Illustrative activity:  What are condensation/humidity/dewpoint?

The following activity is an illustrative activity, not an experiment.  An illustrative activity helps one understand a concept that has already been figured out through experiments by other people.

This activity was developed for Project Atmosphere and published by the American Meteorological Society (1993), who grant permission for its reproduction so long as it is used for non-commercial educational purposes.

Activity: Water Vapor Investigation

Materials
Four 12-ounce clear or translucent drinking cups for every group, enough Styrofoam "peanut" packing material or popped popcorn to fill one cup, scissors, permanent marking pen.

Procedure  (this step is already done for this lab)
Prepare a set of cups for use in this activity. Fill a 12-ounce cup to the brim with water. Pour from that cup into another until the water levels in both are the same. Trace the water line on the outside of both cups with a permanent marking pen. Empty the water from one of these and cut along the traced line to make a 6-ounce cup. Now pour water remaining in the 12-ounce cup into the third until their water levels are the same. Trace their water levels. Cut the third cup down to the water level to make a cup that holds 3 ounces. You should end up with one 3-ounce cup, one 6-ounce cup that has a line marking the 3-ounce level, and one 12-ounce cup with lines marking 6- and 3-ounce levels. Write a large 0 on the side of the smallest cup, 10 on the middle size, and 20 on the largest cup to indicate 0, 10, and 20 degrees Celsius. Use these Cups as guides to preparing other sets. The fourth and unmarked cup will hold the Styrofoam packing "peanuts" or popped corn.

Objectives
After completing this investigation, you should be able to:

• Explain with cups of different sizes how the "capacity" of air to hold water vapor varies with temperature.

• Use the cups and Styrofoam packing "peanuts" as a model to explain relationships between the "capacity" of air to hold water vapor and the actual amount of water vapor in the air.

Method
Cups of different sizes are used in this exercise to represent the "capacity" of air to hold water vapor at 0, 10, and 20 degrees C.  Packing "peanuts" are poured into the cups to represent the water vapor actually in the air.

Start the activity by filling the large unmarked cup approximately level with packing peanuts. Tap the cup gently on your table or desk top to help the material settle as you fill the cup. This is the supply of packing peanuts you will use in this exercise.

This activity involves the use of four cups. The unmarked cup is used to store the packing peanuts. The largest marked cup is twice the capacity of the mid-size cup. The mid-size cup is twice as big as the small cup. The marked cups represent the "capacity" of air to hold water vapor at 0, 10, and 20 degrees Celsius. Each cup is labeled by the temperature related to its capacity.

1. Fill the small 0-degree cup with packing peanuts until the contents are level with the top of the container. Pour the contents into the mid-size 10-degree cup. Repeat this until the 10-degree cup is level full. Now pour filled 10-degree cups into the 20-degree cup until it is full. Assuming the cups represent the capacities of air to hold water vapor at 0, 10, and 20 degrees, complete the following statement:

The capacity of the air to hold water vapor approximately ______________ when the temperature increases 10 Celsius degrees.

2. Starting with a filled 20-degree cup, pour its contents into the 10-degree cup until it is level full. Now pour the contents of the 10-degree cup into the 0 -degree cup until filled to the brim. Now complete the following statement:

The capacity of the air to hold water vapor approximately ________________when the temperature lowers 10 Celsius degrees.

3. Now empty the 20-degree cup and pour filled 0-degree cups into the large cup until it is full. Based on this, complete the following statement:

The capacity of the air to hold water vapor increases approximately ____________times when the temperature rises 20 Celsius degrees.

4. According to the same observations, what happens to the capacity of air to hold water vapor as the temperature falls 20 Celsius degrees?
 
 

5. When air cools, its capacity to hold water vapor decreases, and any excess water vapor must condense. This can be demonstrated by attempting to pour all the packing peanuts from a filled 20-degree cup into the 10-degree cup. Level the top on the 10-degree cup. The overflow represents the water vapor that condensed out. In this example of the 10-degree cooling, how much of the water vapor condensed to liquid as the temperature dropped 10 degrees?
 
 

6. Air filled to its capacity with water vapor is called saturated air. If saturated air at 20 degrees is cooled 20 degrees, how much of its water vapor must condense?
 
 

7. Saturated air has a relative humidity of 100%. Relative humidity is a measure of the amount of water vapor actually in the air compared to the amount it would hold if saturated at the same temperature. Pour a full 0-degree cup into a 10-degree cup to determine what the relative humidity would be if air saturated at 0 degrees is warmed 10 degrees with no addition of water vapor. What is it? What would the relative humidity be if that same air were warmed another 10 degrees to 20 degrees Celsius?
 
 
 

8. Explain in your own words why in cold weather the relative humidities in heated buildings (without humidifiers) are quite low.
 
 
 
 

9. Dew point is another common humidity measure. It is the temperature to which air has to be cooled (without changing the amount of water vapor in the air) to become saturated. Whenever air is saturated, its temperature and dew point will be the same. What is the approximate dew point of air at 20 degrees with a relative humidity of 50%? To find out, fill the 20-degree cup half full. Then, pour it into the 10-degree cup.
 
 
 
 

10. What is the dew point of air saturated at 0 degrees when the air temperature is raised to 10 degrees without the addition of water vapor? To find out, pour a filled 0-degree cup into 10-degree cup and ask yourself whether or not the dew point changed.
 
 

11. If saturated air at 20 degrees is cooled to 10 degrees, what is its final dew point? To help find your answer, attempt to pour a filled 20-degree cup into 10-degree cup while asking yourself how much water vapor the 10-degree cup is holding compared to its capacity.
 
 

12. In general, when saturated air is cooled, what happens to its capacity to hold water vapor, its dew point, and its relative humidity? Refer to your observations made above.
 
 
 
 
 

13. Describe, in your own words, the water vapor and temperature relationships which must exist for cloud, dew, and frost formation.
 
 
 
 
 
 
 

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