Weather. Describing Weather

Transcription

1 CHAPTER 13 Weather LESSON 1 Describing Weather What do you think? Read the two statements below and decide whether you agree or disagree with them. Place an A in the Before column if you agree with the statement or a D if you disagree. After you ve read this lesson, reread the statements to see if you have changed your mind. Before Statement After Key Concepts What is weather? What variables are used to describe weather? How is weather related to the water cycle? 1. Weather is the long-term average of atmospheric patterns of an area. 2. All clouds are at the same altitude within the atmosphere. What is weather? Weather is the atmospheric conditions, along with short-term changes, of a certain place at a certain time. Have you ever been caught in a rainstorm on what began as a sunny day? If so, you know that weather can change quickly. It can also stay the same for days. Weather Variables Variables are things that can change. Temperature and rainfall are two of the variables used to describe weather. Meteorologists are scientists who study and predict weather. They use several variables that describe a variety of atmospheric conditions. These variables include air temperature, air pressure, wind speed and direction, humidity, cloud coverage, and precipitation. Air Temperature Air temperature is a measure of the average kinetic energy of molecules in the air. Kinetic energy is the energy an object has because it is moving. When the temperature is high, molecules have a high kinetic energy. Therefore, molecules in warm air move faster than molecules in cold air. Air temperatures vary with the time of day, season, location, and altitude. Summarize What You Read After you read each paragraph, write a sentence or two in your own words describing what you read. Use your sentences to review the lesson. 1. Define What is weather? Reading Essentials Weather 209

2 Visual Check 2. Apply What happens to air pressure as altitude decreases? Reading Check 3. Name What instrument measures air pressure? Reading Check 4. Compare humidity and relative humidity. Top of atmosphere increasing altitude Air Pressure Air pressure is the pressure that a column of air exerts on the air, or surface, below it. Look at the figure. Air pressure decreases as altitude increases. Therefore, air pressure is greater at low altitudes than it is at high altitudes. Air pressure is measured with an instrument called a Sea level barometer. Air pressure is 1 m 1 m measured in millibars (mb). The term barometric pressure means air pressure. Knowing the barometric pressure of different areas helps meteorologists predict the weather. Wind Wind is created as air moves from areas of high pressure to areas of low pressure. Wind direction is given as the direction from which the wind is blowing. For example, winds that blow from west to east are called westerlies. Meteorologists measure wind speed using an instrument called an anemometer (a nuh MAH muh tur). Humidity The amount of water vapor in the air is called humidity (hyew MIH duh tee). Humidity can be measured in grams per cubic meter of air (g/m 3 ). When the humidity is high, there is more water vapor in the air. On a day with high humidity, your skin might feel sticky because sweat might not evaporate quickly from your skin. Relative Humidity A sponge can absorb water. When it becomes full, it cannot absorb any more water. In the same way, air can hold only a certain amount of gaseous water vapor. When air is saturated, it holds as much water vapor as possible. Temperature determines how much water vapor air can contain. air can contain more water vapor than cold air can. Relative humidity is the amount of water vapor present in the air compared to the maximum amount of water vapor the air could contain at that temperature. Relative humidity is measured using a psychrometer. It is stated as a percent. A relative humidity of 50 percent means that the amount of water vapor in the air is one-half of the maximum the air can hold at that temperature. 210 Weather Reading Essentials

3 Dew Point When a sponge becomes saturated with water, the water starts to drip from the sponge. Likewise, when air becomes saturated with water vapor, the water vapor condenses and forms water droplets. When air near the ground is saturated, water vapor condenses into a liquid. If the temperature is above 0 C, dew forms. If the temperature is below 0 C, ice crystals, or frost, form. Higher in the atmosphere, saturated air forms clouds. When the temperature decreases, air can hold less moisture. Eventually, the air becomes saturated and dew forms. The dew point is the temperature at which air becomes fully saturated because of decreasing temperatures while holding the amount of moisture constant. s and Fog Think about what happens when you exhale warm air on a cold day. The warm air you exhale cools. If it reaches its dew point, you can see the water vapor condense into a foggy cloud in front of your face. This also happens when warm air containing water vapor cools as it rises in the atmosphere. When the cooling air reaches its dew point, water vapor condenses on small particles in the air and forms droplets. The droplets block and reflect light. This makes them visible as clouds. Make a two-tab book and use it to collect information about the similarities and differences between clouds and fog. s Fog s are water droplets or ice crystals suspended in the atmosphere. s can have different shapes. s can form at different altitudes within the atmosphere. Read the table below that describes different types of clouds. As clouds move, water and thermal energy are transported from one location to another. Recall that clouds reflect some of the Sun s incoming radiation. Stratus s flat, white, and layered altitude up to 2000 m Types of s Cumulus s fluffy, heaped, or piled up 2,000 to 6,000 m altitude Cirrus s wispy above 6,000 m A cloud that forms near Earth s surface is called fog. Fog is a suspension of water droplets or ice crystals close to or at Earth s surface. Fog reduces visibility. Visibility is the distance a person can see into the atmosphere. Visual Check 5. Illustrate Look out the window and find a cloud. Sketch the shape of the cloud in the space above. Then highlight in the table the type of cloud you drew. Reading Check 6. Describe What is fog? Reading Essentials Weather 211

4 Types of Precipitation droplets Ice crystals droplets Visual Check 7. Compare What is the difference between snow and sleet? Rain Raindrops Raindrops Cold Snow droplets droplet Ice crystal Sleet Cold Ice Hail Hail 8. Identify What variables are used to describe weather? Precipitation Droplets in clouds form around small solid particles in the atmosphere. These particles might be dust, salt, or smoke. Precipitation occurs when cloud droplets combine and become large enough to fall to Earth s surface. Precipitation is water in liquid or solid form that falls from the atmosphere. Types of precipitation rain, snow, sleet, and hail are shown in the figure above. Rain is precipitation that reaches Earth s surface as droplets of water. Snow is precipitation that reaches Earth s surface as solid, frozen crystals of water. Sleet may start out as snow. The snow melts into rain as it passes through a layer of warm air and refreezes when it passes through a layer of below-freezing air. Other times, sleet is just freezing rain. Hail reaches Earth s surface as large ice pellets. Hail starts as a small piece of ice that is repeatedly caught in an updraft within a cloud. A layer of ice is added with each lifting. When it becomes too heavy, it falls to Earth. The Water Cycle Precipitation is an important process in the water cycle, shown at the top of next page. Evaporation and condensation are also important processes in the water cycle. The water cycle is the series of natural processes in which water continually moves among oceans, land, and the atmosphere. 212 Weather Reading Essentials

5 The Water Cycle formation Snow Surface runoff Precipitation Lake Water vapor condenses Evaporation Ocean Visual Check 9. Identify Circle the name of the process in which liquid water changes into water vapor. Most water vapor enters the atmosphere through evaporation. Water vapor forms as water is heated at the ocean s surface. Water vapor cools as it rises in the atmosphere. The cooled water vapor condenses back into liquid. Eventually droplets of liquid and solid water form clouds. s produce precipitation. The precipitation falls to Earth s surface and later evaporates, continuing the cycle. 10. Describe How is weather related to the water cycle? Reading Essentials Weather 213

6 Mini Glossary air pressure: the pressure that a column of air exerts on the air, or surface, below it dew point: the temperature at which air is fully saturated because of decreasing temperatures while holding the amount of moisture constant humidity (hyew MIH duh tee): the amount of water vapor in the air relative humidity: the amount of water vapor present in the air compared to the maximum amount of water vapor the air could contain at that temperature water cycle: the series of natural processes in which water continually moves among oceans, land, and the atmosphere weather: the atmospheric conditions, along with short-term changes, of a certain place at a certain time precipitation: water, in liquid or solid form, that falls from the atmosphere 1. Review the terms and their definitions in the Mini Glossary. Write a sentence that explains how relative humidity and dew point are related. 2. C omplete the graphic of the water cycle by writing the terms below in the correct order. You may begin with any process. cloud formation condensation evaporation precipitation d. The Water Cycle What do you think Reread the statements at the beginning of the lesson. Fill in the After column with an A if you agree with the statement or a D if you disagree. Did you change your mind? c. a. b. ConnectED Log on to ConnectED.mcgraw-hill.com and access your textbook to find this lesson s resources. END OF LESSON 214 Weather Reading Essentials

7 Arctic Air Masses Arctic air masses form over Siberia and the Arctic. These air masses contain bitterly cold, dry air. During the winter, an arctic air mass can bring temperatures to -40 C. Continental Polar Air Masses Land cannot transfer as much moisture to the air as oceans can. Thus, air masses that form over land are drier than air masses that form over oceans. Continental polar air masses are fast moving. They bring cold temperatures in winter and cool temperatures in summer. Polar air masses that affect North America often form over Alaska and Canada. Maritime Polar Air Masses Air masses that form over the northern Atlantic and Pacific Oceans are maritime polar air masses. These air masses are cold and humid. Maritime polar air masses often bring cloudy, rainy weather. Continental Tropical Air Masses Air masses forming in the tropics over dry, desert land are continental tropical air masses. These hot and dry air masses bring clear skies and high temperatures. Continental tropical air masses usually form only during summer. Maritime Tropical Air Masses These air masses form over the Gulf of Mexico, the Caribbean Sea, and the eastern Pacific Ocean. Maritime tropical air masses are moist air masses. They bring hot, humid air to the southeastern United States in summer. In winter, they can bring heavy snowfall. Air masses can change as they move over the land and ocean., moist air can lose its moisture and become cool. Cold, dry air can move over water and become moist and warm. Fronts In 1918, Norwegian Jacob Bjerknes (BYURK nuhs) and his coworkers developed a new method for forecasting the weather. Bjerknes noticed that specific types of weather occur at the boundaries between different air masses. He used the word front, a military term, to describe this boundary. A military front is the boundary between opposing armies. A weather front is the boundary between two air masses. As wind carries an air mass away from the area where it formed, the air mass will eventually bump into another air mass. Major weather changes often occur at fronts. Changes in temperature, humidity, clouds, wind, and precipitation are common at fronts. Math Skills To convert Fahrenheit ( F) units to Celsius ( C) units, use this equation: ( F - 32) C = 1.8 Example: Covert 76 F to C. a. Always perform the operation in parentheses first. (76 F - 32 = 44 F) b. Divide the answer from Step a by 1.8. (44 F) 1.8 = 24 C To convert Celsius ( C) units to Fahrenheit ( F) units, use this equation: F = ( C 1.8) Conversions a. Convert 86 F to C. b. Convert 37 C to F. 4. Describe What drives weather patterns? Reading Essentials Weather 217