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Name _______________________________ Period ________ Date _________ Interpreting Isobars I can … Explain what isobars are and how are they plotted on a weather map. Create and interpret weather maps with isobars. Identify which symbol is used for high pressure systems on a weather map. Identify which symbol is used for low pressure systems on a weather map. Background Information: Low pressure systems usually produce cloudy, stormy weather. An easy way to remember this is L = Low = Lousy. When a low pressure system is situated over a region, the air begins to rise into the atmosphere. When air rises, it cools down and condenses into precipitation and clouds. This is why an approaching low pressure system means an increased chance for clouds, rain, or snow. It is called "low pressure" because as air rises, the air pressure is lower at the surface. Air flows in a counterclockwise direction with a low pressure system in the Northern Hemisphere. A steady snow or rain can fall north of a warm front where moist warm southern air moves up & over cold air that is in front of a warm front. Thunderstorms and showers often form ahead of a cold front in warm, unstable air. The thunderstorms & showers that form ahead of a cold front are generally shorter in duration than precipitation that forms in front of a warm front. Because of the counterclockwise rotation of a lowpressure system in the northern hemisphere, cold air will usually be located north & west of a low-pressure area and warm air is usually found south & east of a lowpressure area. High pressure systems usually produce fair weather. An easy way to remember this is H = High = Happy. It is called "high pressure" because air sinks around areas of high-pressure, which prevents cloud formation and precipitation. For this reason, high-pressure systems bring sunny days and calm weather. Locations east of an area experiencing high-pressure will experience northerly winds which bring in cooler air whereas areas to the west will experience southerly winds which bring in warmer air. Part I: Introduction - This activity will focus on identifying high or low pressure areas and the weather associated with each. You will also learn about isobars which are lines that connect areas that have the same barometric pressure. The image above shows an example of a weather map containing isobars. The thick black lines are called isobars. They are typically placed at intervals of 4 millibars. Isobars can be used to identify "Highs" and "Lows". The pressure in a high is greater than the surrounding air. The pressure in a low is lower than the surrounding air. For example, the low pressure area is within the 996 millibar isobar marking. Since low pressure areas signify “lousy” weather, you could expect rain and possible storms in this area. Part II: Application 1. What millibar intervals will be necessary to use for this map? (Keep in mind that the isobars are found every 4 millibars such as 996, 1000, 1004, etc.) 2. Draw in the isobars on the map – think “connect-the-dots”. Start with the 1000 mb line, then adding 996 mb, 1004 mb, etc. Note: isobars form sets of curves that do not cross each other. 3. Label with an "H" the center of a high pressure area. 4. Label with an "L" the center of a low pressure area. High pressure regions are usually associated with dry weather because as the air sinks it warms and the moisture evaporates. Low pressure regions usually bring precipitation because when the air rises it cools and the water vapor condenses. 5. Over which state would you expect to see rain or snow? 6. Over which state would you expect to see clear skies? In the northern hemisphere the wind blows clockwise around centers of high pressure. The wind blows counterclockwise around lows. It is the opposite in the southern hemisphere. 7. Draw arrows around the "H" on your map to indicate the wind direction. 8. Draw arrows around the "L" on your map to indicate the wind direction. 9. Imagine that you live in El Paso, TX. If a High were approaching you from the west, describe the shift in wind direction as the High approaches you and then passes to the east. 10. How would pressure measurements change as the High approaches and then passes to the east? 11. Imagine that you live in New York City. If a Low were approaching you from the west, describe the shift in wind direction as the Low approaches you and then passes to the east. 12. The strength of the wind is determined by changes in the pressure - the pressure gradient. A large change in pressure over a short distance would indicate strong winds. On a weather map, isobars that are close together indicate a strong pressure gradient. Where on your map would you expect the strongest winds (the steepest pressure gradient)?