NAME DATE PARTNERS WONDERING ABOUT WEATHER You are lying in the grass enjoying a few relaxing hours in the summer sun. You have your favorite cold drink and magazine close at hand. You close your eyes and fall asleep for a while. When you wake up there are more clouds in the sky, the ice in your drink has melted, and your glass is coated with water but it hasn t rained. Your magazine is too hot to touch. What is going on? Are these events related? How are they associated with weather? We have been studying processes that form and modify the solid Earth and understand that we have a dynamic planet under our feet. Surrounding our planet is a very thin blanket of air, the atmosphere that stretches to about 400 kilometers (250 miles) above the solid Earth. Like the solid Earth, this is also a very dynamic region. Most of the mass of the atmosphere resides in the area from the surface to about 50 km above the surface. Our weather occurs in the densest section closest to the Earth called the troposphere (0-10km). How would you define weather? Weather is often defined as the state of our atmosphere at a given time and place. It is usually described with variables such as temperature, cloudiness, pressure, precipitation, and radiation. Weather is complex but it is based on simple science concepts. To investigate weather we need to understand these. LET S EXPLORE... Activity 1: Does Air Have Mass? Does air have mass? Explain your response. Explorations in Earth and Space Science PSC 1210 Prince George s Community College 145
1. Obtain the following materials: 2 balloons of the same size scissors 1 meter stick tape string, about 1 meter 2. Hang the meter stick from the edge of your student lab bench from a section of string about 30 cm long. Be sure the meter stick is level. Secure with tape on the table top. 3. Loosely attach a 30 cm section of string to each of the empty balloons and suspend the balloons from each end of the hanging meter stick using the same amount of tape. Is the meter stick still balanced? Why or why not? If the meter stick is not balanced adjust it so that it is. Tape all the strings in place so they do not move. It is important the strings do not change position once everything is balanced. 4. Remove and untie one of the balloons. Inflate the balloon and reattach it to the meter stick carefully. What happen? What does this tell you about the air that you put in the balloon? Activity 2: Powerful Air To investigate another property of air, let s try two small experiments. 1. Obtain a sheet of paper and a 30 cm (1 foot) ruler. Set the ruler on the table and place the sheet of paper over the ruler as shown in the diagram here. 2. If you press down QUICKLY with medium force on the exposed end of the ruler, what do you expect to happen to the paper? Try it! 146 Explorations in Earth and Space Science PSC 1210 Prince George s Community College
What happened? Why? 3. Ask the instructor to do a demonstration with the glass of water and the index card. What keeps the card from falling off under the influence of gravity? Even though air particles are tiny and there is a lot of space between the particles, they do exert a pressure. Pressure is the force per unit area that an object applies. Pressure exerted by our atmosphere is called atmospheric pressure or barometric pressure. Because the air particles are moving fast they make collisions that generate force. Because we live close to the Earth s surface, there is a large column of air above us. Do you think the atmospheric pressure would be greater at sea level or on the top of Mount Everest? Why? Activity 3: Sweating It Out 1. Obtain a 250 ml beaker and fill it halfway with water. Add several ice cubes to the beaker of water and allow the beaker to sit for 10 minutes. 2. Observe the outside of the beaker and record your observations here. a. Where is the water on the outside of the beaker coming from? b. How could you prove that the water on the beaker did not come from inside the beaker? c. Would you have had the same results if you used hot water rather than cold water in the beaker? Explain why or why not. Explorations in Earth and Space Science PSC 1210 Prince George s Community College 147
What you observed is the process of condensation or the action of condensing. As water particles in the gas state come in contact with the glass, they transfer their energy to the colder glass, slow down in the process, are attracted to other slower water molecules, and form a liquid. When air reaches its dew point it has reached a temperature where the air is saturated with water vapor and condensation will occur easily. d. Would you have gotten the same results if you had done this activity on a very dry day? Explain. What would happen on a very humid day? Why? e. What do you think happens to the temperature of water in the beaker (or in your glass as you bask in the sun) as water continues to condense on the outside? Why? Activity4: Now You See It, Now You Don t Where did the water come from that condensed on the outside of the beaker in the prior activity? 1. Obtain an evaporating dish and add 5 ml of colored water to the dish. 2. Place the dish and liquid under a light source set at a height designated by the instructor. Turn on the light and allow the dish to remain for about 30 minutes (or until the instructor says to come and observe). 3. Observe and record what the contents of the dish are after the time has passed. 4. a. Why did the water level change? b. Did any of the food coloring escape? Why or why not? 148 Explorations in Earth and Space Science PSC 1210 Prince George s Community College
c. What would happen to the water that escaped if it suddenly met a cold surface? Clouds form when evaporated water meets a cold air mass or when it cools off by expanding when rising to higher, colder altitudes. Water droplets form on dust particles in the air and gradually increase in size. If the condensed water droplets in the clouds become too large (heavy), they fall back to Earth in the form of rain or snow, referred to as precipitation. The evaporation-condensation processes occur cyclically in a process we call the hydrologic cycle or water cycle. Hydrologic or Water Cycle What is the heat source that provides energy for this cycle? Explorations in Earth and Space Science PSC 1210 Prince George s Community College 149
Activity 5: Relative Humidity Maybe, while you were lying out in the grass under the sun, it began to get humid. What do you mean when you say it is humid? We have already seen that water evaporates and condenses in a cycle. The warm air (with some vaporized water) is less dense so it rises from the Earth s surface higher into the atmosphere. However, while it is still near us it influences the way we feel. At any given temperature, air can hold a certain amount of water as vapor. If this amount is exceeded, the extra amount is released as dew or fog. Usually air holds less than the maximum amount. Relative humidity is the percent moisture currently in the air compared to the maximum that the air can hold at the same temperature. When the relative humidity is high, the air is holding a relatively large amount of water vapor. Do you think the amount of water air can hold changes with temperature? Explain why or why not. We can measure relative humidity by looking at how easily water evaporates and how the temperature at the evaporation site changes. As water evaporates, what happens to the temperature of the material left behind? Why? (Hint: what happens to how you feel when you are wet and stand in a breeze?) Relative humidity is calculated using the difference in readings from a dry thermometer bulb and a wet thermometer bulb (where evaporation is taking place). The apparatus used to make the measurement is called a psychrometer or a hygrometer. 1. Locate the psychrometer. The psychrometer is an apparatus that has two thermometers enclased in a plastic fram. One thermometer has a piece of gauze called wicking over the bulb. This is the wet bulb thermometer. The thermometer with no covering serves as the dry bulb thermometer. 2. 3. Place drops of water on the wicking until it is wet being careful not to get any water on the dry bulb thermometer. 4. Holding the psychrometer by the handle (section without the thermometers) away from your body, swing the attached thermometers in the air for at least 2 minutes. 150 Explorations in Earth and Space Science PSC 1210 Prince George s Community College
5. Record the wet and dry bulb temperatures. Repeat the procedure three more times making sure the bulb is wet each time. Calculate an average temperature for the four trials for each thermometer. Reading Wet Bulb Temperature ( o C) Dry Bulb Temperature ( o C) 1 2 3 4 Average 6. Use the psychometric chart on the psychrometer to determine the relative humidity in the classroom during this activity. Average Relative Humidity 7. Does the relative humidity make sense based on the weather conditions for today? Explain why or why not. Explorations in Earth and Space Science PSC 1210 Prince George s Community College 151
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