# Condensation is the process by which water vapor changes from a gas to a liquid. Clouds form by this process.

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2 SECTION 1 Water in the Air continued 3. Identify How does air temperature affect how much water vapor the air can hold? What Is Humidity? Water vapor makes up only a small fraction of the mass of the atmosphere. However, this small amount of water vapor has an important effect on weather and climate. When the sun s energy heats up Earth s surface, water in oceans and water bodies evaporates. The amount of water vapor in the air is called humidity. Warmer air can hold more water vapor than cooler air can. Math Focus 4. Read a Graph How much water vapor can air at 30 C hold? Math Focus 5. Calculate What is the relative humidity of 25 C air that contains 10 g/m 3 of water vapor? Show your work. RELATIVE HUMIDITY Scientists often describe the amount of water in the air using relative humidity. Relative humidity is the ratio of the amount of water vapor in the air to the greatest amount the air can hold. There are two steps to calculating relative humidity. First, divide the amount of water in a volume of air by the maximum amount of water that volume of air can hold. Then, multiply by 100 to get a percentage. For example, 1 m 3 of air at 25 C can hold up to about 23 g of water vapor. If air at 25 C in a certain place contains only 18 g/m 3 of water vapor, then the relative humidity is: 18 g/m % relative humidity 3 23 g/m Interactive Textbook 302 Understanding Weather

3 SECTION 1 Water in the Air continued FACTORS AFFECTING RELATIVE HUMIDITY Temperature and humidity can affect relative humidity. As humidity increases, relative humidity increases if the temperature stays the same. Relative humidity decreases as temperature rises and increases as temperature drops if the humidity stays the same. Critical Thinking 6. Compare How is relative humidity different from humidity? MEASURING RELATIVE HUMIDITY Scientists measure relative humidity using special tools. One of these tools is called a psychrometer. A psychrometer contains two thermometers. The bulb of one thermometer is covered with a wet cloth. This is called a wet-bulb thermometer. The other thermometer bulb is dry. This thermometer is a dry-bulb thermometer. You are probably most familiar with dry-bulb thermometers. Wet-bulb thermometers work differently than dry-bulb thermometers. As air passes through the cloth on a wet-bulb thermometer, some of the water in the cloth evaporates. As the water evaporates, the cloth cools. The wet-bulb thermometer shows the temperature of the cloth. If humidity is low, the water evaporates more quickly. Therefore, the temperature reading on the wet-bulb thermometer is much lower than the reading on the dry-bulb thermometer. If the humidity is high, less water evaporates. Therefore, the temperature changes very little. Dry-bulb thermometer Wet-bulb thermometer 7. Identify What are two parts of a psychrometer? Scientists can use psychrometers like this one to measure relative humidity. The difference in temperature readings between the dry-bulb and wet-bulb thermometers is a measure of the relative humidity. The larger the difference between the readings, the lower the relative humidity. Interactive Textbook 303 Understanding Weather

5 SECTION 1 Water in the Air continued AN EVERYDAY EXAMPLE You have probably seen air become saturated because of a temperature decrease. For example, when you add ice cubes to a glass of juice, the temperatures of the juice and the glass decrease. The glass absorbs heat from the air, so the temperature of the air near the glass decreases. When the air s temperature drops below its dew point, water vapor condenses on the glass. The condensed water forms droplets on the glass. Critical Thinking 10. Apply Concepts People who wear glasses may notice that their glasses fog up when they come indoors on a cold day. Why does this happen? The glass absorbs heat from the air. The air cools to below its dew point. Water vapor condenses onto the side of the glass. 11. Describe Where did the liquid water on the outside of the glass come from? How Do Clouds Form? A cloud is a group of millions of tiny water droplets or ice crystals. Clouds form as air rises and cools. When air cools below the dew point, water droplets or ice crystals form. Water droplets form when water condenses above 0ºC. Ice crystals form when water condenses below 0ºC. DIFFERENT KINDS OF CLOUDS Scientists classify clouds by shape and altitude. The three main cloud shapes are stratus clouds, cumulus clouds, and cirrus clouds. The three altitude groups are low clouds, middle clouds, and high clouds. The figure on the next page shows these different cloud types. 12. Explain How are clouds classified? Interactive Textbook 305 Understanding Weather

6 SECTION 1 Water in the Air continued Say It Observe and Describe Look at the clouds every day for a week. Each day, write down the weather and what the clouds looked like. At the end of the week, share your observations with a small group. How was the weather related to the kinds of clouds you saw each day? 13. Compare How is a nimbostratus cloud different from a stratus cloud? What Is Precipitation? Water in the air can return to Earth s surface through precipitation. Precipitation is solid or liquid water that falls to Earth s surface from clouds. There are four main kinds of precipitation: rain, snow, sleet, and hail. Rain and snow are the most common kinds of precipitation. Sleet and hail are less common. 14. Define What is precipitation? Interactive Textbook 306 Understanding Weather

7 SECTION 1 Water in the Air continued RAIN Water droplets in clouds are very tiny. Each droplet is smaller than the period at the end of this sentence. These tiny droplets can combine with each other. As the droplets combine, they become larger. When a droplet reaches a certain size, it can fall to Earth s surface as rain. SLEET Sleet forms when rain falls through a layer of very cold air. If the air is cold enough, the rain freezes in the air and becomes falling ice. Sleet can make roads very slippery. When it lands on objects, sleet can coat the objects in ice. 15. Explain What happens to water droplets in clouds when they combine? SNOW Snow forms when temperatures are so low that water vapor turns directly into a solid. That is, the water vapor in the cloud turns into an ice crystal without becoming a liquid first. Snow can fall as single ice crystals. In many cases, the crystals join together to form larger snowflakes. HAIL Balls or lumps of ice that fall from clouds are called hail. Hail forms in cumulonimbus clouds. Hail can become very large. Hail grows larger in a cycle, as shown in the chart below. 16. Identify What is a snowflake? 2. Low temperatures at the top of the cloud cause the raindrops to freeze, forming tiny pieces of hail. 3. The hail falls through the cloud, and more raindrops collect on it. This process may repeat many times. 1. Rising air in a cloud carries raindrops to the top of the cloud. 4. Another body of rising air carries the hail into the top of the cloud again. There, the rain freezes to the hail, making the hail larger. 17. Identify When does hail fall to the ground? 5. Eventually, the hail becomes too heavy to be carried by the rising air. It falls to Earth s surface. Interactive Textbook 307 Understanding Weather

8 Section 1 Review NSES ES 1f, 1i SECTION VOCABULARY cloud a collection of small water droplets or ice crystals suspended in the air, which forms when the air is cooled and condensation occurs condensation the change of state from a gas to a liquid humidity the amount of water vapor in the air precipitation any form of water that falls to Earth s surface from the clouds relative humidity the ratio of the amount of water vapor in the air to the amount of water vapor needed to reach saturation at a given temperature weather the short-term state of the atmosphere, including temperature, humidity, precipitation, wind, and visibility 1. Identify Relationships How is dew point related to condensation? 2. Identify What is the main source of energy for the water cycle? 3. Explain How do clouds form? 4. Compare What is the difference between sleet and snow? 5. Apply Concepts Fill in the spaces in the table to describe different kinds of clouds. Name Altitude Shape Precipitation? Cirrostratus high no Altocumulus puffy Nimbostratus Cumulonimbus low to middle Interactive Textbook 308 Understanding Weather

9 CHAPTER 16 2 Air Masses and Fronts SECTION Understanding Weather BEFORE YOU READ After you read this section, you should be able to answer these questions: How is an air mass different from a front? How do fronts affect weather? National Science Education Standards ES 1j What Are Air Masses? Have you ever been caught outside when it suddenly started to rain? What causes such an abrupt change in the weather? Changes in weather are caused by the movement of bodies of air called air masses. An air mass is a very large volume of air that has a certain temperature and moisture content. There are many types of air masses. Scientists classify air masses by the water content and temperature of the air. These features depend on where the air mass forms. The area over which an air mass forms is called a source region. One source region is the Gulf of Mexico. Air masses that form over this source region are wet and warm. Each type of air mass forms over a certain source region. On maps, meteorologists use two-letter symbols to represent different air masses. The first letter indicates the water content of the air mass. The second letter indicates its temperature. The figure below shows the main air masses that affect North America. STUDY TIP Summarize As you read, make a chart comparing the four kinds of fronts. In your chart, describe how each kind of front forms and what kind of weather it can cause. 1. Identify How do scientists classify air masses? 2. Apply Concepts Describe the temperature and moisture content of a ct air mass. Interactive Textbook 309 Understanding Weather

10 SECTION 2 Air Masses and Fronts continued 3. Identify What is the source region for cp air masses? COLD AIR MASSES Most of the cold winter weather in the United States comes from three polar air masses. Continental polar (cp) air masses form over northern Canada. They bring extremely cold winter weather. In the summer, cp air masses can bring cool, dry weather. Maritime polar (mp) air masses form over the North Pacific Ocean. They are cool and very wet. They bring rain and snow to the Pacific Coast in winter. They bring fog in the summer. Maritime polar air masses also form over the North Atlantic Ocean. They bring cool, cloudy weather and precipitation to New England. Critical Thinking 4. Infer Why don t warm air masses form over the North Atlantic or Pacific oceans? WARM AIR MASSES Four warm air masses influence the weather in the United States. Maritime tropical (mt) air masses form over warm areas in the Pacific Ocean, the Gulf of Mexico, and the Atlantic Ocean. They move across the East Coast and into the Midwest. In summer they bring heat, humidity, hurricanes, and thunderstorms to these areas. Continental tropical air masses (ct) form over deserts and move northward. They bring clear, dry, hot weather in the summer. Air mass How it affects weather cp from northern Canada mp from the North Pacific Ocean mt from the Gulf of Mexico 5. Identify Fill in the blank spaces in the table. 6. Define What is a front? ct from the deserts What Are Fronts? The place where two or more air masses meet is called a front. When air masses meet, the less dense air mass rises over the denser air mass. Warm air is less dense than cold air. Therefore, a warm air mass will generally rise above a cold air mass. There are four main kinds of fronts: cold fronts, warm fronts, occluded fronts, and stationary fronts. Interactive Textbook 310 Understanding Weather

11 SECTION 2 Air Masses and Fronts continued COLD FRONTS A cold front forms when a cold air mass moves under a warm air mass. The cold air pushes the warm air mass up. The cold air mass replaces the warm air mass. Cold fronts can move quickly and bring heavy precipitation. When a cold front has passed, the weather is usually cooler. This is because a cold, dry air mass moves in behind the cold front. A cold front forms when a cold air mass pushes a warmer air mass away. The front moves in the direction that the cold air mass is moving. 7. Describe What happens to the warm air mass at a cold front? WARM FRONTS A warm front forms when a warm air mass moves in over a cold air mass that is leaving an area. The warm air replaces the cold air as the cold air moves away. Warm fronts can bring light rain. They are followed by clear, warm weather. Warm air mass Warm front 8. Define What is a warm front? Cold air mass Movement of front A warm front forms when a warm air mass moves in and replaces a cold air mass. The front moves in the direction the warm air mass is moving. Interactive Textbook 311 Understanding Weather

12 SECTION 2 Air Masses and Fronts continued OCCLUDED FRONTS An occluded front forms when a warm air mass is caught between two cold air masses. Occluded fronts bring cool temperatures and large amounts of rain and snow. Warm air mass Warm air mass Occluded front Occluded front 9. Describe What happens to the warm air mass in an occluded front? An occluded front forms when a warm air mass is trapped between two cold air masses. The cold air masses move together and push the warm air out of the way. Cold air mass Movement of front Cold air mass STATIONARY FRONT A stationary front forms when a cold air mass and a warm air mass move toward each other. Neither air mass has enough energy to push the other out of the way. Therefore, the two air masses remain in the same place. Stationary fronts cause many days of cloudy, wet weather. 10. Infer What do you think is the reason that stationary fronts bring many days of the same weather? A stationary front forms when air masses stay in one place. Interactive Textbook 312 Understanding Weather

13 SECTION 2 Air Masses and Fronts continued How Does Air Pressure Affect Weather? Remember that air produces pressure. However, air pressure is not always the same everywhere. Areas with different pressures can cause changes in the weather. These areas may have lower or higher air pressure than their surroundings. A cyclone is an area of the atmosphere that has lower pressure than the surrounding air. The air in the cyclone rises. As the air rises, it cools. Clouds can form and may cause rainy or stormy weather. An anticyclone is an area of the atmosphere that has higher pressure than the surrounding air. Air in anticyclones sinks and gets warmer. Its relative humidity decreases. This warm, sinking air can bring dry, clear weather. Cyclones and anticyclones can affect each other. Air moving out from the center of an anticyclone moves toward areas of low pressure. This movement can form a cyclone. The figure below shows how cyclones and anticyclones can affect each other. Critical Thinking 11. Compare Give two differences between cyclones and anticyclones. Anticyclone Cyclone 12. Identify In which direction does air move: from a cyclone to an anticyclone, or from an anticyclone to a cyclone? Interactive Textbook 313 Understanding Weather

14 Section 2 Review NSES NSES ES 1j SECTION VOCABULARY air mass a large body of air throughout which temperature and moisture content are similar anticyclone the rotation of air around a high pressure center in the direction opposite to Earth s rotation. cyclone an area in the atmosphere that has lower pressure than the surrounding areas and has winds that spiral toward the center front the boundary between air masses of different densities and usually different temperatures 1. Identify Relationships How are fronts and air masses related? 2. Compare Fill in the table to describe cyclones and anticyclones. Name Compared to surrounding air pressure, the pressure in the middle is... What does the air inside it do? What kind of weather does it cause? cyclone...lower than surrounding pressure. anticyclone sinks and warms 3. List What are four kinds of fronts? 4. Identify What are the source regions for the mt air masses that affect weather in the United States? 5. Describe What kind of air mass causes hot, clear, dry summer weather in the United States? Interactive Textbook 314 Understanding Weather

15 CHAPTER 16 3 Severe Weather SECTION Understanding Weather BEFORE YOU READ After you read this section, you should be able to answer these questions: What are some types of severe weather? How can you stay safe during severe weather? National Science Education Standards ES 1i, 1j What Causes Thunderstorms? A thunderstorm is an intense storm with strong winds, heavy rain, lightning, and thunder. Many thunderstorms happen along cold fronts. However, thunderstorms can also happen in other areas. Two conditions are necessary for a thunderstorm to form: warm, moist air near Earth s surface and an unstable area of the atmosphere. The atmosphere is unstable when a body of cold air is found above a body of warm air. The warm air rises and cools as it mixes with the cool air. When the warm air reaches its dew point, the water vapor condenses and forms cumulus clouds. If the warm air keeps rising, the clouds may become dark cumulonimbus clouds. LIGHTNING As a cloud grows bigger, parts of it begin to develop electrical charges. The upper parts of the cloud tend to become positively charged. The lower parts tend to become negatively charged. When the charges get big enough, electricity flows from one area to the other. Electricity may also flow between the clouds and the ground. These electrical currents are lightning. STUDY TIP Describe After you read this section, make a flow chart showing how a tornado forms. Critical Thinking 1. Infer Why does air near the surface have to be moist in order for a thunderstorm to form? 2. Describe How does lightning form? Different parts of thunderclouds and the ground can have different electrical charges. When electricity flows between these areas, lightning forms. Interactive Textbook 315 Understanding Weather

18 SECTION 3 Severe Weather continued 8. Explain What causes hurricanes to rotate in different directions in the Northern and Southern Hemispheres? HOW HURRICANES FORM A hurricane begins as a group of thunderstorms traveling over tropical ocean waters. Winds traveling in two different directions meet and cause the storm to spin. Because of the Coriolis effect, hurricanes rotate counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Hurricanes are powered by solar energy. The sun s energy causes ocean water to evaporate. As the water vapor rises in the air, it cools and condenses. A group of thunderstorms form and produce a large, spinning storm. A hurricane forms as the storm gets stronger. 9. Define What is the eye of a hurricane? The hurricane will continue to grow as long as it is over warm ocean water. When the hurricane moves over colder waters or over land, the storm loses energy. This is why hurricanes are not common in the middle of continents. The storms lose their energy quickly when they move over land. Interactive Textbook 318 Understanding Weather

19 SECTION 3 Severe Weather continued DAMAGE CAUSED BY HURRICANES Hurricanes can cause serious damage when they move near or onto land. The strong winds from hurricanes can knock down trees and telephone poles. They can damage or destroy buildings and homes. Many people think that the winds are the most damaging part of a hurricane. However, most of the damage from hurricanes is actually caused by flooding from heavy rains and storm surges. A storm surge is a rise in sea level that happens during a storm. A storm surge from a hurricane can be up to 8 m high. The storm-surge flooding from Hurricane Katrina in 2005 caused more damage than the high-speed winds from the storm. How Can You Stay Safe During Severe Weather? Severe weather can be very dangerous. During severe weather, it is important for you to listen to a local TV or radio station. Severe-weather announcements will tell you where a storm is and if it is getting worse. Weather forecasters use watches and warnings to let people know about some kinds of severe weather. A watch means that severe weather may happen. A warning means that severe weather is happening somewhere nearby. The table below gives ways to stay safe during different kinds of severe weather. 10. Define What is a storm surge? Severe weather Thunderstorms Tornadoes Floods Hurricanes How to stay safe If you are outside, stay away from tall objects that can attract lightning. If you are in an open area, crouch down. Stay away from water. If you are inside, stay away from windows. During a tornado warning, find shelter quickly in a basement or cellar. If you cannot get to a basement, go to a windowless room in the center of the building (such as a closet or bathroom). If you are outside, lie down in an open field or a deep ditch. Find a high place to wait out the flood. Always stay out of floodwaters. Protect the windows in your home by covering them with wood. Stay inside during the storm. If you are told to leave your home, do so quickly and calmly. 11. Explain Why should you listen to weather reports during severe weather? Interactive Textbook 319 Understanding Weather

20 Section 3 Review NSES ES 1i, 1j SECTION VOCABULARY hurricane a severe storm that develops over tropical oceans and whose strong winds of more than 120 km/h spiral in toward the intensely low-pressure storm center lightning an electric discharge that takes place between two oppositely charged surfaces, such as between a cloud and the ground, between two clouds, or between two parts of the same cloud thunder the sound caused by the rapid expansion of air along an electrical strike thunderstorm a usually brief, heavy storm that consists of rain, strong winds, lightning, and thunder tornado a destructive, rotating column of air that has very high wind speeds and that may be visible as a funnel-shaped cloud 1. Explain Why do thunder and lightning usually happen together? 2. Identify How can severe thunderstorms cause damage? 3. Identify Where do most tornadoes happen? 4. Explain Why do most tornadoes happen in the spring and early summer? 5. Analyze How does energy from the sun power hurricanes? 6. Describe When do hurricanes lose energy? 7. Identify Give three ways to stay safe if you are caught outside in a thunderstorm. Interactive Textbook 320 Understanding Weather

21 CHAPTER 16 4 Forecasting Weather SECTION Understanding Weather BEFORE YOU READ After you read this section, you should be able to answer these questions: What instruments are used to forecast weather? How do you read a weather map? National Science Education Standards ES 1i, 1j What Is a Weather Forecast? Weather affects how you dress and how you plan your day. Severe weather can put people in danger. Therefore, accurate weather forecasts are important. A weather forecast is a prediction of weather conditions over the next few days. Meteorologists make weather forecasts using information on atmospheric conditions. Meteorologists use special instruments to collect data. Some of these instruments are far above the ground. Others are tools you may be familiar with from everyday use. WEATHER BALLOONS Weather balloons carry electronic equipment. The equipment on a weather balloon can measure weather conditions as high as 30 km above Earth s surface. This equipment measures temperature, air pressure, and relative humidity. It transmits the information to meteorologists using radio signals. Meteorologists can track the path of the balloons to measure wind speed and direction. STUDY TIP Compare As you read this section, make a chart comparing the different tools that meteorologists use to collect weather data. 1. Explain What do meteorologists use to forecast the weather? 2. Describe How do meteorologists obtain the information from weather balloons? Weather balloons carry equipment into the atmosphere. They use radio signals to transmit information on weather conditions to meteorologists on the ground. Interactive Textbook 321 Understanding Weather

22 SECTION 4 Forecasting Weather continued THERMOMETERS AND BAROMETERS Remember that air temperature and pressure can affect the weather. Therefore, meteorologists must be able to measure temperature and pressure accurately. They use thermometers to measure temperature, just like you do. They use tools called barometers to measure air pressure. Critical Thinking 3. Infer Why is it important for meteorologists to be able to measure wind direction? WINDSOCKS, WIND VANES, AND ANEMOMETERS Meteorologists can use windsocks and wind vanes to measure wind direction. A windsock is a cone-shaped cloth bag that is open at both ends. The wind enters through the wide end and leaves through the narrow end. The wide end always points into the wind. A wind vane is shaped like an arrow. It is attached to a pole. The wind pushes the tail of the arrow. The vane spins until the arrows points into the wind. An anemometer measures wind speed. It has three or four cups connected to a pole with spokes. The wind pushes on the open sides of the cups. This makes them spin on the pole. The spinning of the pole produces an electric current, which is displayed on a dial. The faster the wind speed, the stronger the electric current, and the further the dial moves. 4. Identify What is an anemometer? Meteorologists use anemometers to measure wind speed. 5. Describe Give two things that meteorologists can use radar to do. RADAR AND SATELLITES Scientists use radar to locate fronts and air masses. Radar can locate a weather system and show the direction it is moving. It can show how much precipitation is falling, and what kind of precipitation it is. Most television stations use radar to give information about weather systems. Weather satellites orbiting Earth produce images of weather systems. Satellites can also measure wind speeds, humidity, and temperatures from different altitudes. Meteorologists use weather satellites to track storms. Interactive Textbook 322 Understanding Weather

23 SECTION 4 Forecasting Weather continued What Are Weather Maps? In the United States, two main groups of scientists collect weather data. One group is the National Weather Service (NWS). The other group is the National Oceanic and Atmospheric Administration (NOAA). These groups gather information from about 1,000 weather stations across the United States to produce weather maps. READING A WEATHER MAP Some weather maps contain station models. A station model is a symbol that shows the weather at a certain location. Station models look like circles with numbers and symbols around them. The numbers and symbols stand for different measurements, as shown below. 6. Identify What are two groups that collect weather data in the United States? 7. Use a Model What is the dew point temperature for the station shown in the figure? 8. Infer Will condensation happen in the air at the station in the figure? Explain your answer. Some weather maps, such as those you see on television, show lines called isobars. Isobars are lines that connect points of equal air pressure. They are similar to contour lines on a topographic map. Isobars that form closed circles represent areas of high (H) or low (L) pressure. Weather maps also show fronts. 9. Read a Map On the map, circle the areas of high pressure. Interactive Textbook 323 Understanding Weather

24 Section 4 Review NSES ES 1i, 1j SECTION VOCABULARY anemometer an instrument used to measure wind speed barometer an instrument that measures atmospheric pressure thermometer an instrument that measures and indicates temperature 1. Compare How is an anemometer different from a windsock or a wind vane? 2. Identify What three atmospheric conditions do weather balloons measure? 3. Describe Give three things that meteorologists use weather satellites for. 4. Apply Concepts Which of the two weather stations below is experiencing higher air temperatures? Which is experiencing higher wind speeds? 5. Apply Concepts In which direction is the wind blowing at station A? In which direction is it blowing at station B? Interactive Textbook 324 Understanding Weather

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