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

Transcription

1 CHAPTER 16 1 Water in the Air SECTION Understanding Weather BEFORE YOU READ After you read this section, you should be able to answer these questions: What is weather? How does water in the air affect the weather? National Science Education Standards ES 1f, 1i What Is Weather? Knowing about the weather is important in our daily lives. Your plans to go outside can change if it rains. Being prepared for extreme weather conditions, such as hurricanes and tornadoes, can even save your life. Weather is the condition of the atmosphere at a certain time and place. Weather depends a lot on the amount of water in the air. Therefore, to understand weather, you need to understand the water cycle. THE WATER CYCLE The movement of water between the atmosphere, the land, and the oceans is called the water cycle. The sun is the main source of energy for the water cycle. The sun s energy heats Earth s surface. This causes liquid water to evaporate, or change into water vapor (a gas). When the water vapor cools, it may change back into a liquid and form clouds. This is called condensation. The liquid water may fall as rain, snow, sleet, or hail on the land. STUDY TIP Outline Before you read, make an outline of this section using the questions in bold. As you read, fill in the main ideas of the chapter in your outline. 1. Define Write your own definition for weather. Evaporation is the process by which liquid water changes into water vapor, which is a gas. Condensation is the process by which water vapor changes from a gas to a liquid. Clouds form by this process. Precipitation is rain, snow, sleet, or hail that falls from clouds to Earth s surface. STANDARDS CHECK ES 1i Clouds, formed by the condensation of water vapor, affect weather and climate. Word Help: affect to change; to act upon 2. Identify By what process do clouds form? Runoff is water that flows over land and into rivers, streams, and eventually the ocean. Interactive Textbook 301 Understanding Weather

2 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 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 TAKE A LOOK 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

4 USING A RELATIVE-HUMIDITY TABLE Scientists use tables like the one below to determine relative humidity. Use the table to work through the following example. The dry-bulb thermometer on a psychrometer reads 10ºC. The wet-bulb thermometer reads 7ºC. Therefore, the difference between the thermometer readings is 3ºC. In the first column of the table, find the row head for 10ºC, the dry-bulb reading. Then, find the column head for 3ºC, the difference between the readings. Find the place where the row and column meet. The number in the table at this point is 66, so the relative humidity is 66%. Relative Humidity (%) Dry-bulb reading (ºC) Difference between wet-bulb reading and dry-bulb reading (ºC) TAKE A LOOK 8. Apply Concepts The dry-bulb reading on a psychrometer is 8ºC. The wet-bulb reading is 7ºC. What is the relative humidity? Explain What happens when the temperature of air is below its dew point? What Is Dew Point? What happens when relative humidity reaches 100%? At this point, the air is saturated with water vapor. The temperature at which this happens is the dew point. At temperatures below the dew point, liquid water droplets can form from the water vapor in the air. Condensation happens when air is saturated with water vapor. Air can become saturated if water evaporates and enters the air as water vapor. Air can also become saturated when it cools below its dew point. Interactive Textbook 304 Understanding Weather

5 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. TAKE A LOOK 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 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? TAKE A LOOK 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 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. TAKE A LOOK 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 Earth Science Answer Key continued 9. toward the west 10. upper troposphere, lower stratosphere 11. The jet stream is in different places. The pilot would want to catch or avoid the jet stream. 12. Global winds blow in one direction, but local winds can blow in any direction. 13. An L should be on the arrow end of each wind path; winds blow from H to L. Review 1. Arrows can go clockwise or counterclockwise; arrows should point from H to L; sinking air should be labeled C ; rising air should be labeled W. 2. westerlies 3. During the day, the mountains warm up and the air above them warms and rises. At night, as the mountains cool off, the air cools down and sinks, producing winds. 4. No. Winds are caused by differences in air pressure, which are caused by differences in temperature. SECTION 4 AIR POLLUTION 1. Ozone forms when other pollutants react with one another and with air in the presence of sunlight. 2. Pollutant Primary pollutant or secondary pollutant? Natural or caused by people? Car exhaust primary human-caused Dust primary natural or human-caused Ozone secondary human-caused Paint chemicals primary human-caused Pollen primary natural Sea salt primary natural Volcanic ash primary natural 3. vehicle exhaust 4. motor vehicles 5. If there is not enough ventilation, pollutants can get trapped inside. 6. Answers will vary. 7. People burn coal for energy. Pollutants are released. Pollutants combine with water in the air. Acid rain falls in the lake. Fish die. 8. Ozone in the stratosphere blocks UV light, which can be harmful to humans. 9. Ozone in the stratosphere Forms naturally Is not a pollutant Protects Earth from UV rays Ozone near the ground from human activity a pollutant harmful to living things 10. Short-term effects happen quickly and go away once the pollution is gone. Long-term effects develop over a long time and do not go away easily. 11. The electricity to run them must be generated. Many electrical power plants burn fossil fuels to generate electricity. This causes pollution in the areas near, and downwind of, the power plants. Review 1. Burning fossil fuels puts primary pollutants into the air, causing air pollution. Some of these pollutants can combine with water in the atmosphere to make acid precipitation. Vehicle exhaust combines with sunlight and forms secondary pollutants such as ozone. 2. Pollutant Source Negative Solutions effects CFCs air conditioners and aerosol sprays holes in the ozone layer ban on CFCs Ozone Sulfur dioxide vehicle exhaust that reacts with air burning of fossil fuels lung damage acid rain driving limits, vehicle emission controls less fossil fuel use 3. Answers include: soil, water, plants, animals, buildings 4. The thinning ozone layer lets harmful UV radiation reach Earth. UV can cause skin cancer. Chapter 16 Understanding Weather WATER IN THE AIR 1. Weather is a description of what is going on in the atmosphere at a certain time and place. 2. condensation 3. Warmer air can hold more water vapor. Interactive Textbook Answer Key 55 Earth Science

10 Earth Science Answer Key continued 4. about 30 g/cm g/m 3 23 g/m % relative humidity 6. Humidity is the absolute amount of water vapor in the air. Relative humidity is a measure of how saturated with water vapor the air is. 7. wet-bulb and dry-bulb thermometers 8. about 87% 9. Water vapor condenses into liquid water. 10. The glasses are as cold as the air outside. They absorb heat from the air inside. As a result, the air inside cools below its dew point. Water condenses on the glasses. 11. water vapor in the air 12. by shape and by altitude 13. A nimbostratus cloud is producing precipitation. 14. water that falls to Earth from clouds 15. They get bigger. 16. several ice crystals joined together 17. when it is too heavy to be carried by updrafts Review 1. Possible answer: Water can condense from air that is below its dew point. 2. the sun 3. Air rises and cools. When it cools below its dew point, water vapor condenses to form liquid water droplets or ice crystals. These droplets or ice crystals form a cloud. 4. Sleet forms when liquid water freezes in clouds and falls to the ground as ice. Snow forms when water vapor turns directly into a solid. 5. Name Altitude Shape Precipitation? Cirro-stratus high layered no Altocumulus middle puffy no Nimbostratus low layered yes Cumulonimbus low to middle puffy yes SECTION 2 AIR MASSES AND FRONTS 1. by water content and temperature of the air 2. dry, warm 3. northern Canada 4. Warm air masses form over warm water or land, and the North Atlantic and Pacific are cold. 5. Air mass How it affects weather cp from northern Canada mp from the North Pacific Ocean mt from the Gulf of Mexico ct from the deserts very cold winter weather and cool, dry summer weather rain and snow in the Pacific in the winter, fog in the summer heat, humidity, hurricanes, thunderstorms in East Coast and Midwest clear, dry, hot weather in the summer 6. a place where two or more air masses meet 7. It rises because the cold air pushes it up. 8. a place where a warm air mass moves over a cold air mass 9. The cold air masses push it out of the way. 10. Stationary fronts do not move, so the weather they bring stays in one place. 11. Cyclones have lower pressures than surrounding areas, but anticyclones have higher pressures. Cyclones bring rainy or stormy weather, but anticyclones bring dry, clear weather. 12. anticyclone to cyclone Review 1. A front is a place where two air masses meet. 2. First row, from left to right: rises and cools; cloudy, rainy, and stormy Second row, from left to right:... higher than surrounding pressure; dry and clear 3. cold, warm, occluded, stationary 4. Pacific Ocean, Atlantic Ocean, Gulf of Mexico 5. ct SECTION 3 SEVERE WEATHER 1. It must contain enough water vapor to form clouds. 2. Electricity travels between an area with a positive charge and an area with a negative charge. 3. sound produced by vibrations in expanding air that was superheated by lightning 4. The center of the tornado has low pressure. 5. when it touches the ground mph to 112 mph 7. Ocean water is too cold there. 8. the Coriolis effect 9. the center of the hurricane that contains warm, fairly calm air with low pressure Interactive Textbook Answer Key 56 Earth Science