Outline for Lecture 16

1. Outline for Lecture 16 (Chapter 7) Recap (Chapter 8) Air Masses Source Regions Classifying Air Masses Air Mass Modification Properties of North American Air Masses (Chapter 9) Fronts 4/03/01 4/1/03

2. Zonal Precipitation Patterns are Related to Pressure Patterns H L H The Subtropical belts are the most complicated. L H L H

3. Intertropical Convergence Zone p. 405 of your book

4. Subtropical High Pressure Cells Subtropical high pressure cells have different characteristics with strong subsidence (which leads to dry regions) on the eastern side of these systems.

5. Locations of Great Deserts H p. 406 of the book

6. Glance back at Ch. 15 p. 395 of your book

7. What is an air mass? Air Mass – A large body of air, usually 1600 km or more across, that is characterized by homogenous physicalproperties (temperature and moisture) at any given latitude. A cold Canadian air mass moving southward bringing very cold air

8. Source Regions • Two things are critical for an area to be a source region. • The region must be an extensive area that is physically uniform. • The region must be characterized by a general stagnation of • circulation. • Locations dominated by slow moving or stationary anticyclones • are ideal areas for source regions. H

9. January Average Surface Pressure Systems and Associated Circulation Siberian High, Azores High Aleutian Low, Icelandic Low

10. July Average Surface Pressure Systems and Associated Circulation Bermuda High

11. cA mP mP cP Your book has a MUCH better figure cT mT mT

12. Classifying Air Masses Air masses are classified by: the surface over which it formed and its latitude Air masses are identified by two-letter codes. The first designates the surface of the region: c continental m maritime The second designates latitude/temperature: P Polar A Arctic T Tropical

13. Air Masses cA continental Arctic cP continental Polar cT continental Tropical mT maritime Tropical mP maritime Polar Note: no maritime Arctic!

14. Continental Drier More extreme: the hot is hotter and the cold is colder Maritime WET Smaller temperature range—hey, it’s over water! General Properties

16. Continental Arctic Air Masses (cA) Forms over continents at high latitude. VERY cold and VERY dry. Source: far north, like Greenland and the Arctic Basin cA cP

17. “Arctic Express” cA VERY cold VERY dry A cold snap

18. Continental Polar Air Masses (cP) Forms over continents at high latitude. Cold and dry, but not as cold as the continental arctic in the winter time. Source: interior Canada and Alaska, anywhere in the interior north of 50º latitude and covered with snow cA is just cP with an attitude problem. cP/cA p. 222 of the Book has a Better figure cP

19. Maritime Polar Air Masses (mP) Forms over oceans at high latitude. Cool to cold, and moist, but not as cold as the continental polar in the winter time. Unstable and stormy inwinter. Two regions are important sources for mP air that influences North America: the North Pacific and the Northwestern Atlantic. nor’easter mP mP Rain, snow, clouds

20. Maritime Polar Air Masses (mP) In summer, mP has different characteristics—it is generally cool, wet and stable. So we have no summer nor’easters. mP mP

21. Maritime Tropical Air Masses (mT) Copious moisture is transported from the tropics to the mid latitudes when mT air masses move northward over land. The mT air mass over the Gulf of Mexico and Western Atlantic is important to weather over the eastern half of the nation. Moist, hot, unstable (on the western side of a subtropical high) Moist, warm, and stable mT mT

22. Average annual precipitation for the eastern two thirds of the United States. Precipitation decreases with distance from the mT source regions. Isohyets are lines of equal precipitation

23. Continental Tropical Air Masses (cT) A summertime-only phenomenon in North America. Forms over Mexico and the desert southwestern U.S. Tends to be hot, dry, and unstable, though it’s so dry that clouds cannot form. Tends not to roam much, but if it goes over the central plains, it will produce drought. cT

24. Dryline HOT dry air over here Warm moist air over here cT mT This leads to differences in lapse rates, and explosive thunderstorm Development in the afternoon over West Texas in summer.

25. Tucson Precip { North American Monsoon mT

26. Air Mass Modification After an air mass forms, it normally migrates from the area where it acquired its distinctive properties to a region with a different surface characteristics. As the air mass migrates it exchanges physical properties with the surface below it. The type of exchange determine cloud types and weather patterns. Cold air over warm surfaces will result in instability () whereas, warm air over cooler surfaces will result in more stable conditions and limited vertical motion ().

27. Lake Effect Snow As continental polar air crosses the Great Lakes in winter, it acquires moisture and is made less stable because of warming from below. A lake-effect snow shower on the lee side of the lakes is often the consequences of the air-mass modification.

28. Differential Heating of Land and Water In the winter time, warm, moist air moves over much colder the land which creates instability (). The snowbelts of the Great Lakes region are the zones that most frequently experience lake-effect snowstorms.

29. Current Air Masses cP cA mP mP cP cT mT mT MRF 850 mb Temperature (°C) and Height (m)

30. Storm of the century (extreme example of two air masses interacting) A very cold continental arctic air (cA) mass collided with a warm, moist maritime tropical (mT) air mass from the Gulf of Mexico.

31. Polar Front over the U.S. A series of mid-latitude cyclones forming along the polar front

32. Fronts • Fronts are boundaries between air masses. • One air mass is usually warmer and often contains more moisture than the other. • Along the zone where the air masses meet, they exchange moisture and temperature characteristics.

33. Warm fronts Warm air rides over colder, more dense air. As the air is lifted it is cooled__________-ly causing moisture in the air to ___________ into clouds. slope = 1/200 1 mi 200 mi 0 Slow-moving warm front and stratus clouds

34. Cold fronts Cold air wedging under warm, moist air. The forceful lifting of warm, moist air along the cold front releases ________ which adds the ______ of the air to create more lifting. Most often, the air behind a front is continental polar. slope = 1/100 1 mi 100 mi 0 Fast moving cold front and cumulonimbus clouds.

35. Thunderstorm development along a cold front over the Great Plains Lightning results when very strong vertical lifting creates a static charge within the cloud.

36. Stationary Fronts The air flow on both sides of a front is parallel to the front. This condition is called a stationary front. This will serve as a focal point for storm formation.

37. Occluded Fronts A rapidly moving cold front can overtake a warm front. As the cold air wedges the warm front upward a new front, known as an occluded front, is formed. There are cold occluded fronts and warm occluded fronts.

38. Cold and Warm Occluded Fronts Air is colder than the air it is overtaking. This is a cold occluded front. Cold occluded fronts are the most common type of occluded front. It is also possible for air behind the advancing cold front to be warmer than the cold air it is overtaking. This is a warm occluded front.

39. Exam 2 • Will be April 15, as scheduled • Will cover Chapters 6-10 (mostly) • Might not include tornadoes if we don’t get to that