Name: Period:

Lab Exercise: Dew Point and Relative Humidity Purpose: To determine the dew point and relative humidity in the classroom, and find the current relative humidity outside. Background: Relative humidity is a measure of how much water vapor is in the air compared with the total amount of water vapor the air is capable of holding at a given temperature. If the relative humidity is 50% at room temperature then the air in the room is at half holding capacity.

Dew point is the temperature in which the air needs to cool for the current water vapor in the air to condense into liquid, thus forming dew, collecting on surfaces, and forming clouds. When the dew point temperature is below the freezing point of water the term Frost Point is used.

Part I: Calculating Relative Humidity from Weather Data

Exercise I: Using Sling Psychrometer Data In this part of the lab, we will find the dew point by using sling psychrometer. Below is a set of reading from a sling psychrometer. Using the dry bulb and wet bulb temperatures and Relative Humidity Table at the end of this lab exercise, find the relative humidity. First find the difference between the dry bulb and the wet bulb by subtracting the wet bulb temperature from the dry bulb temperature. This number is the wet bulb depression. Find the air temperature on the left side of the table and read across the row until you are under your calculated value for the wet bulb depression. (Example: For an air temperature (dry bulb) of 20° C and a wet bulb value of 16° C. Yields a value of 66% for the relative humidity.)

Dry bulb temp 40.0°C 15.0°C 30.0°C -10.0°C -5.0°C

Wet bulb temp.: 20.0°C 14.0°C 26.0°C -11.0°C -8.0°C

Wet Bulb Depression

Rel. humidity:

Exercise II: Finding the Dew Point from weather data. Find the dew points for the following set of data using the same procedure in Exercise I. Use Dew Point Table for find your dew points.

Dry bulb temp.: 40.0°C 30.0° C 38.0° C 30.0° C 0° C

Wet bulb temp.: 30.0° C 24.0° C 31.0°C 22.0° C -2.0° C

Wet Bulb Depression

Dew Point (in C°):

Exercise III: Finding the dew point in the classroom. Using the sling psychrometer, measure the wet bulb and dry bulb temperature. First wet the wick on your sling psychrometer with water and begin whirling. Do this for 30 seconds. Record the wet bulb and dry bulb temperature. Find wet bulb depression. Using a Dew Point Table, estimate the dew point in the classroom. It may be necessary to round the wet bulb and dry bulb temperature to the closest value on the table to find the dew point.

Record your data. Indoor Data

Wet Bulb Dry Bulb Avg. Dry Bulb Temp: ____________________

1st 30 Seconds

2nd 30 Seconds Avg. Wet Bulb Temp: __________________

3rd 30 Seconds Wet Bulb Depression: __________________

4th 30 Seconds

5th 30 Seconds Dew Point Temperature: __________________

Name: Period:

Exercise IV: Finding Indoor and Outdoor Relative Humidity Using the sling psychrometer, measure the wet bulb and dry bulb temperatures. From these two values, determine the relative humidity. First, wet the wick on your sling psychrometer with the water and begin whirling. Do this for 30 seconds. Record the wet and dry bulb temperatures. Repeat at 30-second intervals until you have five separate readings. You may need to moisten the wick after each reading. Record temperature readings in the Data table below. Repeat the same test outdoors. Outdoor Data

Wet Bulb Dry Bulb Avg. Dry Bulb Temp: ____________________

1st 30 Seconds

2nd 30 Seconds

Avg. Wet Bulb Temp: __________________

3rd 30 Seconds

Wet Bulb Depression: __________________ 4th 30 Seconds

5th 30 Seconds

Dew Point Temperature: __________________

Questions 1. Why does a beverage can “sweat” on a hot humid summer day? 2. Why do eye glasses fog over when entering a warm building on a cold day? 3. Why do bathroom mirrors fog while you are taking a shower? 4. Why do you see a cloud form from a clothes dryer exhaust vent? 5. Explain why one may see fog over a lake during the morning, whereas the highway nearby is clear.