1. Air Masses
An air mass is a large body of air with relatively homogeneous character of temperature and humidity.
Classification by latitude, moisture
mT, mP, cA, cP, cT
Movement and transitions-character
of air masses can change as moves over
different land surfaces, or crosses
mountains, ie mP can become cP after
crossing the Rockies, cP can become mT
when moving over warm water.
Source:

2. North American Air Masses & Sources
Source regions and common paths of the principal air masses that affect the continental United States.

3. Lifting Mechanisms That Produce Precipitation
Convergence-when similar air masses
converge, they are forced to rise,
forming clouds
Convection-surface heating causes
warm air parcel to rise and form clouds
Orographic-mountains and large
obstacles force air to rise, forming clouds
Frontal-when warm and cold air masses
meet, cold air lifts warmer air forming clouds
Source:

4. Lifting by Convergence
Along the ITCZ warm trades meet and rise. Precipitation along the ITCZ shifts with season, into the summer hemisphere.
Average daily rainfall rates (mm/day) for January and July based upon measurements from the Tropical Rainfall Measuring Mission (TRMM) satellite for the years

5. Frontal Precipitation
Vertical cross-section through a warm front (top) and cold front (bottom). The vertical scale is greatly exaggerated. Warm fronts typically have a 1:200 vertical-to-horizontal ratio; the ratio for cold fronts is approximately 1:70.

6. Convectional Precipitation
Convection often leads to isolated showers from building cumulus clouds. Occasionally, these clouds grow large enough to form thunderstorms. Each thunderstorm cloud, cumulonimbus, has a life cycle of 3 stages.
The three stages of the life cycle of an air mass thunderstorm: cumulus, mature, and dissipating. Updrafts dominate the developing stage, both updrafts and downdrafts are found during the mature stage, while only downdrafts are found during the dissipating stage. The approximate width at each stage of the thunderstorm is shown at the bottom.

7. Cumulonimbus grows near the Canadian Rockies in an afternoon thunderstorm.
Source:

8. Severe Thunderstorms
Conceptual model of a severe thunderstorm producing sizeable hailstones. The red dashed lines represent the warm updrafts, the blue lines show the cold downdrafts, and the green lines represent the movement of hailstones. The thunderstorm is moving from left to right.

9. Orographic Precipitation
Fig 13.7
Orographic precipitation on the upper windward slope of the Cascade Mountains in west-central Oregon. Note the significant temperature and moisture differences between the windward and leeward sides of this major mountain barrier. (Vertical scale is greatly exaggerated.)

10. Fig 13.8
Oregon’s precipitation pattern, with the distribution of isohyets exhibiting the results of the orographic effect as westerly winds off the Pacific are forced across the north-south-trending Cascade Mountains. The transect across the Cascades between Eugene and Bend, diagrammed in Fig , is marked by a red line.

11. Easterly Waves
Easterly waves are disturbances within the trades that can produce heavy rainfall and potentially can form into tropical storms and hurricanes.
Source:
The trough axis marks where the trades are converging with rising motion behind the westward moving wave, and where divergence and sinking motion occur ahead of the wave axis. Heaviest precipitation occur in the convergence zone, while mostly sunny conditions precede the axis.

12. Tropical Storms and Hurricanes
Tropical cyclones can form over land or water, however hurricanes always develop over warm oceans. As cyclones increase their energy, their winds increase. Storms are defined by wind speed:
Tropical Depression-winds below 40 mph
Tropical Storm-winds 40 to 74 mph
Hurricane-winds exceed 74 mph
Hurricanes may form if
They occur over warm water-80F or 28C
There is an initial disturbance (ITCZ, convergence, mid-latitude cyclone
There is sufficient Coriolis force. Most hurricanes form in 8-15 degrees latitude
There are weak upper air winds. Strong flow destroys formation.
Hurricanes can form over all tropical oceans where these conditions are met.

13. Fig 14.2
The eye of Hurricane Mitch is still over water, but this 1998 storm is poised to strike Central America and will become the costliest natural disaster in the modern history of the western hemisphere. Honduras will be hit hardest, with nearly 10,000 deaths When it is over, nearly one-quarter of the country’s 6.4 million people will be homeless, and most of the agricultural economy will be ruined.

14. Fig 14.3
Cross-sectional view of a hurricane showing its mechanics and component parts.

15. Historic Tropical Storm and Hurricane Tracks
Storm tracks of weak and severe tropical cyclones from 150 years through Weaker systems, shown in blue and yellow, occur near the Equator, during their early stages of development, and over land and in the middle latitudes as they lose energy and weaken. The steering of these systems by the easterly trade winds within the tropics, and the westerlies as they move into the middle latitudes is apparent. Category 4 and 5 hurricanes are most common in the western North Pacific Ocean.

16. Rossby Waves in the Upper Atmosphere Westerlies
500-mb map for North America in November. The lines show the height of the 500-mb level in meters above the surface (ex. 92 equals 5920 m). There is a ridge along the W. coast with troughs over central and E. coast US. Dashed lines are temperatures in Celsius.

17. Polar Jet Stream and Surface Weather
Relationship between the Polar jet stream and surface pressure patterns. (A) Position of the jet stream 9000 m (30,000 ft) above North America. (B) Associated air movement around points X and Y in vertical cross section. (C) Resulting surface pressure conditions.

18. Life Cycle of a Mid-latitude Cyclone
Source:

19. Classifying Climates
The ideal climate classification system would achieve five objectives:
1. It should clearly differentiate among all the major types of climates that occur on Earth.
2. It should show the relationships among these climate types.
3. It should apply to the entire planet.
4. It should provide a framework for further subdivision to cover specific locales.
5. It should demonstrate the controls that cause any particular climate.

20. The Köppen Classification
Simplified version of the modern Koppen climate classification system.

21. Rainforest Climate Af

22. Köppen Classification
Fig 16.2
Distribution of climate types across a hypothetical continent.

23. Fig 16.3
Global distribution of climates according to the modified Köppen classification system.

24. Description of the Köppen Climate Types
A-Tropical climates
Af-tropical rainforest, Am-monsoon forest, Aw-savanna
B-Desert climates
BW-true desert, BS-steppe
C-Mesothermal, moderate temperature
Cf-no dry season, Cs-summer dry, Cw-winter dry
D-Microthermal, snow forest
Df-no dry season, Dw-winter dry, continental
E-Frost climates
ET-tundra, EF-eternal frost
H-Highland climates

25. Changing location of the January 18oC isotherm in S. Florida, 1941-1979.

26. Unit 15: Tropical (A) and Arid (B) Climates

27. Rainforest Climate Af

28. Rainforest Climate Af

29. Monsoon Forest Climate Am

30. Savanna Climate Aw Tropical grassland Summer wet, winter dry
Transition zone between forests and grasslands

31. Savanna Climate Aw
Tree-dotted savanna of Kenya’s Maasai Mara game reserve.

32. Arid Climates B
Dry landscape of western Sahara near Morocco and Algeria.

33. Desert Climate BW

34. BW Climate

35. Steppe Climate (Semi-arid) BS

36. Steppe Winter Wheat Region of North America
Combines harvest winter wheat in western Nebraska.

37. Arid Regions Show the Highest Precipitation Variability