Presentation on theme: "John Huth Harvard University"— Presentation transcript:
1John Huth Harvard University Weather and WavesJohn HuthHarvard University
2Weather 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 equatorThe Coriolis effect causes air moving away from the equator to the pole to deflect to the eastThe Coriolis effect causes air moving from the pole toward the equator to deflect to the west
3Driving Forces Behind Wind Pressure GradientAir flows from high to low pressure (“downhill”)CoriolisCaused by the rotation of the earth, wind deflects to the right in the northern hemisphereCentripitalPresent when winds are in rotationFrictionAir moving along the Earth’s surface is slowed by friction
8Weather Basics IIEmergence of three convection cells in northern and southern hemisphere dominate wind patternsDoldrums – equatorTrades blow from east to westHorse latitudes – 30 degreesWesterlies – 30 to 60 degrees northAs moist air rises, it condenses and gives off heatThe 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
9Smaller scale version: land and sea breezes Temperature contrasts (the result of the differential heatingproperties of land and water) are responsible for the formationof land and sea breezes.
10Land Breeze-Sea Breeze Same effect, but on a much smaller scale
11Wind as direction indicator Good over short periods of time – persistentPrevailing winds generally useful, but seasonally dependentWeather systems and fronts can affect theseSurface winds versus winds aloftUnderstand how weather systems/seasons/diurnal variations affect wind patterns
12Wind roses for Boston Logan International Airport JanuaryJuly
13Local knowledge: summer wind patterns on Cape Cod During the months ofJune/July/August,in the absence of fronts,wind patterns on the Capeare reasonably stable.Little wind in the morning,picking up around 2 PMfrom SW, reaching peakaround 3:30, then subsiding.Mainly a sea breeze effect, coupled with prevailing SW winds
17Beaufort Scale – land indicators ForceStrengthkm/hEffectCalm0-1Smoke rises vertically1Light air1-5Smoke drifts slowly2Light breeze6-11Wind felt on face; leaves rustle3Gentle breeze12-19Twigs move; light flag unfurls4Moderate breeze20-29Dust and paper blown about; small branches move5Fresh breeze30-39Wavelets on inland water; small trees move6Strong breeze40-50Large branches sway; umbrellas turn inside out7Near gale51-61Whole trees sway; difficult to walk against wind8Gale62-74Twigs break off trees; walking very hard9Strong gale75-87Chimney pots, roof tiles and branches blown down10Storm88-101Widespread damage to buildings11Violent Storm12HurricaneOver 119Devastation
19Using 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 windsWinds 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.
20Wind shiftsVeering shifts – clockwise shift – typical for N. hemisphereBacking shifts – counterclockwise – typical for S. HemisphereFor approaching cold front – SW wind steady, veers to N to NW (typical)For approaching warm front – NE to SE winds, veers to SW (typical)
21Warm and Cold Air masses Warm air massesHumid, low pressure, warm - move up from equatorial regionsCold air massesDry, high pressure, cold – move down from polar regionsTransitions between air masses are called “fronts”
22Weather signsCloud 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.
24In mid-latitudes, fronts develop as Rossby waves, Typically seen as undulations in the jet-stream.Isolated pockets can develop as low and highpressure cells
25Warm fronts Slow in coming Sequence of clouds – build up of moisture in upper atmosphere, slowly coming down in heightJet contrails at 40,000 ft tend to stick aroundMoon or sun dogs (rings) – from ice crystalsCirrus clouds (mares’ tails)Cirro-stratus (mackerel scales) – 20,000Alto-cumulus (rollers) 15,000-20,000Stratus (sheet-like) ,000Nimbo-stratus (rain clouds) 5000 or lowerRain usually lasts for a longer time
36Wind shiftsVeering shifts – clockwise shift – typical for N. hemisphereBacking shifts – counterclockwise – typical for S. HemisphereFor approaching cold front – SW wind steady, veers to N to NW (typical)For approaching warm front – NE to SE winds, veers to SW (typical)
37Veering winds as front approaches (typical for NE)
48Air column frequently overshoots tropopause, “bubbles out” high cirrostratusFig. 11.2a
49THUNDERSTORM MATURE STAGE SHARP COOL GUSTS AT SURFACE SIGNAL DOWNDRAFTSUPDRAFTS EXIST SIDE BY SIDE WITH DOWNDRAFTSIF CLOUD TOP REACHES TROPOPAUSE UPDRAFTS SPREAD LATERALLY - ANVIL SHAPETOP OF ICE LADEN CIRRUS CLOUDSGUSTY WINDS, LIGHTNING, HEAVY PRECIPITATION, HAIL
50Multicell 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.
51Estimating distances to storms Base of clouds in thunderstorm is typically 5000 ft.Use range techniques to find distanceDifference between lightning and thunder arrival times (light is faster than sound)5 seconds per mile of distancePrevailing winds –Is the storm track moving toward you, or will it pass by?
52Thunderstorm/squall issues General direction is indicated by high cirrus clouds at top of anvil headNOT 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 issueLightning is less of a hazard, but shouldn’t be ignored.
63What Causes Waves? Wind Submarine disturbance Gravitational attraction of sun and moon (tides – very long wavelength waves)
64Motion of Water Particles Beneath Waves (Figure 7-3b)
65Deep Water Waves (Figure 7-4a) Waves do not interact with the seafloorOrbits of the water molecules are circular.
66Shallow Water Waves (Figure 7-4b) Waves interact with the seafloor are known as Orbits of the water molecules become elliptical.
67Characteristics of water waves Velocity depends on wavelength *or* water depthUnlike sound or light – velocity is independent of wavelength for theseWaves become unstable when height is 1/7th of wavelength – whitecaps (120 degree interior angle)Longer wavelength waves hold more energyDepth for “shallow” versus “deep” is about 2 times wavelength
71h L Instability – when h > 1/7 L OR – when interior angle is less 120 degrees120ohL
72Wind Generation of Waves The type of wave generated by wind is determined by:Wind velocityWind durationFetch (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.
74Fetch Conditions Time and distance Small waves buildup, break Larger waves begin – hold more energy before breakingGenerally a range of wavelengthsHigh wind velocity produces more uniform and longer wavelength wavesTypically for NE waters – fully developed seas only for 10 knot windsLarger seas in open oceanSwells travel huge distances unaffected
76Comments on Swells Product of distant storms Can travel thousands of miles without losing energyPeriod of swell indicates severity of storm –Longer period – more severe storm4 seconds – small8-10 seconds – hurricaneMid ocean – can have multiple swells crossingIn New England, sheltering of coast line limits significant swell directionE.g. Gulf of Maine typically will only see SE swellsRhode Island catches a lot of Atlantic stormsNewport beaches/surfing
77Transformation of Shallow-water Waves (Figure 7-7b)
78Reflecting Swells at Great Wass Island (Jonesport) Angle of incidence equals angle of reflection
79Wave Refraction (Figure 7-8a) Bending of the wave crest as waves enter shallow water. It is due toDrag along the bottom.Differential speed along the crest.
80Wave Refraction at Chatham Inlet Gradual transition between deep and shallow waterShallow waterDeep Water