1. Weather Patterns & Severe Storms
Unit 6 Part 3

2. What is an air mass?
How do air masses affect you?
Can you predict the outcome of two air masses meeting?
Can you explain how air masses are classified?
How are cP and cT similar? How are they different?

3. Air Masses Characteristics Large body of air
1600 km (1000 mi) or more across
Several km thick
Similar temperatures at any given altitude
Similar moisture at any given altitude
Move and affect large part of continent

4. Air Masses
Source region – the place where an air mass acquires it’s properties

5. Air Masses Classification of an air mass
Two criteria are used to classify
By the nature of the surface in the source region
Continental (c)
Form over land
Likely to be dry
Maritime (m)
Originate over water
Humid air

6. Air Masses By the latitude of the source region Polar (P)
High latitudes
Cold
Tropical (T)
Low latitudes
Warm

7. Air Masses Four basic types of air masses Continental polar (cP)
Continental tropical (cT)
Maritime polar (mP)
Maritime tropical (mT)

8. Air Masses

9. Air Masses Air masses and Weather
cP and mT air masses are the most important in North America, especially east of the Rockies

10. Air Masses North America (east of the Rockies) Continental polar
From northern Canada and interior of Alaska
Winter – brings cold, dry air
Summer – brings cool relief
Responsible for Lake effect snows
cP air mass crosses the Great Lakes
Air picks up moisture from the lakes
Snow occurs on the leeward shores of the lakes

11. Air Masses Maritime tropical
From the Gulf of Mexico and the Atlantic Ocean
Warm, moist, unstable air
Brings precipitation to the eastern U.S.

12. Air Masses Continental tropical Southwest and Mexico Hot, dry
Seldom important outside the source region

13. Air Masses Maritime Polar
Brings precipitation to the western mountains
Occasional influence in the northeastern U.S. causes the “northeaster” in New England with its cold temperatures and snow

14. What is an air mass?
How do air masses affect you?
Can you predict the outcome of two air masses meeting?
Can you explain how air masses are classified?
How are cP and cT similar? How are they different?

15. What is a front?
How are fronts related to air masses?
What is likely to happen if a cold front is approaching?
How would you describe the sequence clouds as a warm front approaches?
What do you notice about the weather AFTER a front passes?

16. Fronts Boundary that separates air masses
Air masses retain their identities
Warmer, less dense air forced aloft
Cooler, denser air acts as a wedge

18. Fronts Types of fronts Warm front Warm air replaces cooler air
Shown on a map with semicircles
Small slope
Clouds become lower as front nears
Slow rate of advance
Light-to-moderate precipitation
Gradual temperature increase with the passage of the front

20. Fronts Cold front Cold air replaces warm air
Shown on a map with triangles
Twice as steep as warm fronts

23. Fronts Advances faster than a warm front
Weather more violent than a warm front
Intensity of precipitation is greater
Duration of precipitation is shorter
Weather behind the front is dominated by
Cold air mass
Subsiding air
Clearing conditions

24. Fronts Stationary front
Flow of air on both sides of the front is almost parallel
Surface position of the front does not move

26. Fronts Occluded front Active cold front takes over a warm front
Cold air wedges warm air upward
Weather is often complex
Precipitation is associated with air being forced aloft

30. What is a front?
How are fronts related to air masses?
What is likely to happen if a cold front is approaching?
How would you describe the sequence clouds as a warm front approaches?
What do you notice about the weather AFTER a front passes?

31. Mid-Latitude Cyclones
Middle-Latitude Cyclones
Primary weather producer in the middle latitudes
Life cycle
Originate along a front where air masses are moving parallel to the front in opposite directions
Continental polar (cP) air is often north of the front
Maritime tropical (mT) air is often south of the front

32. Mid-Latitude Cyclones
Frontal surface takes on a wave shape with low pressure centered a the apex of the wave
Warm front and cold front form
Cold front catches up to warm front and produces an occlusion
Warm sector is forced aloft
Fronts discontinue
Storm comes to an end

33. Mid-Latitude Cyclones
General characteristics
Large center of low pressure
Counter clockwise air circulation
Air flows inward toward center

34. Mid-Latitude Cyclones
Travels west to east guided by the westerlies
Last a few days to more than a week
Extending from the center of the low are a
Cold front, and
Frequently a warm front

35. Mid-Latitude Cyclones
Convergence and forceful lifting cause
Cloud development
Abundant precipitation

36. Mid-Latitude Cyclones
Idealized weather
Middle-latitude cyclones move eastward across the U.S.
First signs of their approach are to the west
Require two to four days to pass
Largest weather contrasts occur in the spring

37. Mid-Latitude Cyclones
Changes in weather associated with the passage of a middle-latitude cyclone
Changes depend on the path of the storm
Weather associated with fronts

38. Mid-Latitude Cyclones
Warm front
Clouds become lower, thicker
Light precipitation
Perhaps over a large area
Perhaps of prolonged duration
After the passage of a warm front
Winds become more southerly
Warmer air temperatures (mT air mass)

39. Mid-Latitude Cyclones
Cold front
Wall of dark clouds
Heavy precipitation
Narrow band along the front
Short duration
After the passage of a cold front
Wind becomes north to northwest
Drop in temperature as a cP air mass moves in
Clearing skies

40. Mid-Latitude Cyclones
Role of Air Aloft
Cyclones and anticyclones
Generated by upper-level air flow
Maintained by upper-level air flow

41. Mid-Latitude Cyclones
Low pressure system
Surface convergence
Outflow (divergence) aloft sustains the low pressure

42. Mid-Latitude Cyclones
Anticyclone
High pressure system
Associated with cyclones
Surface divergence
Convergence aloft

43. Thunderstorm Thunderstorm Features Cumulonimbus clouds Heavy rainfall
Lightning
Occasional hail

44. Thunderstorm Occurrence 2000 in progress at any one time
100,000 per year in U.S.
Most frequent in
Florida
Eastern Gulf Coast Region

45. Thunderstorm Stages of development All thunderstorms require Warm air
Moist air
Instability
High surface temps
Most common in the afternoon

46. Thunderstorm Require continual supply of warm air
Each surge causes air to rise higher
Updrafts and downdrafts form
Eventually precipitation forms
Most active stage
Gusty winds, lightening, hail
Heavy precipitation
Cooling effect of precipitation marks the end of the thunderstorm activity

47. Tornados Tornado Local storm of short duration Features
Violent windstorm
Rotating column of air
Extends down from a cumulonimbus cloud
Low pressure inside causes air to rush to center
Winds approach 480 km (300 miles) per hour

48. Tornados Occurrence and development
Average 780 each year in U.S. – more than any other country!
Most frequent from April – June
Associated with severe thunderstorms
Exact cause of tornadoes is not known

49. Tornados Conditions for the formation of tornadoes
Occur most often along a cold front
During spring months
Associated with intense thunderstorms

50. Tornados Characteristics Diameter between 150 and 600 meters
Speed across landscape is about 45 kilometers per hour
Cut about a 10km long path
Most move toward the northeast
Maximum winds are bout 480 km/h
Intensity measured by the Fujita intensity scale

51. Tornados Tornado Forecasting
Difficult to forecast because of their small size
Tornado watch
To alert people
Issued when conditions are favorable
Covers 65,000 square km

52. Tornados
Tornado warning is issued when funnel cloud is sighted or is indicated by radar
Use of Doppler radar helps increase the accuracy by detecting the air motion

53. Hurricanes Hurricane Most Violent Storm on Earth
To be called a hurricane
Wind speed in excess of 119 km/h (72 mph)
Rotary circulation

54. Hurricanes Features Tropical cyclone
Low pressure (lowest pressure system in the Western Hemisphere)
Steep pressure gradient
Rapid, inward-spiraling wind

55. Hurricanes Parts of a hurricane Eyewall Near the center Rising air
Intense convective activity
Wall of cumulonimbus clouds
Greatest wind speeds
Heaviest rainfall

56. Hurricanes Eye At the very center About 20km diameter
Precipitation ceases
Wind subsides
Air gradually descends and heats
Warmest part of the storm

57. Hurricanes Wind speeds reach 300 km/hr Generate 50 foot waves at sea
Cause great damage on land
Known by different names
Typhoon in western Pacific
Cyclone in Indian Ocean

58. Hurricanes Frequency Most (20 per year) occur in the Northern Atlantic
Fewer than 5 occur in the warm North Pacific

59. Hurricanes Hurricane formation and decay
Form in all tropical waters except the
South Atlantic (1 exception) and
Eastern South Pacific
Energy comes from condensing water vapor

60. Hurricanes
Develop most in late summer when warm water temperatures provide energy and moisture
Initial stage is not well understood
Tropical depression – winds do not exceed 61km/h
Tropical storm – winds km/hr

61. Hurricanes Diminish in intensity whenever
They move over cooler ocean water
The move onto land
The flow aloft is unfavorable

62. Hurricanes Destruction from a hurricane
Factors that affect amount of hurricane damage
Strength of storm (the most important factor)
Size and population density
Shape of the ocean bottom near the shore

63. Hurricanes Categories of hurricane damage Wind damage Storm surge
Large dome of water
65-80 km long
Greatest threat along coastline
Where eye makes landfall
Inland freshwater flooding from torrential rains
More people die from flooding each year than anything else!