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# Hygrometers: Ways to Measure the Atmosphere's Water Content (page 2)

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Author: Janice VanCleave

### Try New Approaches

1. Is some paper more hygroscopic than others? Repeat the experiment comparing the results of papers such as construction paper and typing paper with newspaper. Use the most hygroscopic paper for the following experiment.
2. Can the coil be used to indicate changes in air humidity? Place one of the coils inside a dry open jar and the other in a dry closed jar. Stand the jars outdoors on different days with low, medium, and high humidity. (The jars prevent any wind from unwinding the coils.) Compare the tightness of the coils to determine the size of the coils when the air has low, medium, and high humidity. How accurate is this method of measuring humidity?

1. Design a hygrometer with a scale, such as one using a hygroscopic strand of silk or hair. (Silk threads can be found where embroidery floss is sold.) One possible design is to cut a pointer from stiff paper and punch two opposite holes in the pointer. Hang the pointer by the hole near its straight end on a nail secured to a board. Cut a hygroscopic strand about 10 inches (25 cm) long, and tie one end to the other hole in the pointer. Hold the free end of the strand against the board so that the straight end of the pointer is parallel to the edge of the board. Secure a second nail to the board near the end of the strand, and tie the strand to the nail. Use a fine-tipped pen and a metric ruler to mark a few lines at l-cm intervals at the tip of the pointer, as shown in Figure 29.2. Observe the effect of changes in humidity on the hygroscopic strand.
2. Relative humidity is the ratio between the amount of water vapor present in the air and the amount of water vapor the air can hold at that temperature, expressed as a percentage. A psychrometer is a type of hygrometer used to measure relative humidity. A psychrometer has two thermometers, a wet-bulb thermometer and a dry-bulb thermometer. Make a psychrometer by wrapping the bulb of one thermometer with a thin layer of wet cotton and leaving the second thermometer uncovered. Lay the two thermometers on the edge of a table with their bulbs extending over the table edge. With an index card, fan the air near the two thermometers, but do not touch the thermometers with the card. When the wet-bulb thermometer reaches its lowest point, record the temperature on both thermometers. Use the following example and the relative humidity table, Table 29.1, to determine relative humidity from your psychrometer readings. Note: If your thermometer measures in degrees Fahrenheit, use Appendix 2 to convert (change) degrees Fahrenheit to degrees Celsius.
3. Example:

What is the relative humidity if the dry-bulb reading is 16°C and the wet-bulb reading is 13°C?

• Subtract the wet-bulb temperature from the dry-bulb temperature:
16°C – 13°C = 3°C
• Find 16, the dry-bulb temperature, in the column on the left side of Table 29.1, and 3, the difference between the dry-bulb and wet-bulb temperatures, in the horizontal row at the top of the table. Where the column and row meet is the number for the relative humidity. For this example, the number is 71; thus the relative humidity is 71%.

For more information about relative humidity and psychrometers, see National Audubon Society Field Guide to North American Weather (New York: Chanticleer Press, 1995), pp. 607-608.

### Get the Facts

1. Dew point is the temperature at which air cannot hold any more water and condensation occurs. Find out more about dew point. What does the saturation point of air mean? How does temperature affect air's saturation point? See Weather, The Nature Company Guides (New York: Time Life Books, 1996), pp. 40–41.
2. Hygrometry is the science of determining how much water air can hold at a given temperature and pressure. Scientific instruments, such as hygrometers, are used to make these measurements. But nature also provides its own hygrometers. If the humidity is very high, the cone scales of a pinecone close, as do the petals of the flower of the scarlet pimpernel. See Gary Lockhart, The Weather Companion (New York: Wiley, 1988) pp. 27-30. Find out more about how natural hygrometers affect the opening and closing of pinecones. For information, see The Weather Companion, pp. 27–30.
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