Weather and Waves

1. Weather and Waves John Huth Harvard University

2. Weather Basics • Hot air rises (less dense), cold air sinks (more dense) • Atmosphere becomes colder the higher up you go (called adiabatic cooling) • It gets colder as you go away from the equator • The Coriolis effect causes air moving away from the equator to the pole to deflect to the east • The Coriolis effect causes air moving from the pole toward the equator to deflect to the west

3. Driving Forces Behind Wind • Pressure Gradient • Air flows from high to low pressure (“downhill”) • Coriolis • Caused by the rotation of the earth, wind deflects to the right in the northern hemisphere • Centripital • Present when winds are in rotation • Friction • Air moving along the Earth’s surface is slowed by friction

4. Pressure Gradient and Winds

5. Coriolis “Force” causes path of a moving object to be deflected to the right in the NH and to the left in SH relative to the surface of the earth

8. Weather Basics II • Emergence of three convection cells in northern and southern hemisphere dominate wind patterns • Doldrums – equator • Trades blow from east to west • Horse latitudes – 30 degrees • Westerlies – 30 to 60 degrees north • As moist air rises, it condenses and gives off heat • The planet is approximately in an isobaric equilibrium – pressure remains roughly constant – regardless of temperature (density of air changes) • Prevailing winds tend to drive surface ocean currents

9. Smaller scale version: land and sea breezes Temperature contrasts (the result of the differential heating properties of land and water) are responsible for the formation of land and sea breezes.

10. Land Breeze-Sea Breeze Same effect, but on a much smaller scale

11. Wind as direction indicator • Good over short periods of time – persistent • Prevailing winds generally useful, but seasonally dependent • Weather systems and fronts can affect these • Surface winds versus winds aloft • Understand how weather systems/seasons/diurnal variations affect wind patterns

12. Wind roses for Boston Logan International Airport January July

13. Local knowledge: summer wind patterns on Cape Cod During the months of June/July/August, in the absence of fronts, wind patterns on the Cape are reasonably stable. Little wind in the morning, picking up around 2 PM from SW, reaching peak around 3:30, then subsiding. Mainly a sea breeze effect, coupled with prevailing SW winds

14. Wind compass – Taumako, Polynesia

15. Pukapukan wind compass

16. Fijian wind compass

17. Beaufort Scale – land indicators

18. Beaufort scale – at sea

19. Using wind • Winds can be deceiving • Surface winds can blow in different directions from winds aloft – you must follow the motion of high clouds to get prevailing winds • Winds will shift as fronts pass through – knowledge of this is important (for many reasons). • Safety – high winds from thunderstorms can be dangerous when at sea.

20. Wind shifts • Veering shifts – clockwise shift – typical for N. hemisphere • Backing shifts – counterclockwise – typical for S. Hemisphere • For approaching cold front – SW wind steady, veers to N to NW (typical) • For approaching warm front – NE to SE winds, veers to SW (typical)

21. Warm and Cold Air masses • Warm air masses • Humid, low pressure, warm - move up from equatorial regions • Cold air masses • Dry, high pressure, cold – move down from polar regions • Transitions between air masses are called “fronts”

22. Weather signs • Cloud formations and wind directions are the most reliable and predictive (often better than NOAA radio). • Best predictor: tomorrow will be like today (true 80% of the time). You can improve on this by being observant. • Some signs: “red sky at night” are next to useless – unless you know the cloud formations causing them.

23. Important North American Air Masses

24. In mid-latitudes, fronts develop as Rossby waves, Typically seen as undulations in the jet-stream. Isolated pockets can develop as low and high pressure cells

25. Warm fronts • Slow in coming • Sequence of clouds – build up of moisture in upper atmosphere, slowly coming down in height • Jet contrails at 40,000 ft tend to stick around • Moon or sun dogs (rings) – from ice crystals • Cirrus clouds (mares’ tails) • Cirro-stratus (mackerel scales) – 20,000 • Alto-cumulus (rollers) 15,000-20,000 • Stratus (sheet-like) 5000-10,000 • Nimbo-stratus (rain clouds) 5000 or lower • Rain usually lasts for a longer time

26. Profile of a Warm Front

27. Lingering jet contrail against a backdrop of cirrus clouds If contrail breaks up -> low humidity If contrail remains -> high humidity (approaching Warm front)

28. Sundogs – rings around the sun (or moon) Caused by ice crystals in the upper atmosphere Cirro-stratus (high, layered clouds) 22 degree halo around sun/moon

29. Mares tails – cirrus clouds (reading wind: watch cloud motion relative to foreground object) Higher wind speed Lower wind speed

30. Mackerel scales – cirrocumulus clouds Old saying: “mackerel scales and mares tails make lofty ships carry low sails”. -> Approaching warm front

31. Altocumulus clouds – “rollers” Faster moving air Eddies Slower moving air Clouds inside eddies

32. Stratus clouds – means “layered” in latin Flat, grey, clouds, covering large areas of the sky

33. Nimbostratus – rain clouds associated with a warm front

34. Cold Fronts • Abrupt transitions • Veering winds (moving clockwise at front) • Strong downdrafts • Squall-lines • Lightning • Development of storms more rapid, unpredictable, violent, and local than in warm fronts

35. Profile of a Cold Front

36. Wind shifts • Veering shifts – clockwise shift – typical for N. hemisphere • Backing shifts – counterclockwise – typical for S. Hemisphere • For approaching cold front – SW wind steady, veers to N to NW (typical) • For approaching warm front – NE to SE winds, veers to SW (typical)

37. Veering winds as front approaches (typical for NE)

39. Fig. 11.7

40. THUNDERSTORM CUMULUS STAGE • CUMULUS STAGE • REQUIRES CONTINUOUS SOURCE OF WARM MOIST AIR • EACH NEW SURGE OF WARM AIR RISES HIGHER THAN THE LAST • STRONG UPDRAFTS • FALLING PRECIPITATION DRAGS AIR DOWN - DOWNDRAFT • ENTRAINMENT

41. Fair weather cumulus clouds (flat, little vertical structure)

42. General character of convection Rising column of hot air (fluid) Surrounding air is cooler and cooler At higher altitudes Hot air rises, at cold enough Temperatures, it begins to mix

43. Development of vertical structure Rising air column Incoming humid air

44. Building cumulus clouds can be a sign of land – high up, seen from further away

45. Building thunderheads

46. Start of anvil-head formation Air column reaches tropopause and spreads

47. Mature anvil-head

48. Air column frequently overshoots tropopause, “bubbles out” high cirrostratus Fig. 11.2a

49. THUNDERSTORM MATURE STAGE • SHARP COOL GUSTS AT SURFACE SIGNAL DOWNDRAFTS • UPDRAFTS EXIST SIDE BY SIDE WITH DOWNDRAFTS • IF CLOUD TOP REACHES TROPOPAUSE UPDRAFTS SPREAD LATERALLY - ANVIL SHAPE • TOP OF ICE LADEN CIRRUS CLOUDS • GUSTY WINDS, LIGHTNING, HEAVY PRECIPITATION, HAIL

50. Multicell line storms consist of a line of storms with a continuous, well developed gust front at the leading edge of the line. An approaching multicell line often appears as a dark bank of clouds covering the western horizon. The great number of closely-spaced updraft/downdraft couplets qualifies this complex as multicellular, although storm structure is quite different from that of the multicell cluster storm.