Weather – Air Mass

1. Weather – Air Mass • extremely large body of air with similar characteristics of temperature and moisture. Forms when air stagnates for long periods of time over a uniform surface • Characteristic weather of an air mass is determined by the surface above which it forms

2. Air masses affecting North America

3. Air mass modification • An air mass may be modified when advected away from the source region. The modified air mass may be warmed/cooled from below --- remember atmospheric stability! It may also take up water vapor or be dried out by passing over certain terrain etc.

4. The air mass takes up characteristics from the surface of the lake. Heating from below  unstable. Warming + evaporationgains moisture

5. Lecture 18 • Fronts • Life cycle of the extratropical cyclone • Three dimensional view of the extratropical cyclone • The extratropical anticyclone

6. Fronts • Separate different air masses • Cold, warm, occluded fronts

7. Sfc weather associated with a cold front Pressure T Wind Clouds & precip

8. Vertical slice through a cold front

9. Sfc weather with a warm front

10. Vertical structure of a warm front

11. Occluded fronts Cold type Warm type

12. Extratropical cyclone • Forms as a wave on a stationary front separating two different air masses • Goes through a life cycle: • Growing stage • Mature stage • Dissipating stage

13. The extratropical cyclone

14. Growing stage

15. Mature stage

16. Dissipating stage

23. The extratropical cyclone

24. Growing stage

25. Mature stage

26. Dissipating stage

27. Another view of the growth and dissipation

28. Cyclogenesis --- needed ingredients • Surface temperature gradient • A strong jet stream • The presence of mountains (or other surface inhomogenious features, e.g. coastline) • Tilted pattern of rising and sinking air (in frontal zones) releases baroclinic energy

29. Typical cyclone paths and areas of cyclogenesis • Ruled by surface temperature gradient and a strong jet • Nor’easters • Cyclones riding the pineapple express from central Pacific to south and central California • Alberta clippers

30. Well known cyclone tracks

31. Preferred areas of cyclogenesis

32. Cyclones and jet streams • Surface lows tend to move in the direction of the flow at midtropospheric level (500mb) • Surface pressure drops when there is divergence of the wind in the column of air above the surface low. • Speed divergence • diffluence

33. Divergence above a deepening low

34. Three dimensional view of the cyclone

35. Convergence at bottom requires divergence at top and vice versa

36. Depiction of 3D airflow in cyclone/anticyclone

37. The extratropical anticyclone

38. High pressure systems • Come at the heals of the cyclone • Highs are air masses where with mostly homogeneous temperature and humidity • Highs are usually clear, dry and calm • A high looks like a blob on a weather map, as opposed to the bull’s eye that the low is • Highs can last for weeks (especially in summer)

39. Highs • Have divergence at the surface, convergence at upper level • Diverging air weakens temperature and moisture gradients by spreading out the isolines • Weak gradients mean no fronts • Divergence at sfc requires sinking air, dry • Highly stable and cloudless

40. High pressure, low visibility