CHAPTER 21 4 Forecasting the Weather SECTION Weather KEY IDEAS As you read this section, keep these questions in mind: How do weather stations communicate weather data? How do meteorologists create weather maps? How do computer models help meteorologists forecast weather? What are three types of weather that meteorologists have tried to control? When Did Weather Forecasting Begin? People have tried to predict the weather for thousands of years. In many early cultures, people believed gods controlled the weather. Some people tried to use the positions of the moon and stars to predict weather. Scientific weather predictions began with the invention of weather instruments, such as the thermometer. The invention of the telegraph in 1844 helped meteorologists share weather information. Over time, national weather services formed. How Do Meteorologists Collect Weather Data? Weather observers are people who watch and record changes in weather conditions. Weather observers are at weather stations around the world. They report weather conditions regularly, often many times per hour. Weather observers record many things, including atmospheric pressure speed and direction of surface wind precipitation, temperature, and humidity type, amount, and height of clouds visibility and general weather conditions Automatic observing systems also record data. These systems, along with weather observers, send data to a collection center. Weather centers around the world share the data they collect. READING TOOLBOX Make a List As you read this section, list ways that meteorologists collect, display, and analyze weather data. Discuss your list with a partner. 1. Explain When did people begin to make scientific weather predictions? 2. Identify What are two things that weather observers record? Holt McDougal Earth Science 325 Weather
3. Explain Why do meteorologists use weather maps? How Do Weather Maps Show Weather Data? Weather maps display data from weather stations. Weather maps help meteorologists understand the current weather and predict future weather. Meteorologists use symbols and colors to show weather data on a weather map. Meteorologists around the world use and understand these symbols. WEATHER SYMBOLS Some weather maps have groups of symbols that show the weather conditions at weather stations. This group of symbols is called a station model. Weather symbols describe conditions such as cloud cover and wind speed. The figure below shows examples of weather symbols and a station model. Cloud coverage (fraction of sky covered) 1 3 4 5 7 Clear 8 Scattered 8 8 8 Broken 8 Overcast Obscured No data 4. List Identify three pieces of information you can get from a station model. Wind speed (knots) Calm 1 2 3 7 8 12 13 17 18 22 23 27 48 52 73 77 103 107 Wind direction North Northeast East Southeast South Southwest West Northwest Weather conditions Drizzle Freezing rain Fog Hail Haze Rain Shower Smoke Snow Thunderstorm Hurricane Tropical storm Station model Temperature 27993 Weather conditions Dew point 26 Atmospheric pressure Barometric tendency Cloud cover Wind direction Wind speed Talk About It Use Models Draw the symbols for weather conditions on index cards. With a partner, take turns showing the index cards and naming the weather condition. Then discuss which symbol describes the current weather conditions in your area. The station model shows the air temperature and the dew point. The dew point is the temperature at which the condensation rate equals the evaporation rate. The dew point shows the level of humidity in the air. The station model also includes the atmospheric pressure, a three-digit number in the upper right-hand section. This number describes pressure in millibars (mb). The line under the number may be flat or angled. This line shows whether the pressure is rising, falling, or steady. Holt McDougal Earth Science 326 Weather
PLOTTING TEMPERATURE AND PRESSURE Scientists use lines on weather maps to connect points of equal measurement. Isotherms are lines that connect points of equal temperature. Isobars are lines that connect points of equal atmospheric pressure. Scientists use the spacing and shape of isobars to understand the movement of wind, as shown below. Isobar Spacing and Wind Speed Spacing Change in Pressure Wind Speed Close together quick high Far apart gradual low 5. Compare Which would show stronger winds widely spaced isobars or closely spaced isobars? Isobars that form circles show areas of high or low air pressure. High-pressure centers are marked with an H. Low-pressure centers are marked with an L. PLOTTING FRONTS AND PRECIPITATION The table below shows the symbols used to represent fronts on a weather map. Type of Front Warm Cold Occluded Stationary Color red blue purple OR red and blue red and blue Example 6. Draw Next to each front symbol, or on a separate piece of paper, use colored pencils or markers to draw the symbols for the four types of fronts in their correct colors. Weather maps can also show the types and amounts of precipitation in an area. Different weather maps show precipitation in different ways. Some maps use color to show the type of precipitation. Others use symbols to show type of precipitation, and color to show amount of precipitation. How Do Meteorologists Forecast Weather? Meteorologists use computers to mark weather systems on maps. The meteorologists compare the most recent weather map with older maps. This helps them track large weather systems and forecast the weather. Holt McDougal Earth Science 327 Weather
7. Identify Name two weather instruments that provide data for computer models. WEATHER DATA Doppler radar and satellite images give data about weather patterns. Meteorologists put these data into computers and create weather models. These models can show the possible weather conditions for many days. However, the models are based on general patterns. Meteorologists must interpret the models to make accurate forecasts. Meteorologists use many sets of data to make a forecast. Some computer models may be better at predicting certain weather conditions. Other computer models may be better at predicting weather in one area. Meteorologists must compare models to make an accurate forecast. If two or more models give the same information, meteorologists are more certain about the forecast. 8. Identify Which type of forecast predicts tomorrow s weather? TYPES OF FORECASTS Meteorologists make different types of forecasts: Nowcasts use radar to track current weather conditions. Daily forecasts predict weather for a 48-hour period. Extended forecasts look ahead 3 to 5 days. Medium-range forecasts look ahead 3 to 7 days. Long-range forecasts look ahead weeks or months. Meteorologists can make accurate weather forecasts for 0 to 5 days. However, accuracy decreases with each day. Computers help meteorologists make extended forecasts. These computers analyze changes in air movements to predict future weather conditions. Critical Thinking 9. Infer Is severe weather more likely during a severe weather watch or during a severe weather warning? SEVERE WEATHER WATCHES AND WARNINGS Severe weather can destroy buildings and hurt people if they are not ready. For this reason, meteorologists try to forecast severe weather early. Meteorologists may issue, or give, warnings and watches when they forecast severe weather. Meteorologists issue a watch when weather conditions make severe weather possible. They give a warning when they expect severe weather within 24 hours. Meteorologists use these alerts to tell people how to be safe during severe weather. The table on the next page shows some ways to be safe during severe weather. Holt McDougal Earth Science 328 Weather
Severe Weather Safety Tips Type of Weather Thunderstorm Tornado Hurricane Blizzard How to Stay Safe Listen to weather updates. Stay or go inside. Avoid electrical objects, running water, metal pipes, and phone lines. If you are outside, avoid tall objects and bodies of water. Get into a car, if possible. Listen to weather updates. Stay or go inside. Go to a basement or small, inner room or closet with no windows. Avoid areas with flying debris. If you are outside, lie in a low area. Protect your head and neck. Listen to weather updates. Be ready to follow an evacuation route. Stay inside. Avoid areas with flying debris. Listen to weather updates. Stay or go inside. Dress warmly. Avoid walking or driving in icy conditions. 10. Summarize Name two things you should do to stay safe during a tornado. PREPARING FOR SEVERE WEATHER People can prepare for severe weather by having a storm preparedness kit. This kit should include a radio, batteries, flashlights, rain gear, blankets, water, canned food, and medicine. People can also practice safety routes so that they know where to go during severe weather. Can People Control the Weather? Some meteorologists are studying ways to control rain, hail, and lightning. One method for producing rain is cloud seeding. In this process, scientists add certain particles to clouds. These particles cause precipitation to fall from the clouds. Cloud seeding can also prevent severe precipitation. Some scientists have used cloud seeding on possible hail clouds. This caused rain to fall instead of hail. HURRICANE CONTROL Scientists have also seeded hurricanes. The scientists added frozen nuclei to hurricanes to reduce their intensity. This process does not always work. Most scientists have stopped trying to control hurricanes because the technology is limited. 11. Define What is cloud seeding? LIGHTNING CONTROL Some scientists have tried to control lightning. They added silver-iodide nuclei to possible lightning storms. However, scientists are not sure if this process really affects lightning. Holt McDougal Earth Science 329 Weather
Section 4 Review SECTION VOCABULARY station model a pattern of meteorological symbols that represents the weather at a particular observing station and that is recorded on a weather map 1. Explain How do weather stations communicate weather data? Use the term station model in your answer. 2. Organize Complete the concept map below with the terms warning, isotherm, station model, isobar, watch, and weather map. weather forecasts display data on predict storms with severe weather forecasts mark weather conditions with issue 3. Describe How do meteorologists use computer models to forecast weather? 4. List Name three types of weather that meteorologists have tried to control. 5. Make Inferences Why might people want to use cloud seeding to produce rain? Holt McDougal Earth Science 330 Weather