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Weather and Waves. John Huth Harvard University. 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

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Weather and waves l.jpg

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 Earth’s surface is slowed by friction



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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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

Same effect, but on a much smaller scale


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Wind as direction indicator deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

January

July


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Local knowledge: summer wind patterns on Cape Cod deflected to the right in the NH and to the left in SH relative to the surface of the earth

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


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Wind compass – Taumako, Polynesia deflected to the right in the NH and to the left in SH relative to the surface of the earth


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Pukapukan wind compass deflected to the right in the NH and to the left in SH relative to the surface of the earth


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Fijian wind compass deflected to the right in the NH and to the left in SH relative to the surface of the earth


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Beaufort Scale – land indicators deflected to the right in the NH and to the left in SH relative to the surface of the earth


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Beaufort scale – at sea deflected to the right in the NH and to the left in SH relative to the surface of the earth


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Using wind deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth


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In mid-latitudes, fronts develop as Rossby waves, deflected to the right in the NH and to the left in SH relative to the surface of the earth

Typically seen as undulations in the jet-stream.

Isolated pockets can develop as low and high

pressure cells


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Warm fronts deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth


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Lingering jet contrail against a backdrop of deflected to the right in the NH and to the left in SH relative to the surface of the earth

cirrus clouds

If contrail breaks up -> low humidity

If contrail remains -> high humidity (approaching

Warm front)


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Sundogs – rings around the sun (or moon) deflected to the right in the NH and to the left in SH relative to the surface of the earth

Caused by ice crystals in the upper atmosphere

Cirro-stratus (high, layered clouds)

22 degree halo around sun/moon


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Mares tails – cirrus clouds deflected to the right in the NH and to the left in SH relative to the surface of the earth

(reading wind: watch cloud

motion relative to foreground

object)

Higher wind speed

Lower wind speed


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Mackerel scales – cirrocumulus clouds deflected to the right in the NH and to the left in SH relative to the surface of the earth

Old saying: “mackerel scales and mares tails make lofty ships carry low sails”.

-> Approaching warm front


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Altocumulus clouds – “rollers” deflected to the right in the NH and to the left in SH relative to the surface of the earth

Faster moving air

Eddies

Slower moving air

Clouds inside

eddies


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Stratus clouds – means “layered” in latin deflected to the right in the NH and to the left in SH relative to the surface of the earth

Flat, grey, clouds, covering large areas of the sky


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Nimbostratus – rain clouds associated with a deflected to the right in the NH and to the left in SH relative to the surface of the earth

warm front


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Cold Fronts deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth


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Wind shifts deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

(typical for NE)


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Fig. 11.7 deflected to the right in the NH and to the left in SH relative to the surface of the earth


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THUNDERSTORM deflected to the right in the NH and to the left in SH relative to the surface of the earthCUMULUS 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

(flat, little vertical structure)


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General character of convection deflected to the right in the NH and to the left in SH relative to the surface of the earth

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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

Rising air column

Incoming humid air


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Building cumulus clouds can be a sign of deflected to the right in the NH and to the left in SH relative to the surface of the earth

land – high up, seen from further away


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Building thunderheads deflected to the right in the NH and to the left in SH relative to the surface of the earth


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Start of anvil-head formation deflected to the right in the NH and to the left in SH relative to the surface of the earth

Air column reaches tropopause and spreads


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Mature anvil-head deflected to the right in the NH and to the left in SH relative to the surface of the earth


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Air column frequently overshoots tropopause, deflected to the right in the NH and to the left in SH relative to the surface of the earth

“bubbles out” high cirrostratus

Fig. 11.2a


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THUNDERSTORM deflected to the right in the NH and to the left in SH relative to the surface of the earthMATURE 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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.


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Basic Pressure Systems: 1.Low deflected to the right in the NH and to the left in SH relative to the surface of the earth

L


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Basic Pressure Systems: 2.High deflected to the right in the NH and to the left in SH relative to the surface of the earth

H


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Cyclones and Anticyclones deflected to the right in the NH and to the left in SH relative to the surface of the earth

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 deflected to the right in the NH and to the left in SH relative to the surface of the earth


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Low pressure system over NE – March 20 deflected to the right in the NH and to the left in SH relative to the surface of the earthth 08


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Low pressure systems in the N. Pacific deflected to the right in the NH and to the left in SH relative to the surface of the earth


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High and low pressure systems in N. Atlantic deflected to the right in the NH and to the left in SH relative to the surface of the earth

(www.oceanweather.com)


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Advection Fog deflected to the right in the NH and to the left in SH relative to the surface of the earth: formed by movement of warm air over cooler surface


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Radiation Fog deflected to the right in the NH and to the left in SH relative to the surface of the earth: forms when land surface cools as a result of outgoing radiation and in turn, cools overlying air


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Wave Parameters deflected to the right in the NH and to the left in SH relative to the surface of the earth(Figure 7-1a)


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What Causes Waves? deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • Wind

  • Submarine disturbance

  • Gravitational attraction of sun and moon (tides – very long wavelength waves)


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Motion of Water Particles Beneath Waves deflected to the right in the NH and to the left in SH relative to the surface of the earth(Figure 7-3b)


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Deep Water Waves deflected to the right in the NH and to the left in SH relative to the surface of the earth(Figure 7-4a)

Waves do not interact with the seafloor

Orbits of the water molecules are circular.


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Shallow Water Waves deflected to the right in the NH and to the left in SH relative to the surface of the earth(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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

Shallow

Gravitation 32 ft/sec/sec

Water depth (ft)

Wave length (ft)


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Instability – when h > 1/7 L deflected to the right in the NH and to the left in SH relative to the surface of the earth

OR – when interior angle is less 120 degrees

120o

h

L


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Wind Generation of Waves deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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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Cat’s paw deflected to the right in the NH and to the left in SH relative to the surface of the earth


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Fetch Conditions deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

  • 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 deflected to the right in the NH and to the left in SH relative to the surface of the earth(Figure 7-7b)


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Reflecting Swells at Great Wass Island deflected to the right in the NH and to the left in SH relative to the surface of the earth

(Jonesport)

Angle of incidence equals angle of reflection


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Wave Refraction deflected to the right in the NH and to the left in SH relative to the surface of the earth(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 deflected to the right in the NH and to the left in SH relative to the surface of the earth

Gradual transition between deep and shallow water

Shallowwater

Deep Water


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Extreme refraction at Baker Island deflected to the right in the NH and to the left in SH relative to the surface of the earth

(Mt. Desert)


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Swell patterns around an atoll deflected to the right in the NH and to the left in SH relative to the surface of the earth

reflections

Main

swell

Refractions


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Crossing swell patterns between islands deflected to the right in the NH and to the left in SH relative to the surface of the earth


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Multi-swell patterns around island deflected to the right in the NH and to the left in SH relative to the surface of the earth


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Polynesian stick chart – illustrating deflected to the right in the NH and to the left in SH relative to the surface of the earth

swell patterns from two islands


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