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John Huth Harvard University

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1 John Huth Harvard University
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
Force Strength km/h Effect Calm 0-1 Smoke rises vertically 1 Light air 1-5 Smoke drifts slowly 2 Light breeze 6-11 Wind felt on face; leaves rustle 3 Gentle breeze 12-19 Twigs move; light flag unfurls 4 Moderate breeze 20-29 Dust and paper blown about; small branches move 5 Fresh breeze 30-39 Wavelets on inland water; small trees move 6 Strong breeze 40-50 Large branches sway; umbrellas turn inside out 7 Near gale 51-61 Whole trees sway; difficult to walk against wind 8 Gale 62-74 Twigs break off trees; walking very hard 9 Strong gale 75-87 Chimney pots, roof tiles and branches blown down 10 Storm 88-101 Widespread damage to buildings 11 Violent Storm 12 Hurricane Over 119 Devastation

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) ,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


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


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.

51 Estimating distances to storms
Base of clouds in thunderstorm is typically 5000 ft. Use range techniques to find distance Difference between lightning and thunder arrival times (light is faster than sound) 5 seconds per mile of distance Prevailing winds – Is the storm track moving toward you, or will it pass by?

52 Thunderstorm/squall issues
General direction is indicated by high cirrus clouds at top of anvil head NOT surface winds (often blow toward the storm) If a storm misses you (passes to the side), be alert for more storms moving in the same direction. Wind is biggest issue Lightning is less of a hazard, but shouldn’t be ignored.

53 Basic Pressure Systems: 1.Low

54 Basic Pressure Systems: 2.High

55 Cyclones and Anticyclones
High pressure systems “shed” air Low pressure systems “suck” air Coriolis force generates circulation

56 Structure of a Hurricane

57 Low pressure system over NE – March 20th 08

58 Low pressure systems in the N. Pacific

59 High and low pressure systems in N. Atlantic

60 Advection Fog: formed by movement of warm air over cooler surface

61 Radiation Fog: forms when land surface cools as a result of outgoing radiation and in turn, cools overlying air

62 Wave Parameters (Figure 7-1a)

63 What Causes Waves? Wind Submarine disturbance
Gravitational attraction of sun and moon (tides – very long wavelength waves)

64 Motion of Water Particles Beneath Waves (Figure 7-3b)

65 Deep Water Waves (Figure 7-4a)
Waves do not interact with the seafloor Orbits of the water molecules are circular.

66 Shallow Water Waves (Figure 7-4b)
Waves interact with the seafloor are known as Orbits of the water molecules become elliptical.

67 Characteristics of water waves
Velocity depends on wavelength *or* water depth Unlike sound or light – velocity is independent of wavelength for these Waves become unstable when height is 1/7th of wavelength – whitecaps (120 degree interior angle) Longer wavelength waves hold more energy Depth for “shallow” versus “deep” is about 2 times wavelength

68 Deep Shallow Gravitation 32 ft/sec/sec Water depth (ft) Wave length (ft)



71 h L Instability – when h > 1/7 L
OR – when interior angle is less 120 degrees 120o h L

72 Wind Generation of Waves
The type of wave generated by wind is determined by: Wind velocity Wind duration Fetch (distance over which wind blows) Simply put, wave size increases as the strength and duration of the wind, and distance over which it blows increases.

73 Cat’s paw

74 Fetch Conditions Time and distance Small waves buildup, break
Larger waves begin – hold more energy before breaking Generally a range of wavelengths High wind velocity produces more uniform and longer wavelength waves Typically for NE waters – fully developed seas only for 10 knot winds Larger seas in open ocean Swells travel huge distances unaffected


76 Comments on Swells Product of distant storms
Can travel thousands of miles without losing energy Period of swell indicates severity of storm – Longer period – more severe storm 4 seconds – small 8-10 seconds – hurricane Mid ocean – can have multiple swells crossing In New England, sheltering of coast line limits significant swell direction E.g. Gulf of Maine typically will only see SE swells Rhode Island catches a lot of Atlantic storms Newport beaches/surfing

77 Transformation of Shallow-water Waves (Figure 7-7b)

78 Reflecting Swells at Great Wass Island (Jonesport)
Angle of incidence equals angle of reflection

79 Wave Refraction (Figure 7-8a)
Bending of the wave crest as waves enter shallow water. It is due to Drag along the bottom. Differential speed along the crest.

80 Wave Refraction at Chatham Inlet
Gradual transition between deep and shallow water Shallow water Deep Water

81 Extreme refraction at Baker Island
(Mt. Desert)

82 Swell patterns around an atoll
reflections Main swell Refractions

83 Crossing swell patterns between islands

84 Multi-swell patterns around island

85 Polynesian stick chart – illustrating
swell patterns from two islands

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