Activity 13 Planner Students will learn how to measure the dew point of air. Activity 13 Summary Activity Subject and Content Objective Materials Just Dew It! Dew point Determine the dew point of the air. Each group of students will need: Celsius thermometer (alcohol), shiny can with top removed (aluminum cans work well), glass stirring rod or wooden stirrer, water at room temperature, ice (crushed or cubes work best) Time Vocabulary Key Concept Margin Features 50 60 minutes Humidity, Saturation, Dew point II: Factors that contribute to weather Safety Alert!, Fast Fact, What Can I Do?, Connections, Resources Scientific Inquiry Unifying Concepts and Processes Technology Personal/Social Perspectives Historical Context Collecting, analyzing, and interpreting data; asking questions Cause and effect: mechanism; energy and matter: flows Critical thinking, problem solving, and decision making Scientific perspective of nature Changing understanding of science 136 136 National Science Teachers Association
Just Dew It! Activity Background If you watch the local news, you have probably heard meteorologists talk about the humidity as well as the temperature. In many locations, the humidity varies from day to day, with some days humid and other days dry. In other locations, it is always humid or always dry. What does this mean? The humidity represents the amount of moisture (in the form of water vapor) that is in the air. There is always some moisture in the air, even in the desert. Most of the water vapor gets into the air by evaporation, largely from the ocean, but also from lakes, rivers, ponds, and even puddles. Temperature is a major factor in determining how much and how rapidly water will evaporate from these places. The warmer it is, the more water will evaporate. Because of this, at any location, there will probably be more water vapor during warm weather than during cold weather. There is a maximum amount of water vapor that can exist in the air. At that point, it can be stated that the air is saturated with water vapor. The amount of water vapor at saturation depends on the temperature. The warmer it is, the greater the amount of water vapor that can be present. In this Activity, you will use the fact that the amount of water vapor at saturation depends on the temperature to determine how humid the air is. The method involves cooling the air. If you cool the air enough, the water vapor begins to condense and form water droplets. The temperature at which this happens is called the dew point. The less water vapor there is in the air, the more it has to Vocabulary Humidity: A measure of the amount of water vapor. Saturation: The condition where the maximum amount of water vapor is present. Dew point: Temperature at which water vapor will condense if air is cooled. Objective Determine the dew point of the air. Topic: clouds and fog Go to: www.scilinks.org Code: PSCM 008 137 Project Earth Science: Meteorology, Revised 2nd Edition
Activity 13 Materials Each group of students will need: Celsius thermometer (alcohol) shiny can with top removed (aluminum cans work well) glass stirring rod or wooden stirrer water at room temperature ice (crushed or cubes work best) be cooled to make the water vapor condense. Because of this, the dew point provides a measure of the amount of water vapor in the air. On days when the air temperature and the dew point are very close to each other, we say that the air is humid. If, for example, the air temperature is 30 C and the dew point is 25 C, there is much more water vapor in the air than on a day when the air temperature is the same, but the dew point is 5 C. We are most likely to notice water vapor on very humid, warm days, when we feel clammy or sticky because all the water vapor surrounding us prevents our perspiration from evaporating easily. Time 50 60 minutes Figure 13.1 Bromeliads are plants that have overlapping leaves that form a basin for dew to collect. The plant eventually absorbs the water that collects in the basin. SAFETY ALERT! 1. Be careful when working with thermometers. They are made of glass and may break. 2. The tops of the cans may have sharp edges. Be careful when handling them. Procedure 1. Measure the air temperature with your thermometer. Record this value in the column labeled Air Temperature ( C) on BLM 13.1. 2. Fill the shiny can half full with water at room temperature. Allow the can to sit for one minute. If condensation forms on the outside of the can, replace the water with warmer water until no condensation forms. 3. Place the thermometer in the can with the water as shown in Figure 13.2. 4. Slowly add small pieces of ice to the can while using the glass rod or wooden stirrer to slowly and gently stir the ice in the water. Watch the outside of the can closely for the first sign of condensation. 5. When condensation begins, immediately record the temperature of the water in the can in the column labeled Dew Point ( C) on BLM 13.1. 138 National Science Teachers Association
Activity 13 20 30 40 Fast Fact When winter arrives, the cool air from the outside is heated inside our homes. The heated cool air from the outside may only have a relative humidity of 5%, which is as dry as the air in the Sahara Desert. Figure 13.2 The setup uses an aluminum can and a thermometer. 6. If the temperature in the room is fairly constant, repeat steps 2 through 5 twice more. Record your data beside Trial 2 and Trial 3 on BLM 13.1. Before you begin each trial, wipe out the inside of the cans or use warm water to allow them to warm to room temperature. 7. Find the average dew point by adding the three individual dew points and dividing the sum by three. Record the result on BLM 13.1. Questions and Conclusions 1. By how many degrees would the air have to cool to reach the dew point you just determined? 2. If you did this Activity again tomorrow and found that the dew point had increased, would this indicate that there was more or less moisture (water vapor) in the air? Why? (Assume the air temperature is the same.) 3. Under what conditions would the air temperature and the dew point be the same? 4. At what time of day are you most likely to find dew? Why? 5. Where do you normally see dew? 6. How does this Activity show that dew does not fall from the sky like rain? What Can I Do? Mapping regional dew points can give you insight about moving humid air and how this air may change with the passage of fronts and other weather systems. You can get dew point maps of the United States from the Weather Underground at www.wunderground.com/ US/Region/US/Dewpoint. html. 139 Project Earth Science: Meteorology, Revised 2nd Edition
BLM 13.1: Calculating Dew Point Date Activity 13: Just Dew It! Dew Point Data Table Air Temperature ( C) Trial Dew Point ( C) 1 2 3 Average Dew Point ( C) 140 National Science Teachers Association
Teachers Guide to Activity 13 Just Dew It! What Is Happening? The amount of water vapor that can exist depends on the air temperature. For any given temperature, there is a maximum amount of water vapor that can be present before the vapor begins to condense. The maximum amount increases rapidly as temperature increases. (See Figure 13.3.) Maximum Water Vapor (vapor pressure at saturation, mm Hg) 100 90 80 70 60 50 40 30 20 10 20 10 0 10 How Do We Know This? Temperature ( C) 20 30 40 50 How do we know there is water vapor in the air? The first device for measuring humidity was invented by Cardinal Nicholas de Cusa in the 15th century. It used a piece of wool, which became heavier when it was humid due to the water that was absorbed by the wool. At the time, it was not recognized that water vapor was a distinct gas in the atmosphere. That insight did not come until Daniel Bernoulli s kinetic theory of gases in 1738 (although his theory was generally ignored until the following century). Nowadays, there are many different methods for measuring the water vapor content of the air. For example, the air can be sent through a wet cloth. The evaporation of the water into the air cools the air. The lower the humidity, the more the air is cooled. The humidity can be then be determined by comparing the temperature of the air passing through the wet cloth (called the wet bulb temperature) with the actual air temperature (called the dry bulb temperature). Figure 13.3 The maximum amount of water vapor that can exist varies with the temperature. Objective Determine the dew point of the air. Key Concepts II: Factors that contribute to weather Materials Each group of students will need: Celsius thermometer (alcohol) shiny can with top removed (aluminum cans work well) glass stirring rod or wooden stirrer water at room temperature ice (crushed or cubes work best) Time 50 60 minutes 141 Project Earth Science: Meteorology, Revised 2nd Edition
Teachers Guide 13 More water vapor can exist when the air is warm than when the air is cool. Air is saturated when the maximum amount of water vapor is present. This saturation point can be obtained in two ways. One way is to continue evaporating water so that the amount of water vapor increases. The other more common way is to cool the air. The temperature at which air is then saturated is identified as the dew point. When the dew point is very close to the air temperature, the air s high humidity becomes easily noticeable. In areas with many hot and humid days, the dew point is often given by meteorologists not only as a measure of the humidity, but also as a kind of comfort index. On hot days when the air temperature and the dew point are close, working outdoors can be very uncomfortable because the air is nearly saturated. When air is nearly saturated, evaporation is slowed dramatically. As a result, perspiration from our bodies is prevented from evaporating, and our cooling mechanism is impaired. If the air temperature and the dew point are known, relative humidity can be calculated. Relative humidity is the ratio of the amount of water vapor actually in the air to the maximum amount that could possibly be in the air at that temperature. When the dew point and the air temperature are close to each other, the relative humidity will be high. When they are the same, the relative humidity is 100%. Dew forms by essentially the same process as clouds and raindrops. When moist air is cooled enough and condensation nuclei are present, water droplets will form in the atmosphere, creating clouds. Dew is the condensation of water vapor, not around nuclei in the air but on the surfaces of objects around us. Dew typically starts to form after sunset when the air begins to cool. How dew forms was a puzzle for many years. It was solved when scientists realized that air contains water vapor, even the air in deserts. During the day, the Sun warms the ground and the air above it. At night, the ground cools by radiating its heat into the sky. On cloudy nights, this radiant heat is absorbed by the water in the atmosphere and radiated back to the ground, and no dew is formed. On clear nights, the radiant heat is lost to space and the ground cools quickly, cooling the air above it. If the temperature of the air reaches its dew point, then water vapor will condense. Measuring how much water condenses as dew is difficult to determine because the quantity is so small. In dry climates like deserts, dew probably accounts for most of the precipitation in that area. Scientists have used a block of gypsum to measure the amount of water vapor in the air by weighing the dry block before dew formation, and then again after dew has formed. The difference is the amount of water absorbed as dew. 142 National Science Teachers Association
Teachers Guide 13 Preconceptions Students sometimes have a difficult time understanding where dew comes from. The preconceptions may come from observing condensation on the outside of their cup or glass. Many students may think that the water passes through the container to the outside. Students may have the following preconceptions: Dew forms on grass every night. Dew is another form of rain or precipitation. Dew falls from clouds. If water vapor is in the air, you can see it. (Students think that water vapor is visible (like a cloud). Subsequently, they think that clouds are made of water vapor (as opposed to liquid).) The maximum amount of water vapor that can exist depends on how much air is present (i.e., it depends on how much space is available between the air molecules). SAFETY ALERT! 1. Be careful when working with thermometers. They are made of glass and may break. 2. The tops of the cans may have sharp edges. Be careful when handling them. What Students Need to Understand The maximum amount of water vapor that can be present depends on the air temperature. Dew point is a measure of the amount of water vapor in the air. For a specific temperature, a higher dew point means more water vapor is present. Relative humidity is a measure of the actual amount of water vapor in the air relative to the maximum amount of water vapor that can be present at its temperature. When the relative humidity is 100%, the air is saturated. Time Management This Activity can be completed in 50 60 minutes or less. It can be done in one class period or conducted on different days over the course of several weeks, as the humidity conditions change due to outside weather and the presence of storm systems. Preparation and Procedure Prior to beginning the Activity, ask students what they know about dew and its formation. Ask students if they have seen any plants that collect water on the inside of their overlapping leaves. Show them pictures of bromeliads. Ask students if they have ever seen a spider web wet with droplets of water that glistened in the sunlight. Record students background knowledge about dew and its formation in the K column of a K-W-L chart. (See Figure 13.4.) Be sure to accept all comments. 143 Project Earth Science: Meteorology, Revised 2nd Edition
Teachers Guide 13 Ask students what they want to learn about the formation of dew and record their questions in the W column of the K-W-L chart. Return to the K-W-L chart at the conclusion of the Activity and record in the L column what students learned about the formation of dew and dew points. Correct any misinformation in the K column and see which questions they are able to answer from the W column. Topic: What Do You Know? What Do You Want to Know? What Did You Learn? Figure 13.4 Numerous KWL graphic organizers such as this are available online by searching for KWL chart. 144 National Science Teachers Association You should check the dew point yourself, especially during winter months. Indoors, the dew point in winter can be below 0 C at times. When the dew point is below 0 C, this Activity will not work. Be sure the materials are centrally located so that each group of students can easily access them. Soup cans or fruit and vegetable cans work best. Since opened cans often are extremely sharp, you should carefully inspect each can before use and caution students to handle them carefully. Use alcohol-filled thermometers, and warn students about the dangers of breaking the glass surrounding the alcohol. Crushed or cubed ice will work for the Activity. It is somewhat easier to add small amounts of crushed ice. It is best to start with room temperature water because sometimes tap water is cold enough to cause condensation by itself. Alternative Preparation This Activity could be done as a demonstration. If so, have students observe and record the temperatures. An alternate procedure can also be followed, as shown in Figure 13.5: 1. Place crushed ice in a small aluminum can (8 oz.). 2. Place the can inside another larger can, such as a coffee can. Place this can on top of a one-inch piece of cardboard or wooden block. 3. Place a thermometer inside the larger can. 4. When dew forms, record the temperature. This temperature is the dew point.
Teachers Guide 13 Figure 13.5 This setup uses one small can of crushed ice inside a larger can. Extended Learning This Activity leads naturally into a discussion of relative humidity. Have students find the relationship between temperature, dew point, and relative humidity. For more information on this topic, see Activity 15. In fact, there are tables that allow you to determine the relative humidity of the atmosphere from the dew point and the air temperature. Try different methods of determining relative humidity and compare the results. Students could measure dew point and temperature and create their own tables. Have students determine the relative humidity from an online calculator. If students know the temperature and the dew point, the online calculator will give them the relative humidity. Go to www.hpc.ncep.noaa.gov/html/ dewrh.shtml. Have students look at relative humidity as a variable to use for weather forecasting. Go to www.weather.gov/forecasts/graphical/sectors/conus.php. If you click on the relative humidity tab, you can then click on the next tab to see the forecast for relative humidity. 145 Project Earth Science: Meteorology, Revised 2nd Edition
Teachers Guide 13 Connections The presence of water can be very harmful for many instruments and machines. Medical companies must be aware of the relative humidity of the air so that their instruments can function properly. See www.airbestpractices.com/ industries/medical/ importance-dewpointmedical-air-systems. Interdisciplinary Study The poem Snail by Langston Hughes might be used to introduce the topic of dew point. Students can be asked to investigate whether snails actually drink dew. Also in the poem, what does the phrase dew drop s mystery mean? The text for the poem can be found on page 4 at www.poetryfoundation.org/ downloads/bhm_elementary.pdf. Differentiated Learning Have students complete this activity at home. Set two pieces of black paper outside on the ground. Late in the day, when dew is likely to form, set up an umbrella so that it shades one of the black pieces of paper. At sunset, start checking the pieces of paper for any apparent signs of moisture. Continue checking every half hour. Dew may form on one but not the other but which one? If this experiment is done on different and consecutive nights, there may be some nights that no dew is formed, so doing this experiment on several nights may be worthwhile. Patterns in weather are often displayed through maps. These maps are used to show patterns over time and distance. Patterns of data, such as temperature, dew point, wind speed, and pressure, show the presence and movement of storm fronts and other weather patterns. Have students locate maps of various weather data, including water vapor. Have students create a map of dew points across the United States at the National Weather Service JetStream Online School for Weather. This activity can also be used to allow students to develop the idea of isopleths, which show equal values for data. Go to www.srh.noaa.gov/jetstream/synoptic/ll_analyze_dp.htm. 146 146 National Science Teachers Association Answers to Student Questions 1. This depends on students data. The answer is obtained by subtracting the average dew point from the measured air temperature. 2. It would indicate that there is more moisture in the air because the air would not have to be cooled as much to reach the point where it would be saturated. This means that there must be more water vapor in the air to begin with. 3. Air temperature and dew point would be the same in a cloud or fog. It is a preconception that it rains every time the air becomes saturated with water vapor. It is also a preconception that the air at ground level is saturated every time it rains. Often, the air just beneath a raining cloud is not saturated. 4. You are most likely to find dew in the early morning because the objects on which dew collects have cooled overnight while the Sun is not shining on them. These cool surfaces lower the surrounding air temperature to their dew point, just like the can in this Activity.
Teachers Guide 13 5. You normally see dew on any surface on which it is cool enough for water vapor in the air to condense (for example, grass, cars, metal railings, spider webs). 6. Dew formed on the can without any water falling from the air. It simply condensed there from water vapor in the surrounding air. Assessment At the beginning of the Activity, assess students background knowledge about dew and its formation by using a K-W-L chart. (See Figure 13.4.) During the Activity, make sure students record the temperature data properly. If students have varying temperatures on their trials, have them start over with a dry, clean can. At the end of the Activity, you can assess the answers to student questions. At the end of the Activity, you might also want to take two to three concepts from the K column of the K-W-L chart and have students determine if the statement is correct. Resources www.wunderground.com/us/ Region/US/Dewpoint.html www.hpc.ncep.noaa.gov/html/ dewrh.shtml www.weather.gov/forecasts/ graphical/sectors/conus.php www.poetryfoundation.org/ downloads/bhm_elementary. pdf www.airbestpractices.com/ industries/medical/importancedewpoint-medical-air-systems www.srh.noaa.gov/jetstream/ synoptic/ll_analyze_dp.htm 147 Project Earth Science: Meteorology, Revised 2nd Edition