Weather and Waves

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Weather and Waves

• John Huth
• Harvard University

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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

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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 Earths surface is slowed by
  friction

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Pressure Gradient and Winds
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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
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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

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

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Wind roses for Boston Logan International Airport
January
July
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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 330, then
subsiding. Mainly a sea breeze effect, coupled
with prevailing SW winds
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Wind compass Taumako, Polynesia
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Pukapukan wind compass
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Fijian wind compass
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Beaufort Scale land indicators
Force Strength km/h Effect
0 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 102-117 Widespread damage to buildings
12 Hurricane Over 119 Devastation
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Beaufort scale at sea
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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.

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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)

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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

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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.

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Important North American Air Masses
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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
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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

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Profile of a Warm Front
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Lingering jet contrail against a backdrop
of cirrus clouds If contrail breaks up -gt low
humidity If contrail remains -gt high humidity
(approaching Warm front)
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Sundogs rings around the sun (or moon) Caused
by ice crystals in the upper atmosphere Cirro-stra
tus (high, layered clouds) 22 degree halo around
sun/moon
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Mares tails cirrus clouds (reading wind watch
cloud motion relative to foreground object)
Higher wind speed
Lower wind speed
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Mackerel scales cirrocumulus clouds
Old saying mackerel scales and mares tails make
lofty ships carry low sails. -gt Approaching warm
front
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Altocumulus clouds rollers
Faster moving air
Eddies
Slower moving air
Clouds inside eddies
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Stratus clouds means layered in latin Flat,
grey, clouds, covering large areas of the sky
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Nimbostratus rain clouds associated with a warm
front
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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

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Profile of a Cold Front
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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)

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Veering winds as front approaches (typical for NE)
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Fig. 11.7
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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

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Fair weather cumulus clouds (flat, little
vertical structure)
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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
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Development of vertical structure

Rising air column
Incoming humid air
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Building cumulus clouds can be a sign of land
high up, seen from further away
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Building thunderheads
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Start of anvil-head formation
Air column reaches tropopause and spreads

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Mature anvil-head

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

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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.
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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?

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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 shouldnt be
  ignored.

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Basic Pressure Systems 1.Low
L
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Basic Pressure Systems 2.High
H
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Cyclones and Anticyclones
Cyclones and anti-cyclones High pressure systems
shed air Low pressure systems suck
air Coriolis force generates circulation
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Structure of a Hurricane
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Low pressure system over NE March 20th 08
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Low pressure systems in the N. Pacific
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High and low pressure systems in N.
Atlantic (www.oceanweather.com)
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Advection Fog formed by movement of warm air
over cooler surface
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Radiation Fog forms when land surface cools as a
result of outgoing radiation and in turn, cools
overlying air
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Wave Parameters(Figure 7-1a)
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What Causes Waves?

• Wind
• Submarine disturbance
• Gravitational attraction of sun and moon (tides
  very long wavelength waves)

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Motion of Water Particles Beneath Waves (Figure
7-3b)
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Deep Water Waves(Figure 7-4a)

• Waves do not interact with the seafloor
• Orbits of the water molecules are circular.

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Shallow Water Waves(Figure 7-4b)

• Waves interact with the seafloor are known as
  Orbits of the water molecules become elliptical.

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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

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Deep
Shallow
Gravitation 32 ft/sec/sec
Water depth (ft)
Wave length (ft)
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Instability when h gt 1/7 L OR when interior
angle is less 120 degrees
120o
h
L
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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.

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Cats paw
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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

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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

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Transformation of Shallow-water Waves (Figure
7-7b)
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Reflecting Swells at Great Wass
Island (Jonesport) Angle of incidence equals
angle of reflection
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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.

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Wave Refraction at Chatham Inlet Gradual
transition between deep and shallow water
Shallow water
Deep Water
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Extreme refraction at Baker Island (Mt. Desert)
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Swell patterns around an atoll
reflections
Main swell
Refractions
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Crossing swell patterns between islands
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Multi-swell patterns around island
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Polynesian stick chart illustrating swell
patterns from two islands