Note: In this Investigation, we wish to follow up on the discussion of high and low-pressure zones and introduce the idea of fronts that form between these different areas of pressure. In Investigation 2 Lab, students demonstrated that wind was generated as a result of pressure changes. Quickly lifting the pack of papers from the isobar map created an area of low pressure at the map’s surface. When this occurred, surrounding areas of air rushed in to prevent the formation of a vacuum, and “wind” resulted. By examining the direction of movement of the foil tent, students saw that wind direction was drawn toward the area of low pressure. They also saw that the extent or strength of the wind (as judged by the distance the tent moved) was dependent on the distance from the low-pressure zone. Locations closer to the low-pressure zone were shown to be more greatly affected.

The reverse was demonstrated when forming a high-pressure zone by rapidly lowing the pack of papers onto the millibar map. As air was forced down to the table, it pushed in all directions away from the site of high pressure, and wind was generated in a direction away from the site, toward areas of lower pressure. The magnitude of this effect too was shown to be related to the distance from the high-pressure zone.

From these experiments in Investigation 2 Lab, students should have developed some concept of what modern weather maps depict and perhaps predict that areas of high or low pressure will likely involve air movement or wind. In addition, they may also conclude that since millibar lines on weather maps indicate the precise amount of pressure at specific locations, that such maps can have predictive value on the amount of wind that is likely to occur at these locations.

• In Investigation 3, we will continue with the concept of fronts between differing pressure zones and extend the concept to include the involvement of cold and warm fronts in the formation of storms and precipitation.

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• This slide highlights the difference between a cold front and a warm front. Both types of fronts involve the interface (front) and interaction of a warm and cold mass (or parcel) of air. The distinction between the two types of fronts is determined by which of the two parcels of air is moving toward the other.

• A cold front signifies that a cold parcel of air moves into a more stationary parcel of warm air. A warm front signifies that a warm parcel of air moves into a more stationary mass of cold air. As indicated on the weather map at the bottom, cold fronts are typically represented by blue lines with triangular barbs depicting the direction of movement of the front. Warm fronts, on the other hand, are represented on weather maps as red lines with semicircles indicating their direction of movement.

• As can be seen in the two illustrations, warm air rises in both warm and cold fronts. From their experiments in Investigation 2, students may suspect that the rising air will cause wind activity at these fronts.

• At this point, we will turn to a consideration of what happens to the warm air as it rises into the atmosphere. We will see that it can result in condensation of water vapor carried in the warm air followed by precipitation – that is, rain and/or snow, sleet, hail, etc.

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• This slide introduces the first phase of precipitation formation, condensation. The background of this slide depicts a bottle of cold water sitting at room temperature. It illustrates an example of condensation that students are probably most familiar with. Notice the beads of water or “sweat” that coats the outside of the bottle.

• Ask students, “Where does this water come from and why is it forming on the bottle?”

• Explain that there are two key components of condensation. The first is that water is present in the air. The second is that, if chilled, the water in the air will change from a gas to a liquid. This is precisely what happens when warm, moist air rises at a front and enters higher and colder regions of the troposphere.

Note: The insert listing the major components of the atmosphere was seen by students in Investigation 1.

• Notice that water vapor (a gas), may be present at different concentrations. In general, air over warm, moist land or bodies of water will contain a greater concentration of water vapor than air over cool, dry regions.

• Ask students, “Which bottle of cold water do you think would have more condensation on it – one placed in a dessert at 33^oC (~91^oF) or one placed in a tropical jungle at 33^oC?”

• Explain that the bottle in the jungle would have more condensation since the air in the jungle is more “humid”, thus carries more water.

• This slide illustrates the precipitation cycle. Evaporation occurs all the time, not just at fronts.

Note: LabLearner discusses water evaporation much more in elementary CELLs in connection with the Water Cycle.

• As shown in this slide, evaporation, condensation, and precipitation occur in relation to the temperature of the air (shown here as an arrow going from red to blue with increasing altitude and the two thermometers).

Note: For reference, the temperature profile of the atmosphere, first used in Investigation 1, is also presented on this slide.

Note: ALL of the events shown in the illustration to the right occur in the troposphere. Therefore, temperature decreases with altitude.

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• This slide returns to the concept of warm air rising at a front. Just as discussed in the previous slide, as warm air rises and temperatures fall, condensation and precipitation can occur.

• This slide also illustrates that, depending on temperature; precipitation may take the form of snow rather than rain. Thus, it is the difference in temperatures of rising air masses that is important. That is, the temperature at the Earth’s surface may be quite cold in winter months, but so long as the temperatures at higher altitudes are even colder, the surface air will nonetheless rise.

Note: Hopefully, at this point, students will suspect that cold and warm fronts may be important locations when it comes to storms and precipitation. This will be confirmed by examining an actual weather map on the next slide.

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• This slide depicts a weather map like the ones students routinely see on television and the Internet. Notice how the storm and precipitation activity, as highlighted by the colorful radar plots, closely aligns with the various fronts across the country. Although local precipitation activity may be scattered throughout vast areas of the country, the concentration of storm activity at fronts is truly dramatic!

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• This slide is included at this juncture because landforms like the mountain shown here can also force air up into the atmosphere as moving air masses collide with their slanted slopes. As air is forced up into lower atmospheric temperatures, condensation and precipitation can occur.

• On the opposite side of the mountain, dry air and arid conditions may predominate. Deserts often form in such locations.

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• This slide shows the apparatus set up for Investigation 3 Lab. It is a model of all three steps in the precipitation cycle we have discussed in previous slides. The heating of water in the flask represents evaporation from a warm, moist surface of the Earth. The warm, moist air leaves the flask through the tube at the top and then passes into a wrap of ice water on the ring stand. This represents the colder upper atmosphere where condensation and precipitation may occur.

• Finally, the tube continues on and into a 15ml centrifuge tube where “rain” may be collected and its volume approximated. This represents the Earth’s surface once again.

• Remind students to wear goggles, lab coats and use hot hand protectors when touching any hot apparatus. Also, shattering glass is always a potential when working with glassware, particularly when heating it.