Dew Point and Relative Humidity Lab

Dew Point and Relative Humidity Lab

Background:Even above the hottest deserted areas on Earth, water vapor (humidity) exists. Humidity is the source of moisture for clouds and rain. Meteorologists measure both dew point and relative humidity to determine how much humidity is in the air and to predict the chances of precipitation.

Dew point is the temperature at which the air is filled or saturated with water vapor. Relative humidity is the extent to which air is saturated with water vapor. When air cools below the dew point, water vapor in the air condenses. In this lab, you will determine dew point and relative humidity and compare two different methods of figuring this result.

Learning Objective:

• I can understand and calculate relative humidity.

• I can distinguish between dew point and relative humidity.

Part A Dew Point Method

Materials:

Shiny metal cansstirring rodice cubes

celsius thermometerwater

Procedure:

1. Read through the procedure and develop a data table on a separate sheet of paper that meets the needs of the lab, label it Data Table 1.
2. Use a thermometer to measure the air temperature in Celsius. Record this in Data Table 1.
3. Look at Figure 1 to the right. Find the capacity of air to hold water vapor for the temperature in the classroom. Record the capacity in Data Table 1.
4. Fill the metal can halfway with water and place a thermometer in the water.
5. Add a small amount of ice and use a stirring rod to stir the water slowly.
6. Continue adding ice slowly and watch for the first appearance of dew on the outside of the container. DO NOT STIR WITH THE THERMOMETER. At the instant you see dew, record the dew point temperature in Data Table 1.
7. To confirm the accuracy of your first dew point reading, repeat steps 3 through 5 one more time and record in Data Table 1.
8. Average your two dew point values and record the average in Data Table 1.
9. Use the capacity chart (Figure 1) to determine the air capacity to hold water vapor for your average dew point temperature. Your value for capacity at the dew point equals the specific humidity of the air. Record this value in Data Table 1.
10. Use your values and the formula below to compute the relative humidity of air and include this calculation in Data Table 1.

Relative humidity = specific humidity/capacity x 100%

Analysisand ConclusionAnswer questions in complete sentences.

1.Explain dew point.

2.How does relative humidity help describe the weather?

3.How does this compare to NOAA’s relative humidity?

4.Describe the experimental error for this lab?

Part B Psychrometer Method

Materials:

Sling Psychrometer Relative Humidity Chart

Procedure:

1. Read the procedure and create a data table and label it Data Table 2.
2. Obtain a Sling Psychrometer. Record the dry-bulb temperature in Data

Table 2.

1. Wave the Sling Psychrometer for 2 minutes. Record the wet-bulb temperature in Data Table 2.
2. Subtract the wet-bulb temperature from the dry-bulb and record in Data Table 2.
3. Refer to the Relative Humidity Chart and determine the Relative Humidity for your data and record in Data Table 2.

Analysis and ConclusionAnswer questions in complete sentences.

1. Compare the two relative humidity values for the classroom air from Data Tables 1 and 2. Are the two values the same or different? If the values differ, which value do you think will be more accurate? Explain your answer.
2. Describe the experimental error for this lab?

1. Imagine that, early one cool morning, you use a psychrometer outdoors and discover that the wet-bulb and dry-bulb values are the same. Why would this happen?

1. How could you raise the relative humidity of the air in our classroom?

1. Do you think outside would have a higher or lower Relative Humidity than inside?

Extension:

Go outside and calculate the Relative Humidity. Why is it different than the inside Relative Humidity?