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Student Resource Sheet IHA - 11 UNDER PRESSURE “I wonder if Stormin’ Stan has any more information for me?” you think to yourself. After wrestling with your brother for control of the computer, you quickly log on and, sure enough, Stormin’ Stan has sent you an email. From: [email protected] To: [email protected] Subject: Ready for More? Morning my little TV stars! I hope you finished researching the atmosphere, because times a wastin’! Here’s your next topic – air pressure! Sure you’ve felt it before – your ears popping while driving up and over mountains, or during take-off and landing in an airplane. Boy, my ears feel it every time! I’m sure you’re wondering what my ears have got to do with weather. Well, nothing really. But, air pressure has quite a bit to do with weather. I’ve sent along a really cool demonstration and an article from my files on pressure that might help you. So, get your head out of the clouds, and get some more work done! And as always, this is Stormin’ Stan the Weather Man asking, “Do you have what it takes to be a star? “Great!” you think to yourself. “I think I’ll look at the demonstration first, and then read the article.” In this investigation, you will examine the factors that affect air pressure in order to understand its affect on weather. Objective When you have completed this investigation, you should be able to 1. identify the factors that affect air pressure in order to explain how air pressure will affect weather. Day One Activity 1 1. Observe the materials set-up by the teacher. 2. Sketch the set-up. Be sure to label the different materials used in the demonstration. 3. Predict what caused the changes you observed during the teacher demonstration. __________________________________________________________________________ __________________________________________________________________________ IHA - 101 Student Resource Sheet IHA - 11 Activity 2: Finding Out More That demonstration was pretty interesting, but you don’t really understand why it happened. So, you decide you might as well read the article Stormin’ Stan has sent. 4. Read the selection, “Stop Pressuring Me!” in order to gather information about the factors that affect pressure and how this relates to weather. 5. Reread the selection and complete the organizer, “Air Pressure,” to take notes about the factors that affect air pressure. Air Pressure Definition: ___________________________________________________________ ____________________________________________________________________ Measured with: ________________________________________________________ Factors that Affect Air Pressure How? Why? How? Why? IHA - 102 How? Why? Student Resource Sheet IHA - 11 CLASS DISCUSSION: Complete the graphic organizer as your class discusses the information you gathered on the factors that affect air pressure. Make sure to include arrows to explain the direction that the air will move. Warm air _______________ 6. Cool air _______________ ______________ pressure ______________ pressure ______________ dense ______________ dense Review the prediction you made in Activity 1. Determine the accuracy of your prediction. In your response, be sure to identify and describe the factor(s) that caused the can to be crushed during the teacher demonstration. Use information from the investigation to support your response. _________________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ ________________________________________________________________ Day Two Activity 3 7. Review the information you gathered on Day One. Complete Chart 1, “Factors Affecting Air Pressure,” by adding arrows to show how pressure will change based on a change in each factor. An up arrow indicates increasing, and a down arrow indicates decreasing. Factor Chart 1 Factors Affecting Air Pressure Increase or Decrease? How will air pressure change? Temperature Water Vapor Elevation IHA - 103 Student Resource Sheet IHA - 11 Activity 4 “Hmmm…,” you think to yourself. “So, these factors affect air pressure; but, what does that have to do with weather? Why didn’t he send me information about that?” WWW: Stan? Are you there? SS: Yeah, I’m here. What’s up now? Don’t you have enough work to do? WWW: Well, I can’t finish the research you told me to do in that email you sent. I downloaded the article you told me to read, and got a lot of good information about the factors that affect air pressure. But, it didn’t say anything about how air pressure relates to weather? SS: Oops! Sorry about that! I’ve been so busy planning several trips, and I can’t keep everything straight. I’ll send the other article over right away. WWW: Thanks. 8. Read the article, “There’s a lot of Pressure in the Weather,” to gather information on the relationship between pressure and weather. 9. Reread the article and take notes on the type of weather associated with high and low pressure. IHA - 104 Student Resource Sheet IHA - 11 10. Observe the weather map outlining the weather across the continental United States. Bismarck San Francisco Springfield Charleston Houston Monterey 11. Complete Chart 2, “Weather Forecasts,” to identify the type of weather that would be expected in each of the cities listed. Support your response in the space provided. Chart 2 Weather Forecasts City Type of Weather Justification Bismarck, ND San Francisco, CA CLASS DISCUSSION: Explain how characteristics such as elevation, proximity to water, and distance from the equator may affect the type of pressure and weather expected in cities such as: Charleston, South Carolina Springfield, Missouri Monterey, Mexico IHA - 105 Student Resource Sheet IHA - 11 Analysis Use the information from the weather report for Houston, Texas, to complete question 1. “This is Whirlin’ Wally with a quick update on your Houston weather. Wheww… it sure is hot out there, ya’ll! Temperatures are reaching into the high 90’s, and there is some serious moisture in the air. Better keep one eye out behind ya’ for a thunderstorm. They could pop up anytime with weather like this! Back to you Charlene. 1. Identify and describe the type of pressure most likely responsible for the weather described in the weather report. In your response be sure to include the following: A statement supporting the type of pressure responsible for the weather Factors that may affect the type of pressure in the area. __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ __________________________________________________________________________ Forecaster’s Forum: Collect some weather maps for about a week from the city you are researching. Place them in order by date. Identify any patterns you notice with the pressure systems and weather indicated on each map. IHA - 106 Student Resource Sheet IHA - 11 Press down on the palm of your hand. What you feel is a force pressing against your hand called pressure. As you sit here right now, there is a tremendous amount of pressure pushing against you. No, not pressure from your teacher to get your work done. It’s actually the pressure of the air that surrounds you. The weight of the atmosphere exerts a force on all surfaces. Air pressure is a measure of the force of the air pressing down on Earth’s surface. Air pressure can change from day to day and location to location. A denser substance has more mass per unit volume than a less dense one. So, a denser substance exerts more pressure than a less dense substance. The same is true for air – more dense air exerts more pressure than less dense air. Okay, so that makes sense, but how do scientists measure pressure? With a device called a barometer! A barometer works by registering slight changes in the pressure of the air pushing against it. When the air pressure increases, it exerts more force on the barometer causing the meter on the device to increase. The opposite is true when the air pressure decreases. Weather reports use several different units for air pressure. Most weather reports for the general public use “inches of mercury” when reporting air pressure. For example, the local meteorologist on television may report the air pressure as “30 inches.” What he/she means is that the barometer measured 30 inches of mercury. The National Weather Service indicates air pressure in “millibars.” One inch of mercury is approximately 33.87 millibars. So, 30 inches of mercury is equal to approximately 1,016 millibars. Now you know how air exerts pressure and how scientists measure pressure. You’re probably wondering what all this has to do with weather. Well, there’s one more thing you need to know before we can get there – the factors that affect pressure. As you have already learned, less dense air exerts less pressure than more dense air. But, what causes the density of air to change? Three things: temperature, water vapor, and elevation. Let’s start with temperature. When a substance is heated, the molecules begin to move more rapidly and spread further apart. When this happens, the substance becomes less dense. In this way, locations with high temperatures usually register lower pressure than areas with cooler temperatures. Next, water vapor – at a given pressure moist air is less dense than dry air. This is because a water molecule has less mass than either a nitrogen or an oxygen molecule. If there is a lot of water vapor in the air, the air tends to be less dense. Therefore, the more water vapor in the air, the lower the air pressure in the area. The third factor is elevation, or altitude. As elevation increases, the air particles begin to spread further apart. In this way, the air becomes thinner, and less dense. And, just like we’ve said over and over, less dense air exerts less pressure. Adapted from Prentice Hall Science: Exploring Earth’s Weather IHA - 107 Student Resource Sheet IHA - 11 If you look at a weather map, you will see areas marked with an L, and others with an H. These letters tell you the type of air pressure in those areas. The L is short for “low,” and indicates an area of relatively low air pressure. Likewise, the H stands for “high,” and indicates an area where the pressure is relatively high. Sure, that’s simple, but what does that mean in terms of the weather? Let’s take a look. As large masses of air move, they will frequently come together in the upper atmosphere. When this happens, the air pressure may become relatively high. These air masses press down on the layers of air below. This pressure usually prevents warm, moist air from rising into the atmosphere. As a result, clouds do not form. And, no clouds generally mean no precipitation. So, high pressure usually means fair weather. On the other hand, when large air masses move apart in the upper atmosphere, the air pressure may become relatively low. This reduces the pressure on the layers of warm air below. As a result, the warm air is able to rise into the atmosphere. If the warm air is moist, the moisture may condense into clouds as it is cooled in the atmosphere. So, low pressure can lead to cloudy, rainy weather. But, remember, there are always exceptions. Figure 1, “Air Pressure and Weather,” illustrates these ideas. So, understanding the day to day weather in an area relies heavily on understanding the pressure that exists in that area at that time. But remember, air pressure can change from one day to the next. Which might explain why you have to watch the weather report daily! Figure 1: Air Pressure and Weather From Simple Weather Experiments With Everyday Materials by Muriel Mandell Adapted from Prentice Hall: Science Explorer, Weather and Climate IHA - 108