1. Weather
Chapter 21

2. Air Masses

3. How Air Moves Areas of high pressure to areas of low pressure
Temperature and pressure differences on the surface create three wind belts in the Northern Hemisphere and three wind belts in the Southern Hemisphere
The Coriolis effect makes these winds rotate

4. Air Mass: a large body of air throughout which temperature and moisture are similar

5. Formation of Air Masses
If pressure differences are small, then the air will move slowly or remain stationary. If this happens, then the air will take on the characteristics (temperature and humidity) of that region.

7. Types of Air Masses
Classified according to its source. (where it formed)
Source for cold air masses is polar
Source for warm air masses is tropical
Air masses that form over oceans are called maritime
Air masses that form over land are called continental

8. Air masses affecting North America

9. 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; Mexico
North-northwest
Warm and dry mT
Tropical Pacific; tropical Atlantic
Northeast; north-northwest
Warm and moist A
Arctic; northern Canada
Very cold and dry

16. Section 2: Fronts
Cold, dense air masses don’t mix well with less-dense, warm air masses.
Thus a boundary is formed between the two. This is called a front.
Why are there no fronts in the tropics?

17. Fronts form when one air mass collides into another air mass

18. Types of Fronts: Cold Front
Cold air mass overtakes a warm air mass.
The moving cold air lifts the warm air. If the warm air is moist, clouds form.
Why do fast moving cold fronts usually produce heavy thunderstorms while slow moving cold fronts do not?
A long line of heavy thunderstorms is called a squall line

20. Squall line circled in red

21. Warm front Warm air mass overtakes a cold air mass
The less dense warm air rises over cooler air.
The slope of the warm front is gentle and gradual. Because of this, clouds may extend far ahead of the front and usually produce less violent weather

24. Stationary Front
A front in which air masses move very slowly or not at all

26. Occluded front
Fast moving cold front overtakes a warm front and lifts the air completely off the ground

27. Front map symbols

28. Polar front: the boundary where very cold air near the poles meets tropical air masses near the equator
Waves develop along these fronts. Waves are bends that form in cold or stationary fronts

29. Mid-latitude cyclone
The waves are the beginning of mid-latitude cyclones which are areas of low pressure that are characterized by rotating wind that moves toward the rising air

30. Stages of a mid-latitude cyclone

31. Anticyclone Air sinks and flows outward from a center of high pressure
Bring dry weather because of the sinking air.

33. Severe Weather Weather that may cause property damage or loss of life
Includes: heavy rain, lightning, hail, strong winds, tornadoes

34. Thunderstorms
A heavy storm that is accompanied by rain, thunder, lightning, and strong winds
There are three stages

35. Stage 1: Cumulus Stage
Warm, moist air rises, and water vapor within the air condenses to form a cumulus cloud. Latent heat causes the air to continue to rise adiabatically.

36. Stage 2: Mature Stage
Dark, cumulonimbus cloud forms as air continues to rise. Has updrafts and downdrafts capable of producing heavy rain and hail.

37. Stage 3: Dissipating Stage
Updrafts dissipates due to the strong downdrafts. No more latent heat is being released. The cloud cools and disappears.

39. Lightning Lightning is discharged electricity from clouds
The released electricity heats up the air and expands rapidly. This produces the loud sound called thunder.

40. Causes of lightning
In order for a cloud to discharge electricity, it must carry distinct electrical charges. The upper part of a cloud usually carries a positive charge (the rising water droplets) while the lower part carries a negative charge (the heavier ice particles)

42. Lightning strikes the ground when the negative charges in the cloud are attracted to the positive charges on the ground.

45. Stepped leader

48. Weather Instruments
Weather observations are based on measurements from atmospheric pressure, humidity, temperature, wind speed, and precipitation

49. Measuring Lower Atmospheric Conditions
Lower atmospheric conditions change drastically

50. Air Temperature
Measured by thermometers usually in a liquid form such as mercury or alcohol
Rising temperature causes the liquid to expand and rise while cooler temperatures causes the liquid to contract and drop.
Electrical thermometers measure the flow of electricity through a material

51. Air Pressure
The approach of a front is usually indicated by a drop in air pressure.
Barometers measure atmospheric pressure

52. Wind Speed and Wind Direction
Anemometer measures wind speed
Wind vanes measure wind direction

53. Measuring Upper-Atmospheric Conditions
Surface conditions are only part of the picture. The upper atmosphere gives a better understanding of local and global patterns

54. Radiosonde
An instrument package carried by a helium-filled balloon that measures relative humidity, air pressure, and air temperature

55. Radar: Radio Detection And Ranging
Uses radio waves to determine velocity and location of objects

56. Weather Satellites
Provides weather information where observations cannot be made on the ground
There are many types of images satellites can provide

57. Infrared: images made at night that reveal temperatures at the top of clouds

58. Visible Imagery: taken during the day

59. Water Vapor Imagery: combines Infrared and Visible to determine moisture

60. Computers
The use of supercomputers are valuable because they help meteorologists understand and interpret weather data and for forecasting.
Computers also store weather information for quick access

62. Forecasting Weather
Weather maps allow scientists to understand current weather and to predict future weather events.

63. Plotting Temperature and Pressure
Scientists use lines on weather maps to connect points of equal measurements.
Lines of equal temperature are called isotherms
Lines of equal pressure are called isobars
Closer spaced lines indicate a rapid change. Widely spaced lines indicated a more gradual change
H represents high pressure. L represents low pressure

64. Isotherm weather map

65. Isobar weather map

67. Weather Forecasts
Forecasting the weather involves using supercomputers to plot the intensity and path of weather systems on maps.
Studying the current maps with previous maps allow for more accurate weather forecasts.
Meteorologists use different computer models to analyze weather data. Some are better at predicting certain types of weather than others.
Temperature, wind direction, wind speed, cloudiness, and precipitation are more accurate than the timing of the event.

69. Types of Forecasts: Nowcasts
Use radar and enable forecasters to focus on timing precipitation and tracking severe weather

71. Daily Forecasts: predict weather conditions for a 48 hour period

72. Extended Forecasts: looks ahead 3 to 5 days

73. Medium-range forecast: 3 to 7 days

74. Long-range forecasts: 7 or more days ahead; sometimes monthly or season

75. Severe Weather watches and warnings
A primary goal in meteorology is to reduce the amount of destruction caused by severe weather.
Meteorologists use watches and warnings to help
Watch: conditions are ideal for severe weather
Warning: severe weather has been spotted or is expected within 24 hours