Ch 24-Weather

• Objectives– Explain how air mass forms– Main types of air masses– Describe how air masses affect the weather of N

America– Weather patterns of cold and warm fronts– Describe how midlatitude cyclone forms– Development of hurricanes, thunderstorms, tornadoes– Instruments that measure lower atmospheric weather

conditions– Describe how scientists measure conditions in the

upper atmosphere– Explain how computers help scientists understand

weather

• Explain how weather stations communicate weather data

• Explain how a weather map is created

• Explain how computer models help meteorologists forecast weather

• Types of weather that meteorologists have attempted to control

Air Masses

• How does air move?• Formation of air masses

– Air pressure differences are small=air is stationary– Stationary air takes on characteristic temp and

humidity of that region

• Air mass-large body of air throughout which temp and moisture content are similar

• Frozen polar regions=very cold and dry• Tropical=warm and moist

Types of Air Masses

• Classified according to source regions-determines temp and humidity of air mass

• Maritime-air masses that form over ocean– Humidity higher than continental– Maritime polar-moist and cold– Tropical-warm and dry

• Continental-air masses that form over land, dry– Form over large landmasses. N Canada, N Asia, SW

U.S.– Humidity is low, dry weather conditions – Polar-cold and dry– Tropical-warm and dry

N American Air Masses

• Continental tropical-form over desert, hot dry weather in summer, no winter

• Maritime tropical-form over warm water of tropical Atlantic, mild cloudy weather to E U.S. in winter, hot humid weather in summer– Form over Pacific, never reach coast– Winter, maritime tropical air masses bring moderate

precipitation to coast and SW deserts

Polar Air Masses

• Continental polar Canadian- Continental air masses form over ice and snow covered land, move into U.S. and can reach Gulf Coast

• Summer, cool and dry weather• Winter, very cold weather to N U.S.• Maritime Polar Pacific-Very moist during winter

on P coast, cool and foggy in summer• Maritime Polar Pacific- Cold, cloudy and snow to

New England and E Canada. Summer produces cool weather w/ low clouds and fog

Air Masses Of N America

Air Mass Source Location

Movement Weather

cP Polar regions in Canada

South-southeast Cold and dry

mP Polar pacific; polar Atlantic

Southeast; southwest-south

Cold and moist

cT U.S. southwest North-northeast Warm and dry

mT Tropical Pacific; tropical Atlantic

Northeast; north-northwest

Warm and moist

Sec 2-Fronts• Front-boundary between air masses. Cool air mass is dense,

does not mix w/ less dense air of warm air mass• Do not exist in tropics. Why?• Types of fronts

– Cold fronts-front edge of moving mass of cold air pushes beneath warmer air mass like a wedge

• Cumulus and Cumulonimbus• Storms short lived, violent• Squall• http://www.mesoscale.iastate.edu/agron206/animations/05_cnWfronts.h

tml– Warm-front edge of advancing warm air mass replaces colder air w/

warmer air• Precipitation over large area, violent weather• http://ww2010.atmos.uiuc.edu/(Gh)/guides/crclm/act/gifs/fpr2.gif

– Stationary front– Occluded front– Fronts

Midlatitude Cyclone

• Polar front-cold polar air meets tropical air mass of middle latitudes, over ocean– Wave-bend that forms in cold front or stationary front– Wave cyclones-low pressure storm centers when

waves are the beginnings

• Mid latitude cyclones-areas of low pressure, characterized by rotating wind that moves toward the rising air of central low pressure region– Strongly influence weather patterns in middle latitudes

• Stages of Mid latitude cyclone– Lasts several days, 45 km/h in easterly

direction, spin counterclock wise in N America– Move from P coast to A coast, loose moisture

and energy over mts• Anticyclones-sinks and flows outward from

a center of high pressure– Circulation is clockwise. Why?– Bring dry weather, sinking air does not

promote cloud formation– Air pollution. Why?– May cause droughts. Why?– http://www.mesoscale.iastate.edu/agron206/a

nimations/12_CycAntCyc.html

Thunderstorms

• Usually brief, heavy storm that consists of rain, strong winds, lightning, and thunder

• 3 stages– 1st water vapor condenses to form cumulus clouds– Mature stage-Cumulonimbus clouds form, heavy rain

and hailstones may fall– Dissipating stage- thunderstorm dissipates, lightning

and thunder heat the air– http://glencoe.com/sec/science/earthscience/2007/

concept_motion/animated_art/ThunderstormsDevelopment13_1.swf

– http://www.wiley.com/college/strahler/0471480533/animations/ch06_animations/index.html

Hurricanes

• Develops over tropical oceans, winds more than 120 km/h, spiral in toward the intensely low pressure storm center

• Thick cumulonimbus clouds, spiral upward toward center of storm

• Eye of storm=calmest• Saffir-Simpson scale-central pressure, wind

speed, storm surge, 5 categories• http://glencoe.com/sec/science/earthscience/

2007/concept_motion/NGS/Visualizing_Cyclone_Formaiton_13.swf

• http://scifiles.larc.nasa.gov/kids/Problem_Board/problems/weather/hurricanebasics.swf

Tornadoes• Smallest, most violent and shortest lived storm• Destructive, rotating column of air, very high wind

speeds, visible as funnel shaped cloud• Forms when thunderstorms meet high altitude,

horizontal winds cause rising air in thunderstorm to rotate

• Narrow, funnel shaped extension may touch ground, may not

• Tornado Alley• What causes main destructive power?

– Winds-400 km/h– http://glencoe.com/sec/science/earthscience/2007/

concept_motion/animated_art/TornadoFormation13_10.swf

– http://www.usatoday.com/weather/graphics/tornadoes/flash.htm

Sec 3-Weather Instruments

• Measuring weather conditions– Air temp-thermometer uses mercury or

alcohol, electrical thermometer, thermistor– Air pressure-barometers– Wind speed-anemometer– Wind direction-wind vane– Temp, dew pt, wind velocity-radiosone– Velocity and location of objects-radar– Weather satellites

Computers

• Help solve mathematical equations and describe behavior of atmosphere

• Can store weather data from around world

• Helpful in forecasting weather changes, store weather records for quick retrieval

Sec 4-Forecasting the Weather

• When do you think predicting weather forecasts came about?– 1844 by telegraph

• National Weather Service– Shares info about weather conditions – Exchanges weather data w/ other nations

• Global Weather Monitoring– How do weather stations communicate weather data?– Reported hourly by recording barometric pressure, speed and

direction of surface wind, precipitation, temp, and humidity• World Weather Watch

– Exchanges weather info in developing countries– Helps establish or improve meteorological services

Weather Maps

• Use data collected by weather stations and transfer to weather maps

• Use symbols and colors• Cloud cover, wind speed, wind direction,

precipitation, storm activities, air temp, dew pt, atmospheric pressure

• Plot temp and pressure by using isotherms which are lines that connect pts of equal temp. Isobars=atmospheric pressure

• Fronts=sharp changes in wind speed and direction, temp or humidity

• Precipitation=colors or symbols

Weather Forecasts

• Plot intensity and path of weather systems on maps• Weather data-computer models show possible weather

conditions for several days• Temp, wind direction, wind speed, cloudiness and

precipitation-accurately• Types of forecasts

– Nowcasts– Daily forecasts– Extended forecasts– Medium forecasts– Long range

• Watch-issued when conditions are ideal for severe weather

• Warning-severe weather has been spotted or is expected w/in 24 hrs

Controlling the Weather

• Cloud seeding– Most researched method for producing rain– How does this work?

• Hurricane control– Seed w/ freezing nuclei in effort to reduce intensity of

storm– Not attainable w/ existing technology

• Lightning control– Seed potential lightning storms w/ silver iodide nuclei– No conclusive results