or hot. When it cooler than "normal", it will feel cool or cold. If you touch an object and "it" feels warm or cold, that means that your skin has gained or lost heat (probably by conduction of heat from or to the object respectively, which will happen if the object is warmer or cooler than your skin, respectively).
    5. Given the pressure exerted by the air inside the tire and the pressure of the air in the room outside the tire, what should the air inside the tire do when the valve is opened? Why? (What physical principle that we've learned about applies here?)
    6. Position the sensor of a thermometer directly in front of the tire valve, so you can measure the temperature of the air immediately after it leaves the tire. Using the tip of a ball-point pen or other narrow, pointy object, open the valve and let air out of the tire until it stops coming out, measuring its temperature the whole time. Record the temperature at the point when it has changed the most.
    7. Quickly and briefly feel the metal part of the valve again with your fingers. Does it feel warmer, no different, or cooler than it did before the air escaped? Why, do you think? (That is, if it changed temperature, by what mechanism did it gain or lose heat?)
    8. Compare the temperature of the air after it escaped from the tire to the temperature you think the air inside the tire had before the air escaped. If the temperature differs, think about the mechanism(s) by which it might have gained or lost heat to account for the temperature change. In particular, what candidate mechanisms seem possible in this case, at least in principle? Which one(s) is (are) seem the most likely to have acted here?
    9. Using the tire pump, pump air back into the tire. Feel the valve again. Has its temperature changed? If so, how do you think it happened?

  3. Temperature and pressure inside of a bottle.

    You need to know: 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.

    1. Measure and record the temperature of the air inside the bottle.
    2. 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.)
    3. Make sure the digital scale is plugged in and turned on. 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).
    4. "Zero" the weight scale (press the "tare" button), or get it as close to zero as you can. When the weight reading is stable, record it.
    5. 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 holds the stopper in place. Read the temperature of the air inside the bottle while pumping air in. Record the temperature at the point where it differs the most from the initial temperature. (If it differs from the initial temperature, think about how that might have happened.)

      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 pumper 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).

    6. If the rubber stopper doesn't pop off by itself after you've pumped as much air into the bottle as you can, carefully pry the stopper loose while holding onto it loosely to prevent it from flying off (standing back as much as you can and shielding your eyes from a flying object). Make sure that the thermocouple sensor remains inside the bottle.
    7. Immediately after the rubber stopper pops off, reinsert it and remove the bottle from the scale. If you see that anything inside the bottle has changed, record the observation.
    8. After a short time (10-30 seconds), read and record the temperature of the air inside the bottle. (If it differs from the the temperature just before the rubber stopper popped off, think about why that might have happened.)
    9. 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.)

  4. Measuring dew point temperature and relative humidity

    You need to know: 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.)

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

    You need to know:
  6. 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. Read and record the temperature of the air inside the bottle.
    2. [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?]
    3. Have someone hold the rubber stopper firmly in place. While the stopper continues to be held firmly in place to prevent air from leaking (and the stopper from popping off), use the vacuum bulb to pump air into the bottle until you can't pump any more in, as before.
    4. Record the temperature, continuing to hold the stopper in place so that no air leaks out.
    5. Wait perhaps 30 seconds, continuing to hold the stopper in place.
    6. Carefully loosen the stopper until it pops off, but hold it loosely to prevent it from flying away and keep the thermocouple sensor inside the bottle (protecting yourself from the flying stopper if it gets away from you). Immediately reinsert the stopper afterwards.
    7. If anything inside the bottle that you can see has changed, record the observation.
    8. Record the temperature inside the bottle at the point where it had changed the most from where it started.

    9. [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 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?]
    10. 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.
    11. Strike a wooden match to light it. Let it burn perhaps halfway, so that it will produce a fair bit of smoke when extinguished. (What does smoke consist of? Is it a cloud?)
    12. As quickly as you can, remove the rubber stopper, gently blow out the match, drop the smoking match into the bottle, and firmly reinsert the stopper.
    13. Set the bottle upright, hold the stopper in place, and pump air into the bottle until you can't pump any more. (Can you see any smoke inside?)
    14. Wait perhaps 30 seconds, continuing to hold the stopper in place.
    15. Carefully loosen the stopper until it pops off, but hold it loosely to prevent it from flying away and keep the thermocouple sensor inside the bottle (protecting yourself from the flying stopper if it gets away from you).
    16. If anything inside the bottle that you can see has changed, record the observation.
    17. Record the temperature inside the bottle at the point where it had changed the most from where it started.


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