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  1. How to Use This Presentation How to Use This Presentation • To View the presentation as a slideshow with effects select “View” on the menu bar and click on “Slide Show”, or simply press F5 on the top row of your keyboard. • To advance to the next slide click the left mouse button once. • From the Chapter screen you can click on any section to go directly to that section’s presentation. • Blank or “missing” areas of a slide will remain hidden until the left mouse button is clicked. • You may exit the slide show at any time by pressing the Esc key.

  2. Resources Bellringers Chapter Presentation Transparencies Standardized Test Prep Image and Math Focus Bank CNN Videos Visual Concepts

  3. Chapter 16 Understanding Weather Table of Contents Section 1 Water in the Air Section 2 Air Masses and Fronts Section 3 Severe Weather Section 4 Forecasting the Weather

  4. Section1 Water in the Air Chapter 16 Bellringer • Observe two glasses of water. One filled with ice water, and one filled with warm water. Why do water droplets form on the outside of the cold container? Where do the water beads come from? Why don’t the water beads form on the warm container? • Write your answers in your science journal.

  5. Section1 Water in the Air Chapter 16 Objectives • Explain how water moves through the water cycle. • Describe how relative humidity is affected by temperature and levels of water vapor. • Describe the relationship between dew point and condensation.

  6. Section1 Water in the Air Chapter 16 Objectives, continued • Listthree types of cloud forms. • Identify four kinds of precipitation.

  7. Section1 Water in the Air Chapter 16 The Water Cycle • The condition of the atmosphere is affected by the amount of water in the air. Water in liquid, solid, and gaseous states is constantly being recycled through the water cycle. • The water cycle is the continuous movement of water from sources on Earth’s surface into the air, onto and over land, into the ground, and back to the surface.

  8. Section1 Water in the Air Chapter 16

  9. Section1 Water in the Air Chapter 16 Water Cycle • 1st – water on Earth’s surface is heated by the sun, changes from a liquid to a gas • 2nd – the gas rises into the atmosphere, condenses, and changes from a gas to a liquid • 3rd – water falls back to Earth as precipitation into bodies of water or is absorbed by Earth • 4th – excess runs off

  10. Section1 Water in the Air Chapter 16 Humidity • Humidityis the amount of water vapor in the air. • The air’s ability to hold water vapor changes as the temperature of the air changes.

  11. Section1 Water in the Air Chapter 16 Humidity • As air temperatures increase, the amount of humidity increases because the air can hold more water vapor • Once air holds all the water it can, it becomes saturated, this is 100% relative humidity

  12. actual water vapor content (g/m3)  relative humidity (%) saturation water vapor content (g/m3) Section1 Water in the Air Chapter 16 Humidity, continued • Relative Humidityis the amount of water vapor in the air compared to the maximum amount of water vapor that the air can hold at a certain temperature. • Given as a percentage • Calculate the relative humidity by using the formula:

  13. Section1 Water in the Air Chapter 16 Humidity, continued • As water vapor relative humidity • As temperature relative humidity

  14. Section1 Water in the Air Chapter 16 Humidity, continued • Measuring Relative Humidity A psychrometeris an instrument that is used to measure relative humidity. A psychrometer consists of two thermometers, one of which is a wet-bulb thermometer(covered with wet cloth), & dry bulb thermometer • The difference in temperature readings between the thermometers indicates the amount of water vapor in the air (relative humidity).

  15. Section1 Water in the Air Chapter 16 Humidity, continued

  16. Section1 Water in the Air Chapter 16 Humidity, continued • dry bulb measures air temp • the damp cloth causes the bulb to cool as the water evaporates from the cloth • if the air is dry, more water will evaporate, and the thermometer will have a lower temperature • if the air is humid, less water will evaporate, and the thermometer will have a higher temperature

  17. Section1 Water in the Air Chapter 16 Condensation • Condensation is the process by which a gas, such as water vapor, becomes a liquid. • to occur, the air must be saturated and cooling • Dew Point The dew pointis the temperature at which a gas condenses into a liquid.

  18. Section1 Water in the Air Chapter 16 Clouds • A cloud is a collection of small water droplets or ice crystals suspended in the air, which forms when the air is cooled and condensation occurs. • Clouds are classified by form, and by altitude. • if the air temp is above freezing, tiny water droplets form • if the air temp is below freezing, ice crystals form • 3 types – cirrus, stratus, and cumulus

  19. Section1 Water in the Air Chapter 16 Clouds, continued • nimbus – produce precipitation • alto – middle altitudes • cirro – higher altitudes • Cumulus Cloudsarepuffy, white clouds that tend to have flat bottoms. • fair weather, unless they become larger (thunderstorms) • cumulonimbous, cirrocumulus, altocumulous, stratocumulous

  20. Section1 Water in the Air Chapter 16 Clouds, continued • Stratus Clouds are clouds that form in layers, block sun • indicate rain or drizzle, light to heavy continuous rain • nimbostratus, cirrostratus, altostratus, fog • Fog is a stratus cloud formed near the ground • Cirrus Clouds are thin, feathery, white clouds found at high altitudes. • seen in fair weather, but indicate a change in weather

  21. Section1 Water in the Air Chapter 16

  22. Section1 Water in the Air Chapter 16 Clouds, continued

  23. Section1 Water in the Air Chapter 16 Precipitation • Rain is the most common form of precipitation. • produced when water droplets in a cloud become large enough to fall • Snow Snow forms when temperatures are so cold that water vapor changes directly to a solid. Can be single ice crystals or snowflakes. • Sleet Sleetforms when rain falls through a layer of freezing air. • Hail are balls or lumps of ice that fall from cumulonimbous clouds

  24. Section2 Air Masses and Fronts Chapter 16 Bellringer • Think of as many different qualities of air as possible. • Write your answers in your science journal.

  25. Section2 Air Masses and Fronts Chapter 16 Objectives • Identifythe four kinds of air masses that influence weather in the United States. • Describe the four major types of fronts. • Explain how fronts cause weather changes. • Explain how cyclones and anticyclones affect the weather.

  26. Section2 Air Masses and Fronts Chapter 16 Air Masses • Changes in weather are caused by the movement and interaction of air masses. • An air mass is a large body of air where temperature and moisture content are constant throughout. • Characterized by their moisture content & temperature, determined by the area over which the air mass forms • m (maritime) – forms over water, wet • c (continental) – forms over land, dry • P (polar) – forms over polar regions, cool • T (tropical) – forms over the tropics, warm

  27. Section2 Air Masses and Fronts Chapter 16 Cold Air Masses • Cold Air Masses Most of the cold winter weather in the United States is influenced by three polar air masses. • ‘cP – forms over northern Canada, brings cold winter weather & cool, dry summer • ‘mP – forms over North Pacific Ocean, bringing rain & snow in winter, cool & foggy in summer to Pacific coast • forms over the North Atlantic Ocean, bringing cool, cloudy precipitation in winter, & cool foggy weather in summer to the New England coast

  28. Section2 Air Masses and Fronts Chapter 16 Air Masses, continued • Warm Air Masses Four warm air masses influence the weather in the United States. • ‘cT – forms over the deserts of northern Mexico & southwestern US bringing clear, dry, hot summer weather • ‘mT – brings hot, humid weather, thunderstorms, hurricanes in summer & mild, cloudy winters

  29. Section2 Air Masses and Fronts Chapter 16

  30. Section2 Air Masses and Fronts Chapter 16 Fronts • The area in which two types of air masses meet is called afront. • 4 types : cold, warm, occluded, & stationary fronts • Named according to which air mass is the “attacker” • Cold Fronts A cold front forms where cold air moves under warm air, which is less dense, and pushes the warm air up. • brings thunderstorms, heavy rain or snow • followed by cooler weather

  31. Section2 Air Masses and Fronts Chapter 16 Fronts • Warm FrontsA warm front forms where warm air moves over cold, denser air. • brings drizzly rain • followed by clear warm weather

  32. Section2 Air Masses and Fronts Chapter 16

  33. Section2 Air Masses and Fronts Chapter 16 Fronts, continued • Occluded Front An occluded front forms when a warm air mass is caught between two colder air masses. • coldest air mass moves under & pushes up the warm • An occluded front has cool temperatures and large amounts of rain and snow. • Stationary Front A stationary front forms when a cold air mass meets a warm air mass. • neither has enough force to lift the warm over the cold • A stationary front often brings many days of cloudy, wet weather.

  34. Section2 Air Masses and Fronts Chapter 16

  35. Section2 Air Masses and Fronts Chapter 16 Air Pressure and Weather • Air rises near low pressure areas • as it rises, it cools and often condenses into clouds and precipitation • An area of high pressure is a section of air that is sinking • as the air sinks, it warms so it is able to hold more water

  36. Section2 Air Masses and Fronts Chapter 16 Air Pressure and Weather • Cyclones are areas of rising air that have lower pressure than the surrounding areas do. • air direction converges (comes together) • brings stormy weather • Anticyclonesare areas of sinking air that have high pressure. (sinking air is denser and pressure is higher) • air direction diverges (moves apart) • brings dry, clear weather

  37. Section2 Air Masses and Fronts Chapter 16

  38. Section3 Severe Weather Chapter 16 Bellringer • Write a one-paragraph description of a thunderstorm. Describe the weather conditions immediately before, during, and after the storm. How does the storm affect each of your senses? • Record your response in your science journal.

  39. Section3 Severe Weather Chapter 16 Objectives • Describe how lightning forms. • Describe the formation of thunderstorms, tornadoes, and hurricanes. • Describethe characteristics of thunderstorms, tornadoes, and hurricanes. • Explain how to stay safe during severe weather.

  40. Section3 Severe Weather Chapter 16 Thunderstorms • Lightningis an electric discharge that occurs between a positively charged area and a negatively charged area. Thunderstorms are very active electrically. • Thunder is the sound that results from the rapid expansion of air along the lightning strike.

  41. Section3 Severe Weather Chapter 16 Thunderstorms • Produced by warm & moist air near the Earth’s surface, and an unstable atmosphere • Atmospheres are unstable when surrounding air is colder than the rising warm air mass • rises until it reaches its dew point, and condenses to form a cumulonimbus cloud which brings thunderstorms • produces: high winds, flash floods, hail, tornadoes • Flash floods are the leading cause of weather deaths

  42. Section3 Severe Weather Chapter 16 Lightning • The upper part of clouds carry a positive charge while the lower carries a negative charge • Energy is released causing the air to expand rapidly and send out sound waves, thunder • Lightning can cause deaths and forest fires

  43. Section3 Severe Weather Chapter 16 Tornadoes • A tornado is a small, spinning column of air that has high wind speeds and low central pressure and that touches the ground. • A tornado starts out as a funnel cloud that develops in the bottom of a cumulonimbus cloud as spinning columns of air form into funnel clouds that touch the ground • 75% of the world’s tornadoes occur in the US during spring and early summer when cold, dry air from Canada meets the warm moist air from the tropics

  44. Section3 Severe Weather Chapter 16 Tornadoes, continued

  45. Section3 Severe Weather Chapter 16 Hurricanes • How a Hurricane FormsA hurricane begins as a group of thunderstorms moving over tropical ocean waters. Winds traveling in two different directions meet and cause the storm to spin. • Damage Caused by Hurricanes Hurricanes can cause a lot of damage when they move near or onto land. Wind speeds of most hurricanes range from 120 to 150 km/h.

  46. Section3 Severe Weather Chapter 16 Hurricanes • Most powerful storms on Earth because of the total energy involved, also known as typhoons and cyclones • Most form between 5 and 20 degrees latitude over warm tropical oceans, start as several thunderstorms moving around an area of low pressure, Coriolis effect causes it to curve, the air rises as it rotates, thus it cools and condenses forming clouds • Get their energy from condensation, and they continue to grow as long as they are over warm, moist air

  47. Section3 Severe Weather Chapter 16 Hurricanes • 3 Parts: • eye • eye wall – surrounds eye, strongest part, group of cumulonimbus clouds that produce the heavy rains and wind • rain bands – beyond the eye wall, circle the hurricanes center, speed decreases as the distance from the eye wall increases • Causes: flooding, storm surges, & property damage

  48. Section3 Severe Weather Chapter 16 Hurricanes, continued

  49. Section3 Severe Weather Chapter 16 Severe Weather Safety • Thunderstorm Safety Lightning is one of the most dangerous parts of a thunderstorm. If you are outside, stay away from trees, which can get struck down, and water. If you are in the open, crouch down. • Tornado Safety If there is a tornado warning for your area, find shelter quickly. The best place to go is a basement or cellar. • watch (tornado may happen) or warning (tornado seen) • outdoors: lie in a large, open field or deep ditch

  50. Section3 Severe Weather Chapter 16 Severe Weather Safety, continued • Flood SafetyThe best thing to do during a flood is to find a high place to wait out the flood. • stay out of floodwaters (even if shallow) • Hurricane Safety If you live in an area where hurricanes strike, your family should have a disaster supply kit that includes enough water and food to last several days. • Evacuate, if living near shore • Board up windows