1. FRONTS Chapter 12

2. This week onwards Air masses What are fronts Different types of front Weather associated with fronts Mid-latitude Cyclones –Weather and cyclones

3. Synoptic Scale - 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.

4. 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.

5. 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

6. 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

7. 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

8. Classification of Air Masses Arctic Air –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.

9. Classification of Air Masses mP cT mT mP cP Arctic

10. 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

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

12. 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 - drylines). A front is the boundary or transition zone between different air masses.

13. Fronts The transition zone is usually narrow (15 to 200 km in width). Fronts may be over 1000 km long. Fronts are 3-Dimensional and slope On maps, we draw the frontal boundary on the warm side of the transition zone. –This is the position of the front at the surface

14. Different fronts There are 4 different types of front we are going to look at: –Cold front –Warm front –Occluded front –Stationary front

15. Fronts on maps Cold Front Warm Front Occluded Front Stationary Front

16. Fronts Fronts are associated with ascending air (warm over cold) They usually result in cloud formation and frequently produce precipitation This precipitation can be very widespread There is often a change in wind direction (and temperature) at a front

17. 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.

18. Idealized Cold Front Isotherms 0C0C55 10  15  20  25  30  C 100 km Frontal Zone 44 75 COLD FRONT

19. Idealized Cold Front Cross-sectional Characteristics Cold Air Warm Air Strong Lifting near the frontal boundary can produce convective clouds.

20. Idealized Cold Front Cold Air Warm Air

21. Idealized Cold Front Precipitation: –Located on either side of the front. –Convective, showery in nature –If warm air is stable Nimbostratus, Altostratus –If warm air is unstable Cumulus, Cumulonimbus, Thunderstorms

22. Idealized Cold Front © 1999 Prentice-Hall, Inc. -- From: Aguado and Burt, Understanding Weather and Climate Cold Air Warm Air Surface Front