In this investigation you will figure out the dew point temperature for your classroom.

At what temperature in your classroom would it start to rain?

A relative humidity measurement gives a measure of the amount of water in the air as compared with the maximum amount of water the air could hold without condensing out as fog. For example, a relative humidity reading of 35% means that the air is holding 35% of the water it could hold for that air temperature.

What happens when the relative humidity is 100%?

• 1 temperature sensor
• 1 relative humidity sensor
  Note: if a relative humidity sensor is not available, refer to the Relative Humidity Measurement activity, which explains how to use the temperature sensor to measure relative humidity.

This activity addresses NSES standards for earth and space science and inquiry at grades 5-8 for structure of the earth system
(http://books.nap.edu/readingroom/books/nses/6d.html#es).

There are no special safety concerns in this investigation.

1. Water is present in the air all around the earth, but the amount of water in the air varies according to the climate and other factors. The following table shows the historical average afternoon relative humidity and temperature for several locations in the United States during the month of June. (Data from RH table, temperature chart. Temperatures converted from F to C.)

City	Average RH (%)	Average Temp (Celsius)
New York, NY	56%	21.7
Las Vegas, NV	11%	29.7
Mt Washington, NH	86%	6.8
Los Angeles, CA	68%	19.1

2. Using the Water Density Chart, calculate how much water is in the air in each of the locations of the chart. You can use the Ruler Tool to read values on the graph. Remember that the line represents that water density at 100% RH for a given temperature. Figure out how to use the Ruler Tool to find the water density at other values of relative humidity. See Technical Hints for using the Ruler Tool.

New York, NY, Avg. RH = 56%, Avg. Temp = 21.7 degrees C

Water density (g/m3) =

Las Vegas, NV, Avg. RH = 11%, Avg. Temp = 29.7 degrees C

Water density (g/m3) =

Mt Washington, NH, Avg. RH = 86%, Avg. Temp = 6.8 degrees C

Water density (g/m3) =

Los Angeles, CA, Avg. RH = 68%, Avg. Temp = 19.1 degrees C

Water density (g/m3) =

3. Based on your calculations, answer the following questions.

Location with the highest temperature:

Location with the highest relative humidity:

Location with the highest water density:

4. Explain what geographic or other features might lead to these differences.

1. Your classroom air and the outside air all contain water. You’ve seen some situations where water comes out of the air. The reason water comes out of the air has to do with the dew point temperature, which we will now investigate more closely. Measure the temperature and relative humidity in your classroom. Refer to Technical Hints to connect the temperature sensor. Refer to Technical Hints to record a single measurement. Refer To the Relative Humidity Measurement activity to take the relative humidity measurement.

Classroom temperature (deg C) =

Classroom relative humidity (%) =

2. Using the water density graph, you can figure out the dew point temperature. Use the Labeling Tool to mark the point on the graph corresponding to your classroom temperature. Refer to Technical Hints to label a point on the graph.

3. This point represents the most water the air in your classroom could hold (100% relative humidity for your classroom temperature). However, your classroom’s relative humidity is not 100. It is something less than that. (If it were 100 in your classroom, it would be raining inside the room or the room would be filled with fog.) Using the Ruler Tool, mark the point on the graph that represents your actual classroom air temperature and water density. Refer to Technical Hints to use the Ruler Tool.

4. At what temperature would your classroom relative humidity be 100%? This is the dew point temperature. To figure this out, follow the horizontal line from your Ruler Tool mark, and see where it crosses the water density graph. Use the Labeling Tool to label this point. The temperature at this point is your classroom dew point temperature. Fill in this temperature below.

Classroom dew point temperature (deg C) =

5. If the temperature in your classroom dropped to the dew point temperature, what would happen?

Using the water density graph below, answer the following question. You can use the Ruler Tool to read values on the graph. See Technical Hints for using the Ruler Tool.

Which sample of air contains the most water?

• A. Air at 10 degrees Celsius with a relative humidity of 60%
• B. Air at 15 degrees Celsius with a relative humidity of 90%
• C. Air at 30 degrees Celsius with a relative humidity of 55%

Can the dew point ever be higher than the current air temperature? Explain.

• Evaporation is one way in which water goes into the air. Transpiration – plants giving off water – is another. Research the process of transpiration. How much of the water in the earth’s atmosphere is a result of transpiration? How much water does an average tree transpire into the air per day?
• Track temperatures, dew points, humidity levels, and weather patterns in your local area over several months. What weather patterns are associated with high or low humidity?
  (http://www.weather.gov/)