Materials Included In Kit

Additional Materials Required

Prelab Preparation

1. Cut the Precipitation Gauge Master Sheets. Each group receives one Precipitation Gauge Ruler.
2. Cut the Barometric Measurement Card Master Sheet. Each group receives one Barometric Measurement Card.
3. Cut the cotton wick into ½-inch pieces for the students to use with the sling psychrometers.

Safety Precautions

Latex (in balloons) may be an allergen for some individuals. Be sure the thermometers of the sling psychrometers are securely attached to the plastic handle before allowing students to swing them. Have students exercise caution not to break the thermometers. Students should wear protective eyewear. Follow all normal classroom and laboratory guidelines. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

All materials may be saved for future use.

Lab Hints

• Enough materials have been included in this kit for 30 students working in pairs or 15 groups of students. One 50-minute class period is ample time to prepare setups for each DIY instrument.
• Students may use any clear glass container as the collection device for Part 1. Precipitation. Baby food jars or other straight-walled jars work best. Clear packing tape will create a better seal on the rain gauge and ruler.
• This kit includes three sling psychrometers which may be shared among student groups. Extra sling psychrometers are available from Flinn Scientific, Catalog No. FB0543 Sling Psychorometer Kit (single) and FB1582 Sling Psychrometer Classroom Set.
• One foot of cotton wick is included for each Sling Psychrometer. Cut the cotton wick into ½" pieces for use. Extra wick is included as a surplus supply.
• Relative humidity and dew point values are given for temperature ranges between 32 and 94 degrees Fahrenheit. Plan the Relative Humidity and Dew Point portion of this lab accordingly.
• Remind students that the barometer apparatus should not be moved after the first data point is recorded.
• The student-made anemometers will only give a good approximation of wind velocity. More accurate anemometers are available from Flinn Scientific, Catalog No. FB0507 and FB0508.
• The straws used in the student-made anemometers may be stapled instead of glued to the cups, if desired.
• The student-made anemometers may be held by hand or may be pushed into the ground for support.

Teacher Tips

• Determine the length of time desired for student data collection. Data tables include space for 15 days.
• Compare calculated values for relative humidity and dew point to values given by a local weather station or the National Weather Service.

Answers to Prelab Questions

1. The map to the right shows high and low pressure areas over Canada.
   1. Which area would experience strong winds and unfavorable weather conditions?

      The area of low pressure (L) would have more wind than the high pressure (H) area.

   2. Defend your answer.

      Wind results from differences in the air pressure. Air flows from high to low pressure areas creating a pressure gradient. A steep pressure gradient lends itself to a high amount of wind. Isobars that are close together, as seen in the low pressure area, indicate a steep gradient.

2. Describe how fronts cause precipitation.

   Fronts create precipitation when moisture in the area the front is passing through is plentiful. Air on one side of the front moves in a different direction than air on the other side of the front and air will accumulate along the front’s surface. Eventually the air has nowhere to go and the warm air will rise over the cold air. The moisture in the warm air will cool, condense and form clouds and precipitation.

3. What is the relationship between relative humidity and the dew point and air temperatures?

   Comparing the air temperature and dew point can give an indication of the relative humidity. If the dew point and air temperature are close together, relative humidity is high. If the dew point and air temperature are far apart, relative humidity is low. If the dew point is equal to the air temperature, relative humidity is 100%.

Sample Data

Precipitation Data Table

Relative Humidity and Dew Point Data Table*

*Data taken indoors.

Barometric Pressure Data Table

Answers to Questions

1. When collecting precipitation, one inch of rain is equal, on average, to 12 inches of snow.
   1. Calculate the total amount of precipitation that you collected.

      Student answers will vary. Snowfall should be converted into rain and total all precipitation.

   2. During a blizzard that lasted for 36 hours from January 22nd to January 24th, 2016, Glengary, West Virginia, received 42 inches of snowfall. Calculate the total precipitation as rain.
      {11836_Answers_Equation_1}
   3. In August of 2013, Nashville, Tennessee, experienced a historic rainfall of 7.8 inches in a 6-hour time period. Compared to the blizzard of Glengary, West Virginia, which city received more liquid precipitation per hour?

      Glengary, W.V. = 3.5" rain / 36 hr = 0.0972" per hr
      Nashville = 7.8" rain / 6 hr = 1.3" per hr

2. First, predict and then test the results of the following challenges using the sling psychrometer.
   1. Which area in the school will give the highest relative humidity value?

      Student answers will vary. Areas within the school that might include high relative humidity values would include the gym, locker rooms, cafeteria or green house.

   2. Which area in the school will give the lowest relative humidity value?

      Student answers will vary. Areas such as the school office, teacher lounge and areas where water vapor is removed by air conditioning tend to have lower relative humidity values.

3. A student collected data for the dew point, which was 67 °F and air temperature, which was 68 °F. Based on this information, explain whether the relative humidity is considered high or low.

   The dew point and air temperature are close in value, which would mean there is a lot of moisture in the air, so the relative humidity would be high.

4. Using a sling psychrometer, a student gathered the following data:

   dry bulb temperature = 60 °F wet bulb temperature = 55 °F air temperature = 60 °F

   1. Determine the relative humidity.

      RH = 73%

   2. Determine the dew point.

      Dew point = 51 °F

5. The National Weather Service gave a local barometric pressure reading for Batavia, Illinois of 30.27 inHg at 11:53 a.m. on January 27, 2016. When students checked their classroom barometer at 2 p.m., they recorded a drop of 2 mm.
   1. What is the barometric pressure for 2 p.m.?

      1mm movement = 0.03937 inHg
      0.03937 inHg x 2 = 0.07874 inHg
      30.27 inHg – 0.07874 inHg = 30.19126 inHg

   2. A drop in barometric pressure indicates what type of weather is approaching?

      A drop in barometric pressure indicates stormy, cloudy weather is approaching.

6. Looking at your data, what is the correlation between barometric pressure and weather conditions?

   When the barometric pressure is higher/rising, nice weather (sunny, calm) is present and when pressure is dropping the weather usually becomes stormy or cloudy.

7. Calculate the wind speed using an anemometer with a circumference of 1.25 feet that turned 213 revolutions in one minute.

   D = 1.25 ft
   C = π x 1.25 = 3.93

   {11836_Answers_Equation_4}
8. From your data, choose a day with the highest wind speed and a day with the lowest wind speed. Discuss the connection between wind speed and barometric pressure (Hint: use the data from the same day on your Barometric Pressure Data Table).

   Generally, days with higher wind speeds will have lower barometric pressure than days with lower wind speeds.

Introduction

Weather is all around us! Introduce your students to some simple tools used to measure atmospheric conditions.

Concepts

• Wind speed
• Dew point
• Relative humidity
• Barometric pressure

Background

Weather is used to define the atmospheric conditions at a particular place and time. The four main factors that determine weather—pressure, air movement, temperature and moisture—are constantly changing.

Of the major factors contributing to weather, pressure is probably the least noticeable. The air molecules in the atmosphere exert a force on the Earth’s surface. At sea level, the air surrounding the Earth exerts a pressure of 14.7 pounds per square inch. Variations in pressure are responsible for the movement of air and are a significant indicator of future weather patterns. An increase in altitude relates to a decrease in overall pressure. Cold air exerts more force on the surface of the Earth than warm air. Therefore, cold air is associated with high pressure and warm air is said to have a low pressure. Atmospheric pressure is commonly measured in inches of mercury (inHg) and millibars (mb) using a barometer. At sea level, 29.92 inches of mercury is equal to 1013.35 millibars (also equal to 1 atmosphere).

Wind is the result of horizontal differences in air pressure. Air naturally flows from areas of higher pressure to those of lower pressure. Wind is nature’s way of trying to balance the inequality of pressure between these areas of high and low pressure. The pressure gradient is the main force behind wind and has both a magnitude and a direction. A pressure gradient is the amount of pressure change occurring over a given distance. Pressure gradients are commonly shown on a weather map as isobars—lines that connect places of equal pressure (see Figure 1). The closer together isobars are on a map the steeper the pressure gradient. A steep pressure gradient results in a high amount of wind. Widely spaced isobars indicate a weak pressure gradient and a low amount of wind.

Temperature is the degree of “hotness” or “coldness” measured against a scale by means of a thermometer. A German physicist, Gabriel Fahrenheit (1686–1736) created the Fahrenheit scale in 1724 and 18 years later, Anders Celsius (1701–1744), a Swedish astronomer created the Celsius scale. Temperature affects weather in the form of fronts. Fronts are boundary or transition areas between two air masses of different temperatures and moisture levels. A front is typically defined as either a warm front or a cold front. A tropical air mass or warm front defines a region of higher temperatures and a polar air mass is a cold front composed of cooler temperatures. Cold fronts and warm fronts are defined on a weather map using the symbols found in Figure 2. Fronts often create precipitation. When a front passes through an area with enough moisture the chance of precipitation increases greatly. Air on one side of the front blows in a different direction than air on the other side of the front. This causes the air to “accumulate” along the surface of the front. The air eventually has nowhere to go and the warm air will rise over the cold air. As the air rises, moisture in the air will cool, condense and form clouds and precipitation (see Figure 3). Water vapor is the gaseous, invisible form of water in the atmosphere. It is better known as humidity. When the air in the atmosphere contains a large amount of water, the air feels very humid. The opposite is true when the air is relatively void of water vapor—the air feels very dry. When air holds the maximum amount of moisture small droplets will begin to form as clouds and dew or frost will be present. Clouds become saturated with water droplets and eventually become too dense to hold all of the droplets. At this point the droplets will start to fall towards the Earth’s surface in the form of rain or snow. This is known as 100% humidity.

Relative humidity is an indication of the amount of moisture in the air and is expressed as a percentage. When water in the air evaporates, a certain amount of heat is required to convert the air into water vapor. Therefore, a cooling effect takes place when evaporation occurs.

Dew point is defined as the temperature at which air must be cooled (at constant pressure and water vapor content) for saturation (dew formation) to occur. When the dew point is below freezing, (32 °F), it is commonly referred to as the frost point. The dew point is an important measurement used to predict the formation of dew, frost, and fog. Since atmospheric pressure varies only slightly at the Earth’s surface, the dew point is a good indicator of the air’s water vapor content. High dew points indicate high water vapor and low dew points indicate low water vapor content.

The difference between air temperature and dew point temperature indicates whether the relative humidity is low or high. When the air temperature and dew point are dramatically different, the relative humidity is low. When the air temperature and dew point are close to the same value, the relative humidity is high. When the air temperature and dew point are equal, the relative humidity is 100% (see the Dew Point Calculation Chart PDF).

Experiment Overview

The purpose of this activity is to collect atmospheric data using simple, DIY (do-it-yourself) instruments that facilitate and develop further understanding of the weather around you!

Materials

Prelab Questions

1. The following map shows high and low pressure areas over Canada.
   {11836_PreLab_Figure_1}
   1. Which area would experience strong winds and unfavorable weather conditions?
   2. Defend your answer.
2. Describe how fronts cause precipitation.
3. What is the relationship between relative humidity and the dew point and air temperatures?

Safety Precautions

Latex (in balloons) may be an allergen for some individuals. Be sure the thermometers of the sling psychrometer are securely attached to the plastic handle before swinging. Be careful not to drop or break the thermometers. Wear protective eyewear. Follow all normal classroom and laboratory guidelines.

Procedure

Part 1. Precipitation

1. Obtain a Precipitation Gauge Ruler and a clear glass jar. Any clear glass jar may be used as the collection vessel.
2. Using transparent tape, attach the precipitation gauge ruler to the side of the glass jar. The 0" mark of the ruler should be flush with the bottom of the glass jar. The Precipitation Gauge Ruler should be completely covered by tape to ensure it remains dry.
3. The precipitation gauge may be taken home. Record the amount of precipitation over a given period of time specified by the instructor in the Precipitation Data Table.
4. Record the type of precipitation—rain or snow—in the Precipitation Data Table.

Part 2. Relative Humidity and Dew Point

A sling psychrometer can be used to measure the relative humidity of the air. It consists of two thermometers: a dry-bulb and a wet-bulb. The dry-bulb thermometer measures the temperature of the surrounding air while the wet-bulb thermometer records the amount of cooling that is required for the water to evaporate at that specific temperature. If the air is very humid, the differences between the dry-bulb and wet-bulb thermometers will not be large because there is little evaporation. However, if the air is arid or dry, a large amount of evaporation takes place (which causes a cooling effect on the wet- bulb thermometer) and the temperature difference between the two thermometers will be greater.

Assembling a Sling Psychrometer

1. Construct a wet-bulb thermometer by slipping a small piece of cotton wick over the bulb of one of the thermometers. The other thermometer is the dry-bulb thermometer.
2. Attach the two plastic-backed thermometers together back-to-back using a small rubber band (see Figure 4).
   {11836_Procedure_Figure_4_Sling psychrometer}
3. Slide both of the thermometers onto the screw through the hole used to hang the thermometers.
4. Twist the screw carefully into the predrilled hole of the plastic handle until 3–4 mm of the screw’s shaft remains above the handle.
5. Place the rubber cap on the bottom of the psychrometer handle.

Activity

1. Measure the air temperature using the dry-bulb thermometer. Record the temperature in °F in the Dew Point and Relative Humidity Data Table.
2. Using a pipet, place a few drops of water on the cotton wick of the wet-bulb thermometer.
3. Hold the plastic handle in your hand and slowly rotate the thermometers around the screw. The spinning motion will increase the evaporation rate of the water.
4. Spin the thermometers on the sling psychrometer for thirty seconds or until the temperature reading on the wet-bulb thermometer remains constant.
5. Immediately record the temperature (in °F) of both thermometers in the Dew Point and Relative Humidity Data Table. Determine the difference between the dry-bulb and wet-bulb thermometers. Record this value on the Dew Point and Relative Humidity Data Table.
6. Pass the sling psychrometer to the next group.
7. Use the Relative Humidity Table to determine the relative humidity of the air. Record the date, time, and relative humidity value in the Dew Point and Relative Humidity Data Table.
8. Use the relative humidity value and the Dew Point Calculation Chart to determine the dew point. Record the date, time and dew point value in the Dew Point and Relative Humidity Data Table.
9. Record relative humidity and dew point in the Dew Point and Relative Humidity Data Table over the next several weeks as desired or as determined by the instructor.

Part 3. Barometer

Assembly

1. Obtain a balloon, scissors, stirring stick, toothpick, large rubber band, wide-mouthed jar and white glue.
2. Cut the mouth of the balloon as shown in Figure 5.
   {11836_Procedure_Figure_5}
3. Cut a slit in the side of the balloon as shown in Figure 5.
4. Stretch the trimmed latex balloon sheet across the mouth of the wide-mouthed jar. The trimmed balloon must be tightly stretched.
5. Place a doubled rubber band over the edge of the jar to hold the balloon securely around the mouth of the jar (see Figure 6).
   {11836_Procedure_Figure_6}
6. Break a toothpick in half.
7. Apply white glue to one end of the stirring stick.
8. Slide the broken end of the toothpick into the pre-glued end of the stirring stick.
9. Place a drop of white glue on the other end of the stirring stick.
10. Glue the stirring stick to the latex balloon as shown in Figure 7.
    {11836_Procedure_Figure_7_Bird’s-eye view}
11. Hold the stick until the glue has set.

Activity

1. Place the assembled barometer in a place where it will not be disturbed and is out of direct sunlight.
2. Obtain a Barometric Measurement Card and an index card. Fold the index card lengthwise. Glue the Barometric Measurement Card to the center of the index card. Stand the card on end and place it near the point of the toothpick (see Figure 6).
3. Once the barometer is placed near the Barometric Measurement Card make sure neither the barometer nor card are moved or disturbed.
4. Record the current date and time in the Barometric Pressure Data Table.
5. Record the height of the toothpick (in mm) on the Barometric Measurement Card in the data table. This value will be the baseline value.
6. Record the reported barometric pressure for your area in the data table using values obtained from your instructor from the Weather Channel, www.weather.com, or a local weather station. All barometric pressure readings should be rounded to the hundredth place. The pressure is usually reported in inches of mercury.
7. In the following day or two, record the amount of movement (in mm) of the toothpick of the barometer in the data table.
8. Convert the movement in mm values to inches of mercury (inHg). Note: 1 mm of movement equals a change of 0.03937 inches of Hg. Consult the instructor if conversion help is needed.
9. Record the reported barometric pressure and the forecasted weather conditions for the current day in the data table.
10. Record data over the next few weeks as desired.
11. Compare the change in barometric pressure and the current weather conditions after the Barometric Pressure Data Table has been completed.

Part 4. Wind Measurements

1. Obtain four small cups. Use a paper punch to punch one hole in each cup roughly a half inch below each rim.
2. Push a straw through the hole of one of the cups. Hot glue the end of the straw to the inside wall of the cup. Also glue the straw to the side of the cup where the hole was punched (see Figure 8).
   {11836_Procedure_Figure_8}
3. Place and hot glue another cup at the opposite end of the straw used in step 2. Be sure that the mouths of the cups are facing in opposite directions (see Figure 9).
   {11836_Procedure_Figure_9}
4. Repeat steps 2 and 3 with two more cups and another straw.
5. Glue the two straw-and-cup assemblies together at their midpoints (see Figure 10).
   {11836_Procedure_Figure_10}
6. Let the glue dry and punch a hole through the midpoint of the two straws using a thin nail or a pushpin.
7. Partially straighten out a paper clip and place it through the hole where the straws intersect.
8. Push the end of the clip into the eraser of a pencil (see Figure 11).
   {11836_Procedure_Figure_11}
9. Color the back of one of the cups with a marker. This cup will be used to count the number of revolutions per minute of the anemometer.
10. Record the current date and time in the Wind Measurements Data Table.
11. Count the number of times the colored cup spins around in one minute. This value will be the number of revolutions per minute. Record the number of revolutions per minute in the data table.
12. Measure the diameter (D), of the circle produced by the spinning cups in feet. Multiply the diameter by π (3.14) to calculate the circumference (C) of the circle. Record the circumference in the data table. Note: Use the end of the cup to to determine the circumference.
13. Multiply the revolutions per minute by the circumference of the circle to determine the velocity of the wind in feet per minute. This value can than be converted to miles per hour. See the sample calculation below. Record the miles per hour value in the data table.

    Sample Calculation

    D = 0.75 ft
    C = π x 0.75 ft = 2.4 ft

    {11836_Procedure_Equation_1}
14. Record data in the Wind Measurements Data Table over the next several weeks as desired or as assigned by your instructor.

Student Worksheet PDF

11836_Student.pdf