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 regionsdetermines 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; northnorthwest
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/co
ncept_motion/animated_art/ThunderstormsDevelopm
ent13_1.swf
– http://www.wiley.com/college/strahler/0471480533/ani
mations/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_Formai
ton_13.swf
• http://scifiles.larc.nasa.gov/kids/Problem_Board/pr
oblems/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/flas
h.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