8.10C: Oceans and Weather Earth and Space. Background

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1 Background Air masses in the atmosphere are made up of millions of gas molecules that are being pulled towards Earth s surface by its gravity. The force that these air masses exert on Earth s surface is known as surface pressure. Air pressure decreases as you go up in elevation because at higher elevations, there are fewer air molecules above you. In other words, air pressure is the weight of the air above a given level. Air pressure varies with temperature. Warmer air masses are less dense and have less pressure than colder air masses, which are denser and therefore have more pressure. When meteorologists talk about weather systems they talk in terms of high or low pressure. High pressure systems have winds moving away from the high pressure center towards areas of lower pressure. In the northern hemisphere, high pressure winds move in a clockwise direction. These systems are associated with cool, dry air masses and clear skies. Low pressure systems, in the northern hemisphere, have winds moving towards the low pressure center in a counter-clockwise direction. These systems are associated with warm, moist air masses and stormy weather. Extreme examples of this would be hurricanes. Since ocean currents transport thermal energy and moisture around the globe, they play an important role in global weather patterns. As ocean currents move warm water into an area, evaporation increases, moving moisture and energy to the atmosphere, causing an increase in the temperature and humidity in the area. If cold water moves into an area by ocean currents, it can lower surface air temperatures, as well as reduce evaporation rates, leading to colder, relatively drier conditions in the areas nearby. Because oceans play a key role in supplying energy and moisture to the atmosphere, they also influence the formation of weather systems, such as thunderstorms, blizzards, and hurricanes. Along the Texas coast, thunderstorms often develop because of warm, moist air from the Gulf of Mexico. Hurricanes are more complex weather systems, which generate thunderstorms, rains, high winds, hail, and even tornadoes. Hurricanes have sustained winds of over 73 mph and form in warm, tropical waters in the Atlantic and Pacific Oceans, the Caribbean Sea, Indian Ocean and the Gulf of Mexico. Hurricanes need the right conditions in the oceans, warm waters and light winds, in order to form. They are fueled by warm, moist air. As water evaporates from warm ocean waters this warm, moist and less dense air rises in the atmosphere. 1

2 Background, continued This forms a low pressure area. Since winds move from high to low pressure areas, surrounding air moves into the low pressure area. The air is then warmed, evaporates, and rises into the atmosphere. The rising, warm air eventually reaches an elevation where it cools and condenses to form clouds. At the top of the storm, winds flow outwards, allowing the warm air from below to keep rising. In the northern hemisphere, hurricanes spin counterclockwise, due to the rotation of the Earth (the Coriolis effect). As more ocean water evaporates and fuels the hurricane, the low pressure at the surface will get stronger causing the storm to spin faster. This leads to higher sustained wind speeds and a very powerful storm. Hurricanes are pushed across oceans by the prevailing winds. The prevailing winds called Easterlies are between latitudes 0 and 30 North and South. They blow from the east to the west. Westerlies are the prevailing winds between 30 and 60 North and South. They blow from the west to the east. Since hurricanes are dependent on warm ocean waters, the storms only form at certain times of the year. The portion of the year with a relatively high incidence of hurricanes, the hurricane season, in the Atlantic, Caribbean, and Gulf of Mexico runs from June 1 to Nov. 30. Eye Wall / Wall Cloud: An organized band or ring of towering thunderclouds that surround the eye, or lightwind center of a tropical storm or hurricane. Eye wall and wall cloud are used synonymously. Eye: The roughly circular area of comparatively light winds in the center of a hurricane. The eye is either completely or partially surrounded by the eye wall cloud. The lowest air pressure is found in the eye where it can be as much as 15% lower than the pressure outside of the storm. The greater the difference in pressure, the stronger the winds of the storm will be. 2

3 Background, continued Tropical Depression: A tropical storm system with winds less than 39 mph. Hurricane: A tropical storm with winds above 73 mph. Tropical Storm: A tropical storm system with winds between 39 and 73 mph. Complete the Background questions in your Student Journal. Part I: Hurricanes! Procedure: 1. Review the Part I questions for Clip #1: Hurricanes in your Student Journal. 2. Watch the video clip 01: Hurricanes. 3. Complete the Clip #1: Hurricanes section in your Student Journal. 4. Review the Part I questions for Clip #2: Under the Hood in your Student Journal. 5. Watch the video clip 20: Under the Hood. 6. Complete the Clip #2: Under the Hood section in your Student Journal. 7. Review the Part 1 questions for Clip #3: The Hurricane Heat Engine in your Student Journal. 8. Watch the video clip 22: The Hurricane Heat Engine. 9. Complete the Clip #3: The Hurricane Heat Engine section in your Student Journal. 10. Review the Part I questions for Clip #4: The Death of a Hurricane in your Student Journal. 11. Watch the video clip 27: Death of a Hurricane. 12. Complete the Clip #4: The Death of a Hurricane section in your Student Journal. 13. Review the Part I questions for Clip #5: Benefits of a Hurricane in your Student Journal. 14. Watch the video clip 32: Benefits of Hurricanes. 15. Complete the Clip #5: Benefits of a Hurricane section in your Student Journal. Complete the Background questions and Part I in your Student Journal. 3

4 Part II: Tracking a Hurricane On September 20, 2005, Hurricane Rita began to threaten the Texas Gulf Coast. Forming only 3 weeks after the devastating and deadly Hurricane Katrina, fear of the approaching storm caused the largest evacuation in U. S. History. Estimates indicate between 2.5 to 3.7 million people fled the Galveston and Houston area to escape the storm. The poorly planned evacuation resulted in at least 90 deaths. Officials responded by developing a better communicated and well thought out plan with specific instructions for an orderly evacuation. Communication is an important key to any hurricane evacuation plan making the weather alerts by the National Weather Service vital information. When a tropical depression forms, the National Weather Service notifies coastal areas using the following guidelines: Hurricane Watch: issued 48 hours in advance when a tropical storm may reach hurricane conditions of sustained winds of at least 74 mph and affect the area. Hurricane Warning: issued 36 hours in advance when a tropical storm is expected to form into a hurricane with sustaining winds of at least 74 mph and affect the area. Tropical Storm Watch: issued 48 hours in advance when a tropical depression may become a tropical storm with sustained winds between 39 and 73 mph and affect the area. Tropical Storm Warning: issued 36 hours in advance when a tropical depression is expected to form a tropical storm with sustained winds between 39 and 73 mph and affect the area. You are a meteorologist for the National Weather Service. Your job is to track hurricanes in the Atlantic and issue weather alerts in the appropriate areas. You have been assigned to watch a tropical disturbance that has come off the western coast of North Africa. Procedure: 1. Use the Student Reference Sheet: Storm Data, Days 1 to 8 to plot the track of the hurricane on the hurricane tracking chart. 2. Color code the points to indicate the strength of the storm. Include a key on your tracking chart. Tropical Depression blue Category 2 - orange Tropical Storm black Category 3 - red Category 1 Hurricane yellow Category 4 - purple 4

5 Part II: Tracking a Hurricane, continued 3. Connect the data points on your tracking chart to show the path of the storm. 4. Part of a successful evacuation plan is to have time to prepare. Use your tracking chart to determine if any alerts should be issued. (Is the storm likely to reach the coast line of the U.S. in the next 48 hours?) 5. Look at the U. S. weather map for the next day, day 9. Plot the next data points for the 6. Look at the U. S. weather map for the next day, day 10. Plot the next data points for the 7. Look at the U. S. weather map for the next day, day 11. Plot the next data points for the 8. Look at the U. S. weather map for the next day, day 12. Plot the next data points for the 9. Look at the U. S. weather map for the next day, day 13. Plot the next data points for the Complete the Part II questions and the Reflections and Conclusions in your Student Journal. 5