1 Valley Breeze Example ©1997 Prentice-Hall, Inc.

2 Valley Breeze Clouds J.M. Moran -- ©1997 Prentice-Hall, Inc.

3 Heat Island -- Washington D.C. ©1997 Prentice-Hall, Inc.

4 Sea Breeze Example ©1998 Wadsworth Publishing Company

5 Hadley Cell Smooth Earth No Rotation ©1997 Prentice-Hall, Inc.©1998 Prentice-Hall, Inc.

6 Add Rotation We now allow the earth to rotate. As expected, air travelling southward from the north pole will be deflected to the right. Air travelling northward from the south pole will be deflected to the left. ©1997 Prentice-Hall, Inc.

7 Three-Cell Conceptual Model ©1998 Prentice-Hall, Inc.

8 Three-Cell Model By looking at the actual winds, even after averaging them over a long period of time, we find that we do not observe this type of motion. In the 1920’s a new conceptual model was devised that had three cells instead of the single Hadley cell. These three cells better represent the typical wind flow around the globe.

9 Planetary Scale Circulations A very complicated pattern that we shall examine only in a very simple form. To begin, imagine the earth as a non- rotating sphere with uniform smooth surface characteristics. Assume that the sun heats the equatorial regions much more than the polar regions. In response to this, two huge convective cells develop.

10 Horse Latitudes Around 30 o N we see a region of subsiding (sinking) air. Sinking air is typically dry and free of substantial precipitation. Many of the major desert regions of the N.H. are found near 30 o latitude. –Sahara –Middle East –SW United States

11 Intertropical Convergence Zone ITCZ -- Moves south in January ©1998 Prentice-Hall, Inc.

12 ITCZ -- Moves north in July Intertropical Convergence Zone ©1998 Prentice-Hall, Inc.

13 Monsoon Wind systems that exhibit a pronounced seasonal reversal in direction. Best known monsoon is found in India and southeast Asia. Winter -- Flow is predominantly off the continent keeping the continent dry. Summer -- Flow is predominantly off the oceans keeping the continent wet.

14 Monsoon -- Winter Continents are dry -- dry season. ©1998 Prentice-Hall, Inc.

15 Monsoon -- Summer Continents are wet -- rainy season. ©1998 Prentice-Hall, Inc.

16 Air Masses Air Mass –A huge volume of air that is relatively uniform horizontally in temperature and water vapor concentration. –Properties of an air mass are usually defined by the type of surface it develops over ---- the source region.

17 Air Masses Source Regions –Relatively flat –Uniform Surface Compositions Oceans Great expanse of snow covered ground Air needs to be in contact with the source region for a long period of time to develop uniform characteristics.

18 Temperature –Cold Polar (P) –Warm Tropical (T) Moisture –Dry Continental (c) –Moist Maritime (m) Results in four basic air mass types. Classification of Air Masses

19 Classification of Air Masses Continental Tropical (cT) –Hot, Dry –Develops over the deserts of Mexico and the Southwestern United States Maritime Tropical (mT) –Warm, Humid –Develops over the tropical and subtropical oceans and the Gulf of Mexico

20 Classification of Air Masses Continental Polar (cP) –Cold, Dry –Develops over the northern interior of North America -- Central Canada Maritime Polar (mP) –Cold, Moist –Develops over the cold ocean waters of the North Pacific and North Atlantic

21 Classification of Air Masses Arctic Air [(A) or (cA)] –Very Cold and Usually Dry –Develops over the snow or ice covered regions of continents –Similar to polar air masses but usually develops in regions north of 60  N where there is little or no insolation during the winter.

22 Classification of Air Masses mP cT mT mP cP Arctic

23 Air Mass Modification Air masses do not remain over their source region. If the air moves over a region that is different from where it originated, the air mass will be modified, or changed, by the land that the air is travelling over. Changes include: warming, cooling, adding or reducing moisture

24 Air Mass Modification cP Warmer Land The cP air mass will be warmed by the warmer land that it passes over.

25 Air Mass Modification -- Changes in Stability Cold air mass moves over a warmer surface. cP Warm, Moist The lower layers are warmed and may become unstable.

26 Air Mass Modification -- Changes in Stability Temperature (  C) Altitude (m) Temperature (  C) Altitude (m) Sounding Before Modification: Conditionally Unstable After modification: Absolutely Unstable Cold air mass moves over a warmer surface.

27 Air Mass Modification -- Changes in Stability Warm air mass moves over a colder surface. mT Warm, Moist The lower layers are cooled and tend to stablize. Cold

28 Air Mass Modification -- Changes in Stability Temperature (  C) Altitude (m) Temperature (  C) Altitude (m) Sounding Before Modification: Conditionally Unstable After modification: Absolutely stable Warm air mass moves over a colder surface.

29 Air Mass Modification -- Orographic Effects mP “Pacific” Air Pacific Air is warm but not as dry as continental air.

30 Fronts Front –A narrow transition zone between air masses of differing densities. –The density differences usually arise from temperature differences. –Density differences may be a result of humidity differences (summer). A front is the boundary or transition zone between different air masses.

31 Fronts The transition zone is usually narrow (15 to 200 km in width). Fronts may be over 1000 km long. On maps, we draw the frontal boundary on the warm side of the transition zone.

32 Idealized Cold Front Isotherms 0C0C55 10  15  20  25  30  100 km Frontal Zone COLD FRONT

33 Idealized Cold Front Cold Front –A boundary that moves in such a way that the colder (more dense) air advances and displaces the warmer (less dense) air. –The largest temperature differences are normally associated with cold fronts.