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LESSON 20 TEKS: 3.8A, 4.8A, 5.2G, 5.3D, 5.8A Weather Conditions The words in bold are vocabulary words that the student should be aware of. When you turn on a TV or listen to a radio, you can hear a weather forecast. You can look up what the weather will be like in your area on the internet also. How do weather scientists know if it is likely to rain or snow or be windy tomorrow? They use tools to measure temperature, precipitation, and more. These measurements help scientists understand what the weather is now, and how it might change in the future. Temperature Weather is the condition of the atmosphere at a certain time and place. One factor that influences weather is air temperature. A meteorologist is a scientist who studies weather. Meteorologists use thermometers to measure the temperature of the outside air. Air temperature is a measure of how hot or cold the air is. Weather forecasts usually report temperature in degrees Fahrenheit ( ͦF). Precipitation Precipitation is water that falls from the atmosphere as rain, snow, hail, and sleet. You can use a rain gauge to measure the amount of rain that falls. This tool is a can or tube with an opening on top. Rain falls into the rain gauge. Marks and numbers on the side show how much rain has collected in the rain gauge. Snow is measured with a snow gauge, a tool similar to a rain gauge. Humidity Humidity is another important factor in describing weather. Humidity is the amount of moisture, or water vapor, in the air. During the summer, the air feels more humid because warm air can hold more moisture than cold air. During the winter, when temperatures are colder, will the humidity be more or less than during the summer? Explain why. __________________________________________________________ Wind Wind is moving air. Air moves because of differences in temperature and humidity. Weather tools can measure two properties of wind – direction and speed. A wind vane, or weather vane, shows wind direction – where the wind is coming from. A wind vane spins freely in the wind. It points in the direction from which the wind is blowing. If a wind vane points east, the wind is blowing from the east. An anemometer measures wind speed – how fast the wind is blowing. This tool has several arms that stick out from a central rod. Each arm has a cup at its end. Wind blows into the cups and spins the anemometer. The anemometer counts how many times it spins in a given period of time. The faster the wind is blowing, the faster the anemometer spins. Air Pressure Air pressure is the weight of the atmosphere pressing in all directions at a certain place. Air pressure can change because air particles can move farther apart or closer together. Cold air is dense. Dense means that the particles are close together. Cold air sinks because it is dense. Warm air rises because it is less dense. When air rises, it can create an area of low pressure. When air sinks, it can create an area of high pressure. Air moves from areas of high pressure to areas of low pressure. The movement of air from high pressure to low pressure areas is wind. Weather scientists use barometers to measure air pressure. Air pressure information is important for forecasting the weather. Changes in air pressure mean that the weather is about to change. Low air pressure usually means rainy or stormy weather. Suppose the weather is fair but the readings on a barometer are falling. Stormy weather is probably coming. High air pressure usually means fair weather. If the air pressure is rising, sunny weather is most likely on the way. Are the particles of warm air close together or far apart? ______________________________________________________________ Weather and Climate Weather changes from day to day. It also changes from season to season. But weather patterns in an area tend to repeat over many years. A pattern is a repeated set of changes. Climate is the average weather of an area over a long period of time. Scientists use weather data gathered over many years to describe an area’s climate. Temperature is a major factor in determining an area’s climate. The North and South Poles receive little direct sunlight, so those areas have cold polar climates. The equator receives a lot of direct sunlight, so that area has a very warm climate. Precipitation is another major factor in determining an area’s climate. Places near large bodies of water tend to receive a lot of precipitation. Those areas have wet climates. Places such as deserts, don’t receive very much precipitation so they have dry climates. What are two main factors that determine an area’s climate? ______________________________________________________________________________ How are weather and climate different? ______________________________________________________________________________ ________________________________________________________________________ A student wants to know how fast the wind is blowing. What tool should the student use? ______________________________________________________________________________ Information on Weather Maps Weather maps show what the weather is like in an area at a certain time. A weather map of the United States is shown below. Studying weather maps helps scientists make predictions about what the weather will be in the future. A prediction is a statement about what is likely to happen in the future. Weather maps show temperature, precipitation, and other types of weather data. The key tells you what each symbol means. The numbers on the map show temperature in degrees Fahrenheit. Weather maps help scientists follow the movement of air masses. An air mass is a large body of air with the same temperature and humidity throughout. Humidity is the amount of water vapor in the air. Humid air contains a lot of water vapor. An air mass can cover a very large area. There are four basic types of air masses – cold and dry, cold and humid, warm and dry, and warm and humid. Weather maps often show air pressure. You may see circles with H’s or L’s on a weather map. The H stands for “high” and indicates an area of high air pressure. The L stands for “low” and indicates an area of low air pressure. High pressure areas usually bring fair weather. Low pressure areas usually bring storms. Weather maps also show fronts. A front is a place where two different air masses meet. On a weather map, triangles show a cold front. Half circles show a warm front. The triangles and half circles point in the direction that the front is moving. At a cold front, a cold air mass bumps up against a warm air mass. Strong storms happen at cold fronts. Thunderstorms are likely along cold fronts in warm weather. In winter, snowstorms are likely along cold fronts. When a cold front passes over an area, the temperature drops. At a warm front, a warm air mass meets a colder air mass and slowly rides up over it. Sheets of gray clouds form at warm fronts. They often bring rain. After a warm front passes over an area, the temperature rises. A stationary front forms when two air masses meet and stop moving. Clouds and precipitation form at a stationary front. COLD FRONT WARM FRONT Weather Patterns in the United States In the United States, weather usually moves from west to east. Rain that forms over the Pacific Ocean moves eastward over California. Rainstorms that form over the middle of the country often move to the east coast within a day or two. The changing daily weather is due to the movement of air masses. Air masses behave in ways that can be predicted. When a cold, dry air mass bumps into a warm, moist air mass, thunderstorms or snowstorms can occur. When a warm air mass rises up over a cold air mass, the clouds that form can bring rain. Weather forecasters watch air masses across the country. Tracking the air masses helps them predict the weather for the next few days. QUESTIONS 1. What does a rain gauge do? A. Tells what chemicals are in rain B. Measures how fast rain falls C. Measures the amount of rain that falls D. Measures the size of raindrops 2. What is an area’s average weather over a long period of time called? A. B. C. D. Seasonal weather pattern Daily weather pattern Climate Precipitation 3. Which tool measures air pressure? A. B. C. D. Barometer Anemometer Wind vane Thermometer 4. Which would a meteorologist MOST LIKELY study? A. B. C. D. Weathering Wind direction Mixtures Electricity 5. Mrs. Smith checks the internet weather website every morning to see how she should dress for work. She also wants to know if she will need an umbrella. She is checking the – A. B. C. D. Climate Pattern Weather Forecast 6. On Monday, the rain gauge showed 2 cm of precipitation collected. On Wednesday, the rain gauge showed that 1 cm of precipitation had fallen. On Thursday, the rain gauge showed 3 cm of precipitation collected. What is the total amount of precipitation that had fallen during the week? _________________cm. 7. What does a key on a weather map tell you? A. B. C. D. What today’s temperature is What symbols on the map mean Where mountains are located How to predict the weather 8. What does the letter H on a weather map show? A. B. C. D. An area of high air pressure A high front An area of high mountains An area of high temperature 9. What is a front? A. B. C. D. A place where air masses meet The front part of a storm An area of very high wind An area that has fair weather 10. What does the “triangle” symbol stand for on a weather map? A. B. C. D. Stationary front Warm front Warm, dry air mass Cold front 11. When you use a weather map to make a prediction, you are acting like – A. B. C. D. An anemometer A geologist A meteorologist A paleontologist 12. A student uses a wind vane and records data for three days. What data is the student collecting? A. B. C. D. Precipitation Wind speed Temperature Wind direction