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 Thunder 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 Lightning damage Hail damage 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 Weather satellites 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