Chapter 21, guest starring Ch. 20, Section 2 Vocabulary 1. Coriolis effect 21. Stratocumulus 41. Cumulus stage 2. Trade winds 22. Cirrus clouds 42. Mature stage 3. Westerlies 23. Cirrocumulus 43. Dissipating stage 4. Easterlies 24. Cirrostratus 44. Lightning 5. Doldrums 25. Crepuscular rays 45. Thunder 6. Horse latitudes 26. Fog 46. Hurricane 7. Air mass 27. Radiation fog 47. Latent heat 8. Continental polar air mass 28. Advection fog 48. Storm surge 9. Continental tropical air mass 29. Upslope fog 49. Saffir-Simpson scale 10. Maritime polar air mass 30. Steam fog 50. Tornado 11. Maritime tropical air mass 31. Front 51. Barometer 12. Cloud 32. Cold front 52. Anemometer 13. Troposphere 33. Warm front 53. Wind vane 14. Advective cooling 34. Squall line 54. Radar 15. Stratus clouds 35. Stationary front 55. Station model 16. Nimbostratus 36. Occluded front 56. Isotherms 17. Altostratus 37. Polar front 57. Isobars 18. Cumulus clouds 38. Midlatitude cyclones 58. Watch 19. Cumulonimbus 39. Anticyclones 59. Warning 20. Altocumulus 40. Thunderstorm 60. Cloud seeding

I. Air Masses A. Air Movement 1. Uneven heating of Earth’s surface causes differences in air pressure a. Equator gets more solar energy than poles b. Heated air rises – creates low-pressure system c. Cold air sinks – creates high-pressure system 2. Air moves from high-pressure areas to low-pressure areas a. So surface air usually moves from poles to equator b. At high altitudes, air moves from equator to poles 3. Differences in temp & pressure create wind belts a. 3 wind belts in each hemisphere (Northern & Southern) i. Trade Winds: 0° - 30° ii. Westerlies: 30° - 60° iii. Easterlies: 60° - 90° b. Wind belts are influenced by the Coriolis effect – Earth’s rotation causes wind deflection c. Doldrums: low-pressure zone at equator (0°) d. Horse latitudes: high-pressure zones at 30° N and S B. Formation of Air Masses 1. Air mass: large body of air with nearly uniform conditions of temperature and moisture content 2. Temperature of air masses usually depend on the climate where they form a. Polar regions have cold, dry air masses b. Tropical oceans have warm, wet air masses C. Types of Air Masses 1. Classified according to where they form 2. Take on characteristics of areas where they form 3. Continental air masses develop over land & usually have low humidity – bring dry conditions when they travel 1. Continental tropical (cT): form over warm, dry areas of land (southwest U.S.) 2. Continental polar (cP): form over cold, dry areas of land (polar Canada) 4. Maritime air masses develop over water & have high humidity – bring precipitation & fog when they travel 1. Maritime tropical (mT): form over warm, wet areas (tropical Pacific & Atlantic) 2. Maritime polar (mP): form over cold, wet areas (polar Pacific & Atlantic) D. Air Masses in North America 1. Continental air masses a. Continental tropical develop over deserts in southwest U.S. i. Bring hot, dry weather in summer; don’t form in winter b. Continental polar develop over northern Canada where it’s covered with snow/ice & move into the northern U.S. i. Bring cool, dry weather in summer; very cold weather in winter (polar vortex!) 2. Maritime air masses a. Maritime tropical develop in warm areas of Atlantic, Caribbean, & Gulf of Mexico, then travel to eastern U.S. i. Bring hot, humid weather in summer; mild, cloudy weather in winter b. Maritime polar develop in north Pacific & move into northwest U.S. i. Bring cool, foggy weather in summer; rain & snow in winter ii. Sometimes dry out as they move over mountains in western U.S., bringing cool & dry weather to central U.S. Fronts  See Interactive Reading Packet Clouds (Chapter 20, section 2) A. Cloud: collection of small water droplets or ice crystals suspended in the air; forms when air is cooled & condensation occurs 1. Form in troposphere: lowest layer of the atmosphere, about 7 miles thick B. Advective Cooling: process by which temperature of an air mass decreases as it moves over a cold surface; causes air to cool & form clouds C. Types of Clouds 1. Stratus Clouds: found 0 – 6500 ft. above Earth’s surface a. Have a flat, uniform base b. Cover large areas of sky, may block out sun c. Rarely produce precipitation d. Nimbostratus: dark gray, produce heavy precipitation e. Altostratus: type of stratus cloud that occurs at middle altitude; usually doesn’t produce precipitation 2. Cumulus Clouds: vertical clouds with dark bottoms & cotton-y looking top a. Form when warm, moist air rises & cools b. Height depends on stability of troposphere

a. Tallest clouds form on hot, humid days c. Cumulonimbus: high, dark storm clouds; produce thunder, rain, lightning, & other severe weather d. Altocumulus: mid-altitude cumulus clouds; thunderstorms may follow e. Stratocumulus: low-altitude clouds that are combination of stratus & cumulus 3. Cirrus Clouds: thin, feathery clouds formed above 20,000 ft. above surface a. Composed of ice crystals b. Usually indicate fair or pleasant weather c. Cirrocumulus: high-altitude, small puffy clouds; indicate upcoming snowfall or rainfall d. Cirrostratus: long, thin clouds covering most of sky; may cause halos! 4. Crepuscular rays: rays of sunlight that stream through gaps in clouds, especially stratocumulus 5. Fog: water vapor condensed near Earth’s surface; essentially clouds that form on the ground a. Radiation fog: forms from nightly cooling of earth; thickest in valleys & low elevations b. Advection fog: forms when warm, moist air moves across a cold surface; common along coasts & over oceans c. Upslope fog: forms by lifting & cooling of air as air rises over land slopes (hills, mountains, etc.) d. Steam fog: shallow layer formed when cool air moves over a warm body of water (rivers) III. Weather Instruments A. Lower Atmosphere Weather 1. Thermometer: indicates & measures temperature a. Usually uses mercury or alcohol in a sealed glass tube b. Rise in temp. causes liquid to expand & fill more of the tube c. Electric thermometers are based on electrical currents that increase with temperature 2. Barometer: measures air pressure, can indicate fronts approaching

3. Anemometer: measures wind speed

a. Small cups are attached to a rotating base -- wind pushes the cups & causes the base to rotate, which triggers an electrical signal to register wind speed

b. Used at airports to determine flight conditions 4. Wind vane: determines wind direction

a. Arrow-shaped device rotates as tail catches the wind

b. Described using compass directions B. Upper Atmosphere Weather 1. Radiosonde: group of instruments carried by balloons to measure conditions in the upper atmosphere a. Sends measurements as radio waves b. Path of balloon is tracked to determine speed & direction of winds 2. Radar: uses reflected radio waves to determine the velocity & location of objects a. Can track precipitation & thunderstorms through how radar pulses reflect off of water particles 3. Satellites are used to monitor weather that cannot be tracked from the ground 4. Computers store weather data, solve complex equations about atmospheric tendencies, and provide more reliable forecasts

IV. Forecasting Weather A. Global Weather 1. Monitored by the World Meteorological Organization a. World Weather Watch enables the rapid exchange of weather data around the world b. Helps establish & improve forecasting in developing countries c. Offers insight about how weather impacts human activities 2. Weather Maps a. Used to communicate a lot information at once b. Station model: group of symbols indicating weather conditions at a given location i. Cloud cover, wind speed, wind direction, dew point, temperature, atmospheric pressure, etc. c. Different types of lines are used on weather maps to indicate conditions i. Isotherms: connect points of equal temperature ii. Isobars: connect points of equal atmospheric pressure 3. Forecasts a. Weather data gathered from radar, satellites, and other instruments are input into computer models to create forecasts b. Meteorologists compare models to create more reliable predictions c. Relatively easy to determine temperature, pressure, wind speed & direction, but can be difficult to anticipate precise timing & amount of precipitation d. Severe weather predictions have different levels: i. Watch: conditions are ideal for severe weather ii. Warning: severe weather has been spotted or is expected within 24 hours 4. Controlling Weather

a. Scientists are currently investigating if there are ways to control rain, hail, and lightning

b. Cloud seeding: produces rain through adding particles to clouds that cause them to produce precipitation

i. Can be used to prevent more severe precipitation, like cause rain instead of hail

c. Attempts at hurricane control have been generally unsuccessful

d. It is possible to generate artificial lightning, but there is no conclusive information about modifying preexisting lightning