Weather Patterns Earth Science Chapter 17
Air mass • Huge body of air with similar temperature, humidity, and air pressure at any height • Can be huge • Classified by temperature and humidity • Tropical or polar • Temperature • Maritime or continental • humidity Earth Science chapter 17
Maritime Tropical • Warm and humid • Form over tropical oceans • Then move across land • Hot and humid in summer • Bring heavy rain or snow in winter Earth Science chapter 17
Maritime Polar • Cool and humid • Form over polar oceans • Affect west coast more than east coast • Bring fog, rain, and cool temperatures Earth Science chapter 17
Continental Tropical • Hot and dry • Form over dry land areas – southwest and Mexico • Smaller than other air masses • Bring hot, dry weather Earth Science chapter 17
Continental Polar • Cool and dry • Form over Canada and Alaska • Can bring bitterly cold and dry weather in winter • Milder in summer • Cause storms when colliding with maritime tropical masses Earth Science chapter 17
Air mass movement • Caused by prevailing westerlies and jet streams (in US) • Prevailing westerlies push from west to east • Jet streams push from west to east faster Earth Science chapter 17
Fronts • Air masses don’t easily mix • Front – boundary where air masses meet Earth Science chapter 17
Discuss • What two characteristics are used to classify air masses? • What are the four types of air masses? • What is a front? Earth Science chapter 17
Cold fronts • Rapidly moving cold air mass runs into a slowly moving warm air mass – from opposite directions • Denser cold air slides under • Warm air pushed up and cooled • Causes water vapor to condense • Clouds form • Can cause heavy rain or snow • Cause abrupt weather changes • After front moves through, usually clear skies, cooler, and wind shifts Earth Science chapter 17
Warm fronts • Fast-moving warm air mass overtakes a slowly moving cold air mass • Moving same direction • Warm air moves over • Light rain or snow • Cloudy skies • After front usually warm and humid Earth Science chapter 17
Stationary fronts • Warm and cold meet, but don’t move each other • From opposite directions • Rain, snow, fog, or clouds • Can last many days Earth Science chapter 17
Occluded fronts • Most complex • Warm air mass caught between two cold air masses • Warm air moves up • Cold air masses may mix • Surface temperature becomes cool • Cloudy, rain, or snow Earth Science chapter 17
Cyclones • Air at a frontal boundary gets distorted, bends and swirls • Caused by mountains or jet stream • Swirling area of low pressure • Winds spiral in • Spin counterclockwise in northern hemisphere • Clouds, wind, and precipitation Earth Science chapter 17
Anticyclones • High pressure and dry air • Winds spiral out • Spin clockwise in northern hemisphere • Dry, clear weather Earth Science chapter 17
Discuss • Name the four types of fronts and they types of weather each one brings. • Compare and contrast cyclones and anticyclones. • What type of front would most likely be responsible for several days of rain and clouds? Earth Science chapter 17
Storm • Violent disturbance in atmosphere • Sudden changes in air pressure • Rapid air movements Earth Science chapter 17
Thunderstorms • Small storm with heavy precipitation and frequent lightning and thunder • Form in cumulonimbus clouds (thunderheads) • Heavy rain, maybe hail Earth Science chapter 17
Lightning and thunder • Lightning • Sudden spark between areas of different electrical charge • Within a cloud • Cloud to cloud • Cloud to ground • Thunder • Lightning heats air rapidly and makes it expand and explode Earth Science chapter 17
Thunderstorm damage • Flooding • Overloaded rivers/streams • Saturated ground • Urban areas with little access to ground • Lightning damage • Hail damage Earth Science chapter 17
Thunderstorm safety • Safer inside • Avoid touching electrical appliances or plumbing • Avoid metal or bodies of water • If outside, find a low area away from anything tall Earth Science chapter 17
Tornadoes • Rapidly whirling, funnel-shaped cloud • Usually brief, but can be deadly • Wind speeds up to 500 km/hr • Most often form in thunderheads when cold air and warm air meet Earth Science chapter 17
Tornado damage • Strong winds and flying debris • Low pressure sucks object up into cloud • May deposit many miles away • Unpredictable path • Tornadoes ranked from F0 to F5 by damage Earth Science chapter 17
Tornado safety • Tornado watch – conditions are favorable for tornado formation • Watch for thunderstorms and stay near radio, TV or internet for weather info • Tornado warning – a tornado has been seen • Get to a safe place ASAP • Basement of a well-built building • Middle of ground floor, away from windows and doors • Lie flat in a ditch Earth Science chapter 17
Discuss • What weather conditions are most likely to produce tornadoes? • Why do tornadoes occur most often in “tornado alley”? • What safety precautions should be taken in a thunderstorm? • What safety precautions should be taken in a tornado? Earth Science chapter 17
Hurricane • Tropical cyclone with winds of 119 km/hr or higher • About 600 km across Earth Science chapter 17
Hurricane formation • Forms as a low-pressure area over warm ocean water • Warm air from water rises and forms clouds, draws more air up • Has bands of very high winds and heavy rains • Strongest winds around calm “eye” Earth Science chapter 17
Hurricane motion • Last longer than most storms – a week or more • Form in Atlantic and move towards Caribbean and southeastern US – carried by trade winds • Lose energy when hit land – no more warm, moist air to draw up • Can cause several days of heavy rain over land Earth Science chapter 17
Hurricane damage • High waves, severe flooding, and wind damage • Low pressure and high winds raise ocean level up to six meters • Storm surge – dome of water that hits coast where hurricane lands • Major flooding and erosion Earth Science chapter 17
Hurricane safety • Hurricane watch – possible within 36 hours • Prepare to evacuate • Hurricane warning – expected within 24 hours • Leave area immediately if told to evacuate • If you stay in a house, move away from windows Earth Science chapter 17
Winter storms • Precipitation falls as snow if the air is below freezing all the way down to the ground • Heavy snow – can block roads • Extreme cold – can damage crops and freeze pipes • High winds – reduce visibility and break trees and power lines • For safety – find shelter from wind and try to stay dry Earth Science chapter 17
Lake – effect snow • Falls south and east of Great Lakes • Cold dry air moves across warmer water and gets humid • Moves across land, cools, and can’t hold all the humidity Earth Science chapter 17
Discuss • How do hurricanes form? • What is lake-effect snow? • What should you do if you are caught in a snowstorm? Earth Science chapter 17
Weather Forecasting • First, make observations and collect data • Simple observations • Watch the clouds • Cumulus clouds growing taller may mean a thunderstorm • High thin cirrus clouds may mean an approaching warm front Earth Science chapter 17
More complex data • Meteorologists use • Local observers • Radar • Instruments in balloons, satellites, and global weather stations • Mostly from national weather service Earth Science chapter 17
Weather Technology • Weather balloons • Carry instruments into lowest layers of atmosphere • Measure temperature, air pressure, and humidity • Weather satellites • Orbit in uppermost atmosphere • Send images back to Earth • Can show temperature, humidity, solar radiation, wind speed, and wind direction Earth Science chapter 17
More Weather Technology • Automated weather stations • Surface locations • Temperature, air pressure, relative humidity, rainfall, wind speed, and wind direction • Computer forecasts • Process large amounts of data quickly • Make predictions using mathematical models Earth Science chapter 17
Discuss • What tools do meteorologists rely on to forecast the weather? • How does the large amount of weather data gathered by various modern technologies affect the accuracy of weather forecasts? • What types of weather information is gathered by satellites? Earth Science chapter 17
Weather Service maps • Assemble data from all over the country • Show temperature, wind speed and direction, fronts, air pressure • Isobars – lines connecting areas with the same air pressure • Isotherms – lines connecting areas with the same temperature Earth Science chapter 17
Newspaper Weather Maps • Simpler than weather service maps • Use standard symbols Earth Science chapter 17
Limits of weather forecasts • A small change in one part of the atmosphere can cause a large change in the forecast a few days later • The butterfly effect Earth Science chapter 17
Doppler Radar • Uses the Doppler effect • Uses changes in radio waves as they reflect off moving objects • Radio waves bounce off particles in the air, like raindrops, snowflakes, hail, and dust Earth Science chapter 17
Doppler radar effectiveness • Can give people more advance warning for storms like tornadoes • Can be blocked by mountains or buildings • Doesn’t always pick up drizzle • Dust can interfere with it • Meteorologists must use more than just Doppler to forecast the weather Earth Science chapter 17
Discuss • According to the weather map on page 604, what is the weather like in Chicago? How might it change in a few hours? Earth Science chapter 17
