1. Air Masses and Fronts Chapter 8

2. AIR MASSES
The troposphere can be further divided into separate regions known as air masses.
In most cases, these air masses do not gradually merge with each other, but remain separated by relatively narrow transition zones or fronts.

3. Definitions
Air Mass - Large body of air (portion of the troposphere) that has fairly uniform properties of temperature and moisture in the horizontal.
Source Region - The region which an air mass acquires its particular properties of temp and moisture.

4. SOURCE REGIONS
These areas can be large snow or ice covered polar regions, cold northern oceans, tropical oceans, and large desert areas.

5. CLASSIFICATION OF AIRMASSES
Air Masses are classified according to the temperature and moisture characteristics of their source region. Based on moisture content, air masses can be considered either continental (dry) or Maritime (moist). According to their temperature, they are either Tropical (warm), Polar (cold), or Artic (extremely cold)

6. CLASSIFICATION OF AIRMASSES
In Meteorology everything is shorthand including the identification of Air Masses.
A small letter such as c or m indicates the moisture conditions of the air mass.
Following the small letter a capital letter T, P, or A represents the temperature characteristics of the air mass.

7. CLASSIFICATION OF AIRMASSES
What your left with is 6 possible types of air masses.
cP, cA, cT, or; mA, mP, mT

8. Continental Air Masses
Continental Polar (cP) and Continental Artic (cA) Air Masses form over large, high-latitude land masses such as northern Canada or Siberia. In the winter, these regions have short days and low solar angles. They also are usually snow-covered during the winter and therefore reflect much of whatever solar radiation does reach the surface

9. Continental Air Masses
This combination of circumstances virtually guarantees that the air will lose more radiant energy in the winter than it recieves. The cooling of the air from below leads not only to low temperatures but also to radiation inversions and highly stable conditions.

10. Continental Air Masses
Low temps in the winter also very dry, sunny days. In cities pollutants hard to dispurse very little mixing.
cA air mass differes from the Polar airmass in that it has much less of a vertical extent. (much shallower)

11. Maritime Polar Air Masses
Similar to continental polar air masses but are more moderate in both temperature and dryness. Maritime polar air forms over the North Pacific as cP air moves out from the interior of Asia. The warm Japan current adds heat and mositure to the cold, dry air and converts it from cP to mP. The air masses then moves eastward across the Pacific. Low Pressure

12. Continental Tropical Air Masses
Forms during the summer over hot, low-latitude areas, such as the southwestern United States and Northern Mexico. Usually form in the dessert regions where there is little surface water and vegetation.
Very high ground temperatures, very dry often cloud free.

13. Maritime Tropical Air Masses
Develop over warm tropical waters (Gulf of Mexico & Atlantic). Warm but not as hot as cT.
mT air is very moist, and unstable near the surface - ideal for the development of clouds and precipiation TS.
Enormous influence on the southeastern U.S., especially during the summer.

14. Air Masses & Fronts

15. Air Mass Modification pg.64
When an air mass moves away from its source region, it takes on the characteristics of its new region
Degree of Modification depends on 3 factors
Speed of the air mass
nature of the region
temp. difference between the new surface and the air mass

16. Air Mass Modification 4 ways air masses are modified
Warming from below (instability and showers)
Cooling from below (stable, if cooled to much fog)
Subtraction of water vapor (condensation and precipitation)
Addition of water vapor (cold air over warm water i.e. evaporation)

17. Definitions
Front - Zone between 2 different air masses. Or a boundary that separates the two unlike air masses.
Frontolysis - when a front dissipates or merges into the adjacent air mass (it DIES)
Frontogenesis - formation of a front (it is CREATED)

18. Definitions
Occluded Front - A composite of two fronts as a cold front overtakes a warm front or quasi-stationary front
Quasi-stationary front - is stationary or nearly is moving at a speed of less than 5 knots.
Trowal - Trough of warm air aloft

19. Chart Symbols

20. Fronts
Discontinuities are changes in properties from 1 front to another
Temperature - usually a temp change
Dew Point - Temp dew point spread will usually change
Wind always changes across a front usually both in speed and direction
Beware of wind shear in this area

21. Fronts
Also Pressure
Cold front pressure drops as the front approaches and with passage pressure rises
Warm front pressure generally falls until frontal passage then pressure remains steady or may fall slightly in the warm air.
Generally speaking with frontal passage pressure increases.

22. Pressure

23. Warm Frontal Slope
Slope below is 1 in An aircraft flying at around 6000 feet would encounter the frontal surface 150 nm past the surface based front

24. Fronts
Follow Low Pressure Systems.
Low pressure Systems follow the Jet Streams

25. Warm Front
Warm fronts, leading edge of a warm air mass

26. Warm Front Wx Much more stable Shallow slope Stratiform clouds and fog
continuous precipitation
Smooth air
Fair to poor visibility

27. Clouds Associated with a Warm Front

28. Cold Front
Cold fronts, leading edge of a cold air mass

29. Cold Front Weather Generally speaking cold fronts are: fast moving
More severe weather
Cumuliform clouds
Showery Precipitation
Rough Air (turbulence)
Good Visibility

30. Cold Front Weather in Flight
Windshifts
The windshift occurs at the frontal surface
A change in temperature tells you when you have passed through the frontal surface.
The windshift is such that an alteration to starboard (to the right) is required to stay on course, no matter which way you fly through the front.
Windshift more significant at lower levels

31. Cold Front Weather in Flight
Windshifts - to right need the crab.

32. Cold Front Weather in Flight
Ceiling and visibility - Fast moving cold front typically brings good visibility the faster it is moving.
Turbulence - can be a problem. If you know where the frontal boundary is slow to Va or below in turbulence for the crossing
Precip and icing - can be severe however you are typically in clouds for a shorter period of time.

33. Warm Front Weather in Flight
Windshift - same as in a cold front
Ceiling and visibility - low ceilings and visibility common
Turbulence - more stable less turbulence compaired to cold front
Precip and icing - precip is steady icing is lighter however you are in the icing layer typically longer resulting in more ice.

34. Naming Fronts A front is named two ways.
1. The temperature of the advancing air
2. The colder of the 2 airmass cA, mA, mP

35. Warm Front Occlusion

36. Type of wx associtated with the warm and cold front occlusion

37. Upper Fronts (basin)

38. Frontal Waves
They usually form on slow moving cold fronts or stationary fronts
A. Winds blowing parallel cause a disturbance (remember stationary, so winds are parallel)
B. Wave starts
C. Start of a Cyclonic (counterclockwise) circulation developes

39. Frontal Waves
D. At the peak pressure falls it then transforms into a Low which reinforces the cylconic circulatory pattern
E. Cold front catches up to the warm front they from an occluded front

40. Frontal Waves
F. As it grows in length the circulatory pattern diminishes, the air masses start to mix
G. The fronts merge, break off & disappear

41. Frontal Waves

42. Frontal Weather Weather along the front depends upon numerous things:
The amount of moisture
Must be present for clouds and precip to form
Dry cold front meeting a moist warm front

43. Frontal Weather
Weather along the front depends upon numerous things continued:
Degree of stability
Stable stratiform
Unstable Cumuliform

44. Frontal Weather The slope of the front
Shallow gives large areas of precipitation and or fog
Steep front gives thinner line of precip but usually more intense cumiliform

45. Frontal Weather The speed of the front
Faster it moves the more energy and intensity experienced usually
Dry fronts may only have cirroform clouds

46. Frontal Weather Upper wind flow pattern (Jet Stream)
When perpendicular the front moves same direction
Parallel to the front it moves very little
Watch the Jet it is the primary moving force

47. Questions
#1. When a cold front passes, the temperature generally _____________
Falls
#2. When a cold front approaches and passes, the pressure at a station generally______.
Falls continuously as the front approaches, and rises rapidly after it has passed.

48. Questions
#3. Cold air behind a cold front is generally unstable because?
It is warmed from below as it crosses relatively warm regions (warm air rises)
#4. The horizontal extension of clouds and precip. Associated with a slow-moving cold front is generaly ____ than in the case of a rapidly moving cold front.
greater

49. Questions
#5. You can tell when you are crossing a cloud-free frontal surface at a low altitude because_____.
The winds will vear (need to crab to the right)
#6. Given two air masses: one warm and one cold. Generally the ______ air mass will contain more water vapor
warmer

50. Questions
#7. Name 7 things you can plan to maybe experience when crossing a cold front
icing severe over short period of time
precipitation heavy
Turbulence
thunderstorms
wind shifts
low ceiling
low visibility

51. Questions
#8. What is the effect on a station’s altimeter setting while a warm front approaches and passes over?
As the warm front approaches, the thickness of the cold air over the station decreases, and so the MSL pressure and the altimeter setting fall. After the warm front has passed, the uniform warm air mass has uniform thickness and characteristics; the MSL pressure and altimeter remain constant

52. Questions
#9. A warm front indicates that the cold air mass is (stationary/retreating/ advancing)
retreating
#10. The slope of a warm front is generally (steep/gentle)
gentle
#11. When a front passes a station, the wind (backs/veers)
Veers

53. Questions #12. A trowal is what? An upper-air trough of warm air
#13. In an air mass ______ and _______ are distributed fairly uniformly in the horizontal
Temperature and Moisture
#14. An Airmass that forms over an expanse of water is called ______
Maritime

54. REVIEW FOR TEST #1
TAKE NOTES

55. Troposphere Where is the troposphere the thickest and when?
How can you identify the tropopause?
What is the driving force of the Earth’s weather?

56. What is relative humidity and dew point temperature?
When the temp/dew point temp spread decreases what happens to relative humidity?
How can you increase relative humidity?

57. What is a temperature inversion?
Is a temperature inversion a stable or unstable phenomenon?
When is it common for an inversion to be produced?
What would you expect the visibility to be like beneath a low level inversion?

58. What is the approximate height of the atmosphere?
Where is the atmosphere approximately 1/2

59. Name two situations when your true altitude may be lower than your indicated altitude

60. What is the name of the force in the Northern hemisphere that deflects the winds to the right until it is parallel to the isobars?

61. What is ISA?
How much does pressure decrease per 1000ft?

62. What is needed to create a cloud?
Define condensation

63. What is the main factor in determining how much water vapor air can hold?

64. Cumuliform cloud bases
Surface temp = 31 degrees C
D.P.T.= 23 degrees C
At sea level
What would the cloud base be approximately?

65. Cumuliform cloud bases
Surface temp = 31 degrees C
D.P.T.= 22 degrees C
At sea level
What would the cloud base be approximately?
Take spread = 8 degrees /2.2 or multiply by .45 = answer of around 3600 feet

66. Cumuliform clouds = stable or unstable atmosphere?
What are the four cloud families?
What does nimbus mean?

67. What kind of clouds are found around mountain waves?
What kind of turbulence can be expected around mountain waves?

68. How thick do clouds usually have to be to produce significant precip?
What are the approximate bases of low, mid and high clouds?

69. How can you determine atmospheric stability?
What is the DALR? Approximately what is the SALR? Is the SALR a constant?

70. What determines the structure of a cloud formation?
What would you expect the visibility to be in an unstable air?
What are some characteristics of stable and unstable air

71. If you have a good lifting agent and an unstable air mass what type of clouds would you expect?
Name an atmospheric process that would increase the stability of an air mass? Name process that would decrease the stability

72. Which clouds are the meanest on the block?
Name a type of cloud that forms due to convection?

73. When can you expect more convective updrafts over what types of terrrain?
What can you always expect with frontal passage?
What is a cold front?

74. Be able to identify or classify an airmass.
If the airmass is moist you would call it what?
If the airmass is warm you would call it what?

75. Be able to explain a Chinook